ThermochemistryThermochemistry

Unit theme: EnergyUnit theme: Energy

Thermochemistry

Heat capacityEndothermic and exothermic reactions

Specific heat

calorimetryUnits of heat

calorie

joule

Heats of changes of state

Thermochemical equations

Hess’ Law

enthalpy

entropy

Gibb’s Free energy

Potential energy diagrams

What evidence can you see What evidence can you see that energy is involved?that energy is involved?

EnergyEnergy

Defined as the capacity to do work or Defined as the capacity to do work or supply heatsupply heat

Chemical potential energy:Chemical potential energy: aka chemical energyaka chemical energy Energy stored in chemicals because of their Energy stored in chemicals because of their

compositionscompositions Different substances store different amount of Different substances store different amount of

energiesenergies

HeatHeat

A form of energy that flows from a warmer A form of energy that flows from a warmer object to a cooler objectobject to a cooler object QQ Can’t be measured directlyCan’t be measured directly

Can only measure its effect on temperatureCan only measure its effect on temperature

When heat is added to a system, its When heat is added to a system, its temperature risestemperature rises

Temperature is not heat!Temperature is not heat! Temperature is a measure of average kinetic energyTemperature is a measure of average kinetic energy

ThermochemistryThermochemistry

The study of heat changes that occur The study of heat changes that occur during chemical reactions and physical during chemical reactions and physical changes of statechanges of state

Units of HeatUnits of Heat

caloriecalorie The quantity of heat needed to raise the The quantity of heat needed to raise the

temperature of 1 g of pure water by 1 degree temperature of 1 g of pure water by 1 degree CelsiusCelsius 1 kcal = 1000 calories = 1 Calorie (nutrition)1 kcal = 1000 calories = 1 Calorie (nutrition)

joulejoule SI unit, named after British physicistSI unit, named after British physicist newtonnewtonmetermeter

1 cal = 4.186 joules1 cal = 4.186 joules 1000 J = 1 kJ 1000 J = 1 kJ

Commonly used because joules are so smallCommonly used because joules are so small

Heat CapacityHeat Capacity

The amount of heat it takes to change an The amount of heat it takes to change an object’s temperature by 1 Celsius degreeobject’s temperature by 1 Celsius degree Depends partly on massDepends partly on mass

More mass More mass greater heat capacity greater heat capacity

ProblemProblem

How many calories are required to heat How many calories are required to heat 32.0 g of water from 25.032.0 g of water from 25.0ooC to 80.0C to 80.0ooC? C? How many joules is this?How many joules is this?

Answer:Answer: 1760 calories1760 calories 7360 joules = 7.36 kJ7360 joules = 7.36 kJ

Specific HeatSpecific Heat

Not all substances Not all substances respond the same respond the same way to the input of way to the input of heatheat Some get hot much Some get hot much

more quickly than more quickly than others!others!

Specific HeatSpecific Heat

The amount of heat required to raise the The amount of heat required to raise the temperature of 1 g of a substance by 1temperature of 1 g of a substance by 1ooCC

A measure of how well a substance A measure of how well a substance stores heat energystores heat energy Substances with low specific heat (ex. Substances with low specific heat (ex.

metals) heat quickly, cool quicklymetals) heat quickly, cool quickly Substances with high specific heat (ex. Substances with high specific heat (ex.

water) take a long time to heat and coolwater) take a long time to heat and cool

Using Specific HeatUsing Specific Heat

C or CC or Cpp

Units: J/gUnits: J/gooC or cal/gC or cal/gooCC

Q = m C Q = m C TTwherewhereQ = total heat changeQ = total heat changem = massm = massT = temperature changeT = temperature change

Phase ChangesPhase Changes

Melting, freezing Melting, freezing Boiling, condensingBoiling, condensing Sublimation, depositionSublimation, deposition

Phase changes occur without Phase changes occur without temperature changes temperature changes

Calculating Energy Changes Calculating Energy Changes in Phase Changesin Phase Changes

Can’t use specific heat, because no Can’t use specific heat, because no temperature change is involvedtemperature change is involved

