Chapter .Hess’s Law: add up reactions, add up enthalpy ”Ho f is the energy...

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Transcript of Chapter .Hess’s Law: add up reactions, add up enthalpy ”Ho f is the energy...

Chapter5HesssLaw

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Energyunits:

WhichstatementADaboutenergyunitsisnot correct?a. TheSIunitofenergyistheJoule(J).b. IntermsofSIbaseunits,1J=1kgm2 s2.c. Anutritionalcalorie(1Cal)is4.184kJ.d. Anutritionalcalorie(1Cal)isequalto1,000cal.e. StatementsADallarecorrect.

Joule

TheSIunitforEnergy1J=1kgm2 s2

1J=1kgm2/s2

calories

AcalorieistheenergyrequiredtoraiseonegramofwateronedegreeCelcius.1calorie=4.184J

DietaryCalories:1Calorieofdietpepsi =1kcal=1000calories100CaloriesofOreos=100kcal=1x105 calories1kcal=4.184kJ

WhichstatementADaboutenergyunitsiscorrect?a. TheSIunitofenergyisthekilojoule(kJ).b. IntermsofSIbaseunits,1J=1kgm2 s2.c. Anutritionalcalorie(1Cal)is4.184kJ.d. Anutritionalcalorie(1Cal)isequalto100cal.e. StatementsADallarecorrect.

Whathasthehigherenthapy content?A. Aswimmingpool375m3 at27CB. Acupofcoffee(237mL)at80CC.They arethesame.

Whathasthehigherenthapy content?A. Aswimmingpool375m3 at27CB. Acupofcoffee(237mL)at80CC.They arethesame.

H =massxCsxT375m3 x1000L x1000mL x1g x4.184J x(27C-25C)=3.13x109 J

m3 LmLgC

237mLx x1g x4.184 J x(80C-25C)=5.45x104 JmLgC

HesssLaw:addupreactions,addupenthalpyHof is the energy gained or lost when a compound or molecule is formed from the elements in their natural state.Calculate Hof for SO3 in kJ/mol given the following data:

Need:S(s) + 1.5*O2(g) SO3(g) *Can have fractions of molecules

Have:S(s) + O2(g) SO2(g) Ho = -296.8 kJ

SO2(g) + O2(g) SO3(g) Ho = -98.9 kJ

Adding these:

S(s) + O2(g) + SO2(g) + O2(g) SO2(g) + SO3(g) Ho= -395.7kJ

Cancel out if on both sides, add up common molecules:

S(s) + 1.5*O2(g) SO3(g) Ho= -395.7kJ

Usethefollowinginformationtodeterminethestandardenthalpychangewhen1mol ofPbO(s)isformedfromleadmetalandoxygengas:Pb(s)+!

"O2(g) PbO(s)

PbO(s)+C(graphite) Pb(s)+CO(g)Ho =107kJ2C(graphite)+O2(g) 2CO(g) Ho =222kJ

Need:Pb(s)+!"O2(g) PbO(s)

Have:PbO(s)+C(graphite) Pb(s)+CO(g)Ho =107kJ2C(graphite)+O2(g) 2CO(g) Ho =222kJ

Pb(s)+CO(g)PbO(s)+C(graphite) Ho =-107kJC(graphite)+!

"O2(g) CO(g) Ho =111kJ

Ho =-218kJ

Determine thechangeinenthalpyforthefollowingreactionfromtheenthalpiesofformationforthereactantsandproducts.(Hf forpureelements iszero.)

Cl2(g)+C2H2Cl2(g) C2H2Cl4(g)C2H2Cl2,Hf =4.27kJ/molC2H2Cl4,Hf =155.6kJ/mol

AddupHof for reactantsAdd up Hof for products

Products Reactants = Horxn(rxn = reaction)

Determinethechangeinenthalpyforthefollowingreactionfromtheenthalpiesofformationforthereactantsandproducts.(Hf forpureelementsiszero.)

Cl2(g)+C2H2Cl2(g) C2