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Calorimetry
Calorimetry Calorimetry - the accurate and
precise measurement of heat change for chemical and physical processes.
The device used to measure the absorption or release of heat in chemical or physical processes is called a Calorimeter
Calorimetry Foam cups are excellent heat
insulators, and are commonly used as simple calorimeters
A Cheap Calorimeter
For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system
Calorimetry Changes in enthalpy = H q = H These terms will be used
interchangeably in this textbook Thus, q = H = m x C x T H is negative for an exothermic
reaction H is positive for an endothermic
reaction
Calorimetry Calorimetry experiments can be
performed at a constant volume using a device called a “bomb calorimeter” - a closed system
9
In terms of bonds
COO C
O
O
Breaking this bond will require energy.
CO
OOO C
Making these bonds gives you energy.In this case making the bonds gives you more energy than breaking them.
10
Exothermic The products are lower in energy
than the reactants Releases energy
2Al (s) + 3Cl2 (g) --> 2 AlCl3 (s) + 1408 kJ
∆H=1408 kJ
11
C + O2 CO2E
nerg
y
Reactants Products
C + O2
C O2
395kJ
+ 395 kJ
12
Endothermic The products are higher in energy
than the reactants Absorbs energy
2 H2O + 575 kJ ------> 2 H2 + 1 O2 (g)
∆H = + 572 kJ
13
CaCO3 CaO + CO2E
nerg
y
Reactants Products
CaCO3
CaO + CO2
176 kJ
CaCO3 + 176 kJ CaO + CO2
14
Chemistry Happens in
MOLES An equation that includes energy is
called a thermochemical equation CH4 + 2O2 CO2 + 2H2O + 802.2 kJ
1 mole of CH4 releases 802.2 kJ of energy.
When you make 802.2 kJ you also make 2 moles of water
What is the molar enthalpy of CO2 (g) in the reaction for the burning of butane below?
2 C4H10 +13 O2 8 CO2 +10 H2O
∆H=-5315 kJ
Answer: Molar enthalpy is the enthalpy change in equation divided by the balance of CO2
Molar enthalpy, ∆H substance = 5315 kJ ÷ 8 mol
= 664 kJ / mol.
For each of the following rewrite the equation in " H " notation, for one mole of the underlined substance.
Fe2O3 (s)+3CO(g)→3CO2(g)+2Fe(s)+25kJ
Answer:1/3 Fe2O3 (s)+CO(g)CO2(g)+2/3 Fe(s) ∆H = - 8.3 KJ
4 NH3(g)+5O2 (g)→4 NO(g)+6H2O(l)+1170kJ
2 HCl (g)+96 KJ → H2 (g)+Cl2 (g)
N2 (g)+3 H2 (g) → 2 NH3 (g)+92 KJ
2 CO2 (g)+566 KJ →2 CO (g)+ O2 (g)
4 Al (s) +3 O2 (g) →2 Al2O3 (s)+3360 KJ
18
Thermochemical Equations A heat of reaction is the heat
change for the equation, exactly as written• The physical state of reactants
and products must also be given.
• Standard conditions for the reaction is 101.3 kPa (1 atm.) and 25 oC
19
CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ
If 10. 3 grams of CH4 are burned completely, how much heat will be produced?
10. 3 g CH4
16.05 g CH4
1 mol CH4
1 mol CH4
802.2 kJ
= 514 kJ
20
CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ
How many liters of O2 at STP would be required to produce 23 kJ of heat?
How many grams of water would be produced with 506 kJ of heat?
How much heat will be released if 65 grams of butane is burned in a lighter according the equation:
2 C4H10 +13 O2 8 CO2 +10 H2O
∆H=-5315 kJ
= 2976.4 kJ
= 3.0 MJ
104
104104 2
5315
14.58
165
HmolC
kJ
g
HmolCHgC
Calculate the heat released when 120 grams of Iron (III) oxide is formed by the following equation
2 Fe2O3 (s) → 4 Fe(s)+3 O2 (g)
∆H=1625 kJ
mol
kJ
g
OmolFeOgFe
2
1625
70.159
1120 32
32
= 610.5 kJ= 610 kJ
Q = n ∆H (substance)
Where n = # of moles
What mass of carbon dioxide must form to create 1200 kJ of heat when the following reaction occurs?
C6H12O6(s)+6O2(g)→6CO2(g)+6H2O(l)
∆H=- 2808kJ
Answer: 110 grams
3) What mass of oxygen is needed to completely react and release 550 kJ of heat in the following reaction?
4Fe (s)+3O2 (g) → 2 Fe2O3 (s)
∆H=- 1625 kJ
Answer: 32 grams
Summary, so far...
27
Enthalpy The heat content a substance has at a
given temperature and pressure Can’t be measured directly because
there is no set starting point The reactants start with a heat content The products end up with a heat content So we can measure how much enthalpy
changes
28
Enthalpy Symbol is H Change in enthalpy is H (delta H) If heat is released, the heat content of
the products is lower
H is negative (exothermic) If heat is absorbed, the heat content
of the products is higher
H is positive (endothermic)
29
Ene
rgy
Reactants Products
Change is down
H is <0
30
Ene
rgy
Reactants Products
Change is upH is > 0
31
Heat of Reaction The heat that is released or absorbed in a
chemical reaction Equivalent to H C + O2(g) CO2(g) + 393.5 kJ
C + O2(g) CO2(g) H = -393.5 kJ
In thermochemical equation, it is important to indicate the physical state
H2(g) + 1/2O2 (g) H2O(g) H = -241.8 kJ
H2(g) + 1/2O2 (g) H2O(l) H = -285.8 kJ
32
Heat of Combustion The heat from the reaction that
completely burns 1 mole of a substance
33
OBJECTIVES:
• Classify, by type, the heat changes that occur during melting, freezing, boiling, and condensing.
34
OBJECTIVES:
• Calculate heat changes that occur during melting, freezing, boiling, and condensing.
35
Heats of Fusion and Solidification
Molar Heat of Fusion (Hfus) - the heat absorbed by one mole of a substance in melting from a solid to a liquid
Molar Heat of Solidification (Hsolid) - heat lost when one mole of liquid solidifies
36
Heats of Fusion and Solidification
Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies
• Thus, Hfus = -Hsolid
37
Heats of Vaporization and Condensation
When liquids absorb heat at their boiling points, they become vapors.
Molar Heat of Vaporization (Hvap) - the amount of heat necessary to vaporize one mole of a given liquid.
38
Heats of Vaporization and Condensation
Condensation is the opposite of vaporization.
Molar Heat of Condensation (Hcond) - amount of heat released when one mole of vapor condenses
Hvap = - Hcond
39
Heats of Vaporization and Condensation
The large values for Hvap and Hcond are the reason hot vapors such as steam is very dangerous
• You can receive a scalding burn from steam when the heat of condensation is released!
40
Heats of Vaporization and Condensation
H20(g) H20(l) Hcond = - 40.7kJ/mol
41
Heat of Solution Heat changes can also occur when
a solute dissolves in a solvent. Molar Heat of Solution (Hsoln) -
heat change caused by dissolution of one mole of substance
Sodium hydroxide provides a good example of an exothermic molar heat of solution:
42
Heat of Solution
NaOH(s) Na1+(aq) + OH1-
(aq)
Hsoln = - 445.1 kJ/mol The heat is released as the ions
separate and interact with water, releasing 445.1 kJ of heat as Hsoln thus becoming so hot it steams
H2O(l)