Chapter 6Energy and Chemical Change
Brady and Senese
5th Edition
Index
61 An object has energy if it is capable of doing work62 Internal energy is the total energy of an objectrsquos
molecules63 Heat can be determined by measuring temperature
changes 64 Energy is absorbed or released during most chemical
reactions65 Heats of reaction are measured at constant volume or
constant pressure66 Thermochemical equations are chemical equations that
quantitatively include heat67 Thermochemical equations can be combined because
enthalpy is a state function68 Tabulated standard heats of reaction can be used to
predict any heat of reaction using Hessrsquos law
61 An object has energy if it is capable of doing work 3
Energy is the Ability to do Work
bull Energy is the ability to do work (move mass over a distance) or transfer heat
bull Types kinetic and potential Kinetic - the energy of motion
Potential - the stored energy in matter
bull Internal energy (E) - the sum of the kinetic and potential energy for each particle in the system
61 An object has energy if it is capable of doing work 4
Kinetic Energy The Energy of Motion
bull KE = frac12mv2
bull Energy can be transferred by moving particles
bull Collision of fast particles with slower particles causes the slow particle to speed up while the fast molecule slows This is why hot water cools in contact with cool air
61 An object has energy if it is capable of doing work 5
Potential Energy Depends on Position
bull Potential energy increases when Objects that attract move apart or
Objects that repel move toward each other
bull Stored energy that can be converted to kinetic energy
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
Index
61 An object has energy if it is capable of doing work62 Internal energy is the total energy of an objectrsquos
molecules63 Heat can be determined by measuring temperature
changes 64 Energy is absorbed or released during most chemical
reactions65 Heats of reaction are measured at constant volume or
constant pressure66 Thermochemical equations are chemical equations that
quantitatively include heat67 Thermochemical equations can be combined because
enthalpy is a state function68 Tabulated standard heats of reaction can be used to
predict any heat of reaction using Hessrsquos law
61 An object has energy if it is capable of doing work 3
Energy is the Ability to do Work
bull Energy is the ability to do work (move mass over a distance) or transfer heat
bull Types kinetic and potential Kinetic - the energy of motion
Potential - the stored energy in matter
bull Internal energy (E) - the sum of the kinetic and potential energy for each particle in the system
61 An object has energy if it is capable of doing work 4
Kinetic Energy The Energy of Motion
bull KE = frac12mv2
bull Energy can be transferred by moving particles
bull Collision of fast particles with slower particles causes the slow particle to speed up while the fast molecule slows This is why hot water cools in contact with cool air
61 An object has energy if it is capable of doing work 5
Potential Energy Depends on Position
bull Potential energy increases when Objects that attract move apart or
Objects that repel move toward each other
bull Stored energy that can be converted to kinetic energy
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 3
Energy is the Ability to do Work
bull Energy is the ability to do work (move mass over a distance) or transfer heat
bull Types kinetic and potential Kinetic - the energy of motion
Potential - the stored energy in matter
bull Internal energy (E) - the sum of the kinetic and potential energy for each particle in the system
61 An object has energy if it is capable of doing work 4
Kinetic Energy The Energy of Motion
bull KE = frac12mv2
bull Energy can be transferred by moving particles
bull Collision of fast particles with slower particles causes the slow particle to speed up while the fast molecule slows This is why hot water cools in contact with cool air
61 An object has energy if it is capable of doing work 5
Potential Energy Depends on Position
bull Potential energy increases when Objects that attract move apart or
Objects that repel move toward each other
bull Stored energy that can be converted to kinetic energy
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 4
Kinetic Energy The Energy of Motion
bull KE = frac12mv2
bull Energy can be transferred by moving particles
bull Collision of fast particles with slower particles causes the slow particle to speed up while the fast molecule slows This is why hot water cools in contact with cool air
61 An object has energy if it is capable of doing work 5