Instead, use this formulaInstead, use this formula

Q = m Q = m Heat of fusion (or heat of Heat of fusion (or heat of vaporization)vaporization) use heat of fusion for freezing, meltinguse heat of fusion for freezing, melting use heat of vaporization for boiling, use heat of vaporization for boiling,

condensationcondensation

Heating/Cooling CurvesHeating/Cooling Curves

Regions A, C, E Regions A, C, E have temperature have temperature changechange Q = mCQ = mCTT Choose specific heat Choose specific heat

to match state of to match state of mattermatter

Regions B, D are Regions B, D are phase changesphase changes B: Q = mHB: Q = mHfusfus

D: Q = mHD: Q = mHvapvap

Enthalpy of the reactionEnthalpy of the reaction

The total energy change associated with The total energy change associated with a chemical or physical changea chemical or physical change

Given the symbol Given the symbol HHrxnrxn

HHrxnrxn = (energy of products) – (energy of reactants) = (energy of products) – (energy of reactants)

Classifying Heat ChangesClassifying Heat Changes

Thermite reactionThermite reaction Produces molten ironProduces molten iron Formerly used in Formerly used in

welding railroad welding railroad tracks, shipbuildingtracks, shipbuilding

Gives off light and Gives off light and heat! heat!

Highly exothermicHighly exothermic

Exothermic reactionsExothermic reactions

Energy is released to Energy is released to the surroundingsthe surroundings

The temperature of The temperature of the surroundings the surroundings increasesincreases

The products have The products have less energy than the less energy than the reactantsreactants

Endothermic ReactionsEndothermic Reactions

Energy is absorbed Energy is absorbed from the from the surroundingssurroundings

The temperature of The temperature of the surroundings the surroundings decreasesdecreases

The products have The products have MORE energy than MORE energy than the reactantsthe reactants

Thermochemical Thermochemical EquationsEquations

Exothermic reactionsExothermic reactions Energy released by systemEnergy released by system Can treat energy as a productCan treat energy as a product H is negativeH is negative

2 equivalent ways to write equation:2 equivalent ways to write equation: CaO + HCaO + H22O O Ca(OH) Ca(OH)22 + 65.2 kJ + 65.2 kJ

CaO + HCaO + H22O O Ca(OH) Ca(OH)22 H = - 65.2 kJH = - 65.2 kJ

Thermochemical Thermochemical EquationsEquations

Endothermic reactionsEndothermic reactions Energy absorbed by systemEnergy absorbed by system Can treat energy as a reactantCan treat energy as a reactant H is positiveH is positive

2 equivalent ways to write equation:2 equivalent ways to write equation: 2 NaHCO2 NaHCO33 + 129 kJ + 129 kJ Na Na22COCO33 + H + H22O + COO + CO22

2 NaHCO2 NaHCO33 Na Na22COCO33 + H + H22O + COO + CO22 H = + 129 kJH = + 129 kJ

2 ways to manipulate 2 ways to manipulate thermochemical thermochemical equationsequations

1) Write equation backwards 1) Write equation backwards Sign of Sign of H must changeH must change A + B A + B C C H = + 123 kJH = + 123 kJ C C A + B A + B H = - 123 kJH = - 123 kJ

2) Multiply everything by a coefficient2) Multiply everything by a coefficient Must multiply Must multiply H by coefficient, too!H by coefficient, too! 3 A + 3B 3 A + 3B 3C 3C H = 3 (+123 kJ) = + 369 kJH = 3 (+123 kJ) = + 369 kJ

Problems with Problems with thermochemical thermochemical equationsequations

Consider the equation: Consider the equation: 2 NaHCO2 NaHCO33 Na Na22COCO33 + H + H22O + COO + CO22 H = + 129 kJH = + 129 kJ

How many kJ would be released if 4.5 How many kJ would be released if 4.5 moles NaHCOmoles NaHCO33 reacted? reacted?

Hess’ LawHess’ Law

If you add two or more thermochemical If you add two or more thermochemical equations to give a final equation, then equations to give a final equation, then you can also add the heat changes to you can also add the heat changes to give the final enthalpy of reaction.give the final enthalpy of reaction.