Potential Energy Depends on Position
bull Potential energy increases when Objects that attract move apart or
Objects that repel move toward each other
bull Stored energy that can be converted to kinetic energy
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 5
Potential Energy Depends on Position
bull Potential energy increases when Objects that attract move apart or
Objects that repel move toward each other
bull Stored energy that can be converted to kinetic energy
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 6
Your Turn
Which of the following is not a form of kinetic energy
A A pencil rolls across a desk
B A pencil is sharpened
C A pencil is heated
D All are forms of kinetic energy
E None are forms of kinetic energy
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 7
Law of Conservation of Energy
bull Energy cannot be created or destroyed but can be transformed from one form of energy to another
bull Also known as the first law of thermodynamics
How does water falling over a waterfall demonstrate this law
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 8
Heat and Temperature are Not the Same
bull The temperature of an object is proportional to the average kinetic energy of its particlesmdashthe higher the average kinetic energy the higher the temperature
bull Heat is energy (also called thermal energy) transferred between objects caused by differences in their temperatures until they reach thermalequilibrium
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
61 An object has energy if it is capable of doing work 9
Units of Energy
bull SI unit is the Joule J J = kgm2s2
If the calculated value is greater than 1000 J use the kJ
bull Another unit is the calorie cal cal = 4184 J (exact)
bull Nutritional unit is the Calorie (note capital C) which is one kilocalorie 1 Cal = 1 kcal = 4184 kJ
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
62 Internal energy is the total energy of an objectrsquos molecules 10
bull 1st Law of Thermodynamics For an isolated system the internal energy (E) is constant
Δ E = Ef - Ei = 0
Δ E = Eproduct- Ereactant= 0
bull We canrsquot measure the internal energy of anything so we measure the changes in energy
bull E is a state function
Internal Energy is Conserved
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
62 Internal energy is the total energy of an objectrsquos molecules 11
bull Temperature (T) is proportional to the average kinetic energy of all particle units degC degF K KEaverage= frac12mvaverage
2
bull At a high temperature most molecules are moving at higher average
What is Temperature
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
62 Internal energy is the total energy of an objectrsquos molecules 12
State Function
bull A property whose value depends only on the present state of the system not on the method or mechanism used to arrive at that state
bull Position is a state function both train and car travel to the same locations although their paths vary
bull The actual distance traveled does vary with path
New York
Los Angeles
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 13
Heat Transfer is a State Function
bull Transfer of heat during a reaction is a state functionbull The route taken to arrive at the products does not
affect the amount of heat that is transferredbull The number of steps does not affect the amount of
heat that is transferred
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 14
Heat Transfer q
bull Heat (q) - the transfer of energy from regions of high temperature to regions of lower temperature
bull Units J cal kgmiddotm2s2
A calorie is the amount of energy needed to raise the temperature of 100 g water from 145 to 155 degC
bull A metal spoon at 25 degC is placed in boiling water What happens
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 15
Surroundings System Universe
bull System - the reaction or area under studybull Surroundings - the rest of the universebull Open systems can gain or lose mass and energy
across their boundaries ie the human body
bull Closed systems can absorb or release energy but not mass across the boundary ie a light bulb
bull Isolated systems (adiabatic) cannot exchange matter or energy with their surroundings ie a closed Thermos bottle
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 16
The Sign Convention
bull Endothermic systems require energy to be added to the system thus the q is (+)
bull Exothermic reactions release energy to the surroundings Their q is (-)
bull Energy changes are measured from the point of view of the system
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 17
Your Turn
A cast iron skillet is moved from a hot oven to a sink full of water Which of the following is not true
A The water heats
B The skillet cools
C The heat transfer for the skillet has a (-) sign
D The heat transfer for the skillet is the same as the heat transfer for the water
E None of these are untrue
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 18
Heat Capacity and Transfer
bull Heat capacity (C) - the (extensive) ability of an object with constant mass to absorb heat Calorimeter constant
Varies with the sample mass and the identity of the substance
Units J degC-1
bull q = Ctimes ∆t q = heat transferred
C = heat capacity of object
Δt = Change in Temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 19
Learning Check
A cup of water is used in an experiment Its heat capacity is known to be 720 J degC-1 How much heat will it absorb if the experimental temperature changed from 192 degC to 235 degC
q = Ctimes ∆t
q = 720 J degC-1times (235 - 192 degC)
q = 31 times 103 J
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 20
Heat Transfer and Specific Heat
bull Specific heat (s) - The intensive ability of a substance to store heat
C = mtimes s
Units J g-1degC-1 or J g-1 K-1 or J mol-1 K-
1
bull q = mtimes ∆ttimes s
q = heat transferred
m = mass of object
Δt = change in temperature (tfinal - tinitial)
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 21
Specific Heats
SubstanceSpecific Heat
J g-1degC-1
(25 degC)
Carbon (graphite) 0711
Copper 0387
Ethyl alcohol 245
Gold 0129
Granite 0803
Iron 04498
Lead 0128
Olive oil 20
Silver 0235
Water (liquid) 418
bull Substances with high specific heats resist temperature changes
bull Note that water has a very high specific heat This is why coastal
temperatures are different from inland temperatures
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 22
Learning Check
Calculate the specific heat of a metal if it takes 235 J to raise the temperature of a 3291 g sample by 253 degC
235 J J282
3291 g 253 degC g degC
= times times ∆
= = =times ∆ times
q m s t
qs
m t
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 23
The First Law of Thermodynamics Explains Heat Transfer
bull If we monitor the heat transfers (q) of all materials involved and all work processes we can predict that their sum will be zero
bull By monitoring the surroundings we can predict what is happening to our system
bull Heat transfers until thermal equilibrium thus the final temperature is the same for all materials
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 24
Learning CheckA 4329 g sample of solid is transferred from boiling water (t = 998 degC) to 152 g water at 225 degC in a coffee cup The twaterrose to 243 degC Calculate the specific heat of the solid
qsample+ qwater+ qcup= 0
qcup is neglected in problem
qsample= -qwater
qsample= mtimes stimes ∆t
qsample= 4329 g times stimes (243 - 998 degC)
qwater= 152 g times 4184 J g-1 degC-1times (243 ndash 225 degC)
4329 g times stimes (243 - 998 degC) = -(152 g times 4184 J g-1degC-1times (243 ndash225 degC))
stimes (-327 times 103) g-1degC-1 = -114 times 103 J
s = 0349 J g-1degC-1
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
63 Heat can be determined by measuring temperature changes 25
Your Turn
What is the heat capacity of the container if 100 g of water (s = 4184 J g-1degC-1) at 100 degC are added to 100 g of water at 25 degC in the container and the final temperature is 61 degC
A 870 JdegC
B 35 JdegC
C -35 JdegC
D -870 JdegC
E None of these
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
64 Energy is absorbed or released during most chemical reactions 26
bull Chemical bond - net attractive forces that bind atomic nuclei and electrons together
bull Exothermic reactions form stronger bonds in the product than in the reactant and release energy
bull Endothermic reactions break stronger bonds than they make and require energy
Chemical Potential Energy
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 27
Work and Pistons
bull Pressure = forcearea
bull If the container volume changes the pressure changes
bull Work = -PtimesΔV Units L bull atm
1 L bull atm = 101 J
bull In expansion ∆V gt 0 and is exothermic
bull Work is done by the system in expansion
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 28
How does work relate to reactions
bull Work = Force middot Distance
bull Is most often due to the expansion or contraction of a system due to changing moles of gas
bull Gases push against the atmospheric pressure so Psystem= -Patm
w = -Patm timesΔV
bull The deployment of an airbag is one example of this process
1C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g)
6 moles of gasrarr 7 moles of gas
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 29
Learning Check P-V workEthyl chloride is prepared by reaction of ethylene with HCl How much P-V work (in J) is done if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L (1 L bull atm = 101 J)
Calculate the work (in kilojoules) done during a synthesis of ammonia in which the volume contracts from 86 L to 43 L at a constant external pressure of 44 atm
w = 19 kJ
w = 722 times 103 J
w = -44 atm times (43 - 86) L = 19 Latm
w = -1atm times -715 L = 715 Latm
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 30
Energy can be Transferred as Heat and Work
bull ∆E = q + w
bull Internal energy changes are state functions
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 31
Your TurnWhen TNT is combusted in air it is according to the following reaction
4C6H2(NO2)3CH3(s) + 17O2(g) rarr 24CO2(g) + 10H2O(l) + 6N2(g)
The reaction will do work for all of these reasons except
A The moles of gas increase
B The volume of gas increases
C The pressure of the gas increases
D None of these
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 32
Calorimetry is Used to Measure Heats ofReaction
bull Heat of reaction - the amount of heat absorbed or released in a chemical reaction
bull Calorimeter - an apparatus used to measure temperature changes in materials surrounding a reaction that result from a chemical reaction
bull From the temperature changes we can calculate the heat of the reaction q qv heat measured under constant volume conditions qp heat measured under constant pressure conditions qp termed enthalpy ∆H
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 33
Internal Energy is Measured with a Bomb Calorimeter
bull Used for reactions in which there is change in the number of moles of gas present
bull Measures qv
bull Immovable walls mean that work is zero
bull ΔE = qv
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 34
Learning Check Bomb CalorimeterA sample of 500 mg naphthalene (C10H8) is combusted in a bomb calorimeter containing 1000 g of water The temperature of the water increases from 2000 degC to 2437 degC The calorimeter constant is 420 JdegC What is the change in internal energy for the reaction swater= 4184 JgdegC
qcal= 420 Jgtimes (2437 - 2000) degC
qwater= 1000 g times (2437 - 2000) degC times 4184 JgdegC
qv reaction+ qwater+ qcal = 0 by the first law
qv reaction= ∆E = -20 times 104 J
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 35
Enthalpy of Combustion
bull When one mole of a fuel substance is reacted with elemental oxygen a combustion reaction can be written
bull Is always negative
bull Learning Check What is the equation associated with the enthalpy of combustion of C6H12O6(s)
C6H12O6(s) + 9O2(g) rarr 6CO2(g) + 6H2O(l)
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 36
Your Turn
A 252 mg sample of benzoic acid C6H5CO2H is combusted in a bomb calorimeter containing 814 g water at 2000 degC The reaction increases the temperature of the water to 2170 degC What is the internal energy released by the process
A -711 J
B -285 J
C +711 J
D +285 J
E None of these
swater= 4184 Jg degC
qw + qcal + qv reaction= 0 qcal is ignored by the problemqv reaction= -qw = -814 g times (2170 - 2000) degC times 4184 J g-1degqv reaction= -5789 J
-579 kJ
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 37
Enthalpy Change (ΔH)
bull Enthalpy is the heat transferred at constant pressure
ΔH = qp
ΔE = qp - PΔV = ΔH - PΔV
ΔH = ΔHfinal -ΔHinitial
ΔH = ΔHproduct-ΔHreactant
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 38
Enthalpy Measured in a Coffee Cup Calorimeter
bull When no change in moles of gas is expected we may use a coffee cup calorimeter
bull The open system allows the pressure to remain constant
bull Thus we measure qp
bull ∆E = qp + w or ∆E = ∆H ndash P∆V
bull Since there is no change in the moles of gas present there is no work
bull Thus we also are measuring ∆E
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 39
Learning Check Coffee Cup CalorimetryWhen 500 mL of 0987 M H2SO4 is added to 500 mL of 100 MNaOH at 250 degC in a coffee cup calorimeter the temperature of the aqueous solution increases to 317 degC Calculate heat for the reaction per mole of limiting reactant
Assume that the specific heat of the solution is 418 JgdegC the density is 100 gmL and that the calorimeter itself absorbs a negligible amount of heat
H2SO4(aq) + 2NaOH(aq) rarr 2H2O(l) + Na2SO4(aq)
mol NaOH = 00500 mol is limitingmol H2SO4 = 00494 mol
qsoln = 100 g soln times (317 - 250) degC times 418 JgdegC
qp rxn = -56 times 104 J
qp rxn + qcal + qsoln = 0 thus qp rxn = -qsoln
qp rxn = -28 times 103 J
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 40
Your TurnA sample of 5000 mL of 0125 M HCl at 2236 degC is added to a 5000 mL of 0125 M Ca(OH)2 at 2236 degC The calorimeter constant was 72 J g-1degC-1 The temperature of the solution (s = 4184 J g-1degC-1 d = 100 gmL) climbed to 2330 degC Which of the following is not true
A qcal = 677 JB qsolution = 3933 JC qrxn = 4610 JD qrxn = -4610 JE None of these
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
65 Heats of reaction are measured at constant volume or constant pressure 41
Calorimetry Overview
bull The equipment used depends on the reaction type
bull If there will be no change in the moles of gas we may use a coffee-cup calorimeter or a closed system Under these circumstances we measure qp
bull If there is a large change in the moles of gas we use a bomb calorimeter to measure qv
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 42
Thermochemical Equations
bull Relate the energy of a reaction to the quantities involved
bull Must be balanced but may use fractional coefficients bull Quantities are presumed to be in molesbull Example
C(s) + O2(g) rarr CO2(g) ∆Hdeg = -3935 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 43
2C2H2(g) + 5O2(g)rarr 4CO2(g) + 2H2O(g)
ΔH = -2511 kJ
The reactants (acetylene and oxygen) have 2511 kJ more energy than the products How many kJ are released for 1 mol C2H2
Learning Check
1256 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 44
Learning Check
6CO2(g) + 6H2O(l) rarr C6H12O6(s) + 6O2(g) ∆H = 2816 kJ
How many kJ are required for 44 g CO2 (molar mass = 4401 gmol)
If 100 kJ are provided what mass of CO2 can be converted to glucose
938 g
470 kJ
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 45
Learning Check Calorimetry of Chemical ReactionsThe meals-ready-to-eat (MRE) in the military can be heated on a flameless heater Assume the reaction in the heater is
Mg(s) + 2H2O(l) rarr Mg(OH)2(s) + H2(g)
ΔH = -353 kJ
What quantity of magnesium is needed to supply the heat required to warm 25 mL of water from 25 to 85 degC Specific heat of water = 4184 J g-1degC-1 Assume the density of the solution is the same as for water at 25 degC 100 g mL-1
qsoln = 25 g times (85 - 25) degC times 4184 J g-1degC-1 = 63times 103 J
masssoln = 25 mL times 100 g mL-1 = 25 g
(63 kJ)(1 mol Mg353 kJ)(243 g mol-1 Mg) = 043 g
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 46
Your TurnConsider the thermite reaction The reaction is initiated by the heat released from a fuse or reaction The enthalpy change is -848 kJ mol-1 Fe2O3 at 298 K
2Al(s) + Fe2O3(s) rarr 2Fe(s) + Al2O3(s)
What mass of Fe (molar mass 55847 g mol-1) is made when 500 kJ are released
A 659 g
B 0587 g
C 328 g
D None of these
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
66 Thermochemical equations are chemical equations that quantitatively include heat 47
Learning Check Ethyl Chloride Reaction RevisitedEthyl chloride is prepared by reaction of ethylene with HCl
C2H4(g) + HCl(g) rarr C2H5Cl(g) ΔHdeg = -723 kJ
What is the value of ΔE if 895 g ethylene and 125 g of HCl are allowed to react at atmospheric pressure and the volume change is -715 L
Ethylene = 2805 gmol HCl = 3646 gmol
mol C2H4 319 mol is limitingmol HCl 343 molΔHrxn =319mol times-723 kJmol
=-2306 kJ
w = -1 atm times -715 L
= 715 Latm = 7222 kJΔE= -2306kJ+72kJ= -223 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 48
Enthalpy Diagram
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 49
Hessrsquos Law
The overall enthalpy change for a reaction is equal to the sum of the enthalpychangesfor individual steps in the reaction
For example
2Fe(s) + 32O2(g) rarr Fe2O3(s) ∆H = -8222 kJ
Fe2O3(s) + 2Al(s) rarr Al2O3(s) + 2Fe(s) ∆H = -848 kJ
32O2(g) + 2Al(s) rarr Al2O3(s) ∆H = -8222 kJ + -848 kJ
-1670 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 50
Rules for Adding Thermochemical Reactions
1 When an equation is reversedmdashwritten in the opposite directionmdashthe sign of H must also be reversed
2 Formulas canceled from both sides of an equation must be for the substance in identical physical states
3 If all the coefficients of an equation are multiplied or divided by the same factor the value of H must likewise be multiplied or divided by that factor
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 51
Strategy for Adding Reactions Together
1 Choose the most complex compound in the equation (1)
2 Choose the equation (2 or 3 orhellip) that contains the compound
3 Write this equation down so that the compound is on the appropriate side of the equation and has an appropriate coefficient for our reaction
4 Look for the next most complex compound
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 52
Hessrsquos Law (Cont)
5 Choose an equation that allows you to cancel intermediates and multiply by an appropriate coefficient
6 Add the reactions together and cancel like terms
7 Add the energies together modifying the enthalpy values in the same way that you modified the equation
bull If you reversed an equation change the sign on the enthalpy
bull If you doubled an equation double the energy
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 53
Learning Check
How can we calculate the enthalpy change for the reaction 2 H2(g) + N2(g) rarr N2H4(g) using these equations
N2H4(g) + H2(g) rarr 2NH3(g) ΔHdeg = -1878 kJ
3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
Reverse the first reaction (and change sign)2NH3(g) rarr N2H4(g) + H2(g) ΔHdeg = +1878 kJ
Add the second reaction (and add the enthalpy)3H2(g) + N2(g) rarr 2NH3(g) ΔHdeg = -924 kJ
2NH3(g) + 3H2(g) + N2(g) rarr N2H4(g) + H2(g) + 2NH3(g)
2H2(g) + N2(g) rarr N2H4(g) (1878 - 924) = +954 kJ
2
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 54
Learning CheckCalculate ΔH for 2C(s) + H2(g) rarr C2H2(g) using
bull 2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992 kJ
bull C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ
bull H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ
-frac12(2C2H2(g) + 5O2(g) rarr 4CO2(g) + 2H2O(l) ΔHdeg = -25992)
2CO2(g) + H2O(l) rarr C2H2(g) + 52O2(g) ΔHdeg = 12996
2(C(s) + O2(g) rarr CO2(g) ΔHdeg = -3935 kJ)
2C(s)+ 2O2(g) rarr 2CO2(g) ΔHdeg = -7870 kJ
-1(H2O(l) rarr H2(g) + frac12 O2(g) ΔHdeg = +2859 kJ)
H2(g) + frac12 O2(g) rarrH2O(l) ΔHdeg = -2859 kJ
2C(s) + H2(g) rarr C2H2(g) ΔHdeg = +2267 kJ
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
67 Thermochemical equations can be combined because enthalpy is a state function 55
Your Turn
What is the energy of the following process6A + 9B + 3D + F rarr 2G
Given that C rarr A + 2B ∆H = 202 kJmol2C + D rarr E + B ∆H = 301 kJmol3E + F rarr 2G ∆H = -801 kJmolA 706 kJB -298 kJC -1110 kJD None of these
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
56
State Matters
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(g) ΔH = -2043 kJ
bull C3H8(g) + 5O2(g) rarr 3CO2(g) + 4H2O(l) ∆H = -2219 kJ
bull Note that there is a difference in energy because the states do not match
bull If H2O(l) rarr H2O(g) ∆H = 44 kJmol
4H2O(l) rarr 4H2O(g) ∆H = 176 kJmol
-2219 + 176 kJ = -2043 kJ
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
57
Most stable form of the pure substance at
bull 1 atm pressure
bull Stated temperature If temperature is not specified assume 25 degC
bull Solutions are 1 M in concentration
bull Measurements made under standard state conditions have the deg mark ΔHdeg
bull Most ΔH values are given for the most stable form of the compound or element
Standard State
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
58
Determining the Most Stable State
bull The most stable form of a substance below the melting point is solid above the boiling point is gas between these temperatures is liquid
What is the standard state of GeH4 mp -165 degC bp -885 degC
What is the standard state of GeCl4 mp -495 degC bp 84 degC
gas
liquid
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
59
Allotropes
bull Are substances that have more than one form in the same physical state
bull You should know which form is the most stable
bull C P O and S all have multiple allotropes Which is the standard state for each C ndash solid graphite
P ndash solid white
O ndash gas O2 S ndash solid rhombic
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
60
Enthalpy of Formation
bull Enthalpy of formation is the enthalpy change ΔHdegf for the formation of 1 mole of a substance in its standard state from elements in their standard states
bull Note ΔHdegf = 0 for an element in its standard state
bull Learning CheckWhat is the equation that describes the formation of CaCO3(s)
Ca(s) + C(graphite) + 32O2(g) rarr CaCO3(s)
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol
68 Tabulated standard heats of reaction can be used to predict any heat of reaction using Hessrsquos law
61
Calculating ΔH for Reactions Using ΔHdegdegdegdegf
ΔHdegrxn = [sum of ΔHdegf of all products]
ndash [sum ofΔHdegf of all reactants]
2Fe(s) + 6H2O(l) rarr 2Fe(OH)3(s) + 3H2(g)
0 -2858 -6965 0
CO2(g) + 2H2O(l) rarr 2O2(g) + CH4(g)-3935 -2858 0 -748
ΔHdegrxn = 3218 kJ
ΔHdegrxn = 8903 kJ
Your TurnWhat is the enthalpy for the following reaction
A 965 kJ
B -965 kJ
C 482 kJ
D -482 kJ
E None of these
2H2CO3(aq) + 2OH-(aq) rarr 2H2O(l) + 2HCO3- (aq)
∆Hfordm -69965
kJmol
-2300 kJmol
-2859 kJmol
-69199 kJmol