ExampleExample

What is the enthalpy change, What is the enthalpy change, HHrxnrxn, for , for

the decomposition of hydrogen peroxide?the decomposition of hydrogen peroxide? Target: 2 HTarget: 2 H22OO22(l) (l) 2 H 2 H22O(l) + OO(l) + O22(g)(g)

Given:Given: HH22(g) + O(g) + O22(g) (g) H H22OO22((l) l) H = -187.9 kJH = -187.9 kJ

HH22(g) + ½ O(g) + ½ O22(g) (g) H H22O(l) O(l) H = -285.8 kJH = -285.8 kJ

Standard Heats of Standard Heats of Formation, Formation, HHoo

ff

The standard heat of formation of a The standard heat of formation of a compound is the change in enthalpy that compound is the change in enthalpy that accompanies the formation of one mole accompanies the formation of one mole of a substance from its elements in their of a substance from its elements in their standard states. standard states.

The heat of formation of elements in their The heat of formation of elements in their standard states is arbitrarily set to zero.standard states is arbitrarily set to zero.

Using heats of formationUsing heats of formation

Thermodynamic stability: a measure of the Thermodynamic stability: a measure of the energy required to decompose the compoundenergy required to decompose the compound Compounds with large, negative enthalpies of Compounds with large, negative enthalpies of

formation are thermodynamically stableformation are thermodynamically stable Many heats of formation have been measured. Many heats of formation have been measured.

(see Appendix A-6 in textbook)(see Appendix A-6 in textbook) Another way to do Hess’ Law!Another way to do Hess’ Law!

HHrxnrxn = = nnHHff(products) – (products) – mmHHff(reactants)(reactants) where where

n represents the coefficients for the productsn represents the coefficients for the products m represents the coefficients for the reactantsm represents the coefficients for the reactants

Example ProblemsExample Problems

Compute Compute HHrxnrxn for the following reaction. for the following reaction. (Refer to Appendix A-6)(Refer to Appendix A-6) 2NO(g) + O2NO(g) + O22(g) (g) 2 NO 2 NO22(g)(g)

Compute Compute HHrxnrxn for the following reaction. for the following reaction. (Refer to Appendix A-6)(Refer to Appendix A-6) 4 FeO(cr) + O4 FeO(cr) + O22(g) (g) 2 Fe 2 Fe22OO33(cr) (cr)

Ans: -144.14 kJ, -560.0 kJAns: -144.14 kJ, -560.0 kJ

EntropyEntropy

Symbol: SSymbol: S A quantitative measure of the degree of A quantitative measure of the degree of

disorder in a systemdisorder in a system The greater the disorder, the larger the value The greater the disorder, the larger the value

of Sof S Solids have a high degree of order (low entropy)Solids have a high degree of order (low entropy) Gases have a low degree of order (high entropy)Gases have a low degree of order (high entropy) More particles (moles) results in higher entropyMore particles (moles) results in higher entropy

Entropy, cont.Entropy, cont.

Systems tend to proceed to higher Systems tend to proceed to higher disorderdisorder ExamplesExamples

Stirring sugar into your coffeeStirring sugar into your coffee The neatness of your lockerThe neatness of your locker The order of cards in a pack of playing cards The order of cards in a pack of playing cards

after shufflingafter shuffling

J. Willard GibbsJ. Willard Gibbs

1839-19031839-1903 First American to First American to

earn a Ph.D. in earn a Ph.D. in science from a US science from a US universityuniversity Yale, 1863Yale, 1863

One of nation’s best One of nation’s best scientistsscientists

Will a reaction occur Will a reaction occur spontaneously?spontaneously?

The answer depends on the balance The answer depends on the balance between enthalpy (heat changes) and between enthalpy (heat changes) and entropy entropy

Gibb’s Free EnergyGibb’s Free Energy The energy available from the system to do The energy available from the system to do

useful workuseful work

Gibb’s Free EnergyGibb’s Free Energy

G = G = H – TH – TSS If If G is negative, the reaction will occur G is negative, the reaction will occur

spontaneously and can proceed on its own.spontaneously and can proceed on its own. If If G is positive, the reaction is G is positive, the reaction is

nonspontaneous and needs a sustained nonspontaneous and needs a sustained energy input to proceed.energy input to proceed.

If If G is zero, the reaction is at equilibrium G is zero, the reaction is at equilibrium (both the forward and reverse reactions take (both the forward and reverse reactions take place!)place!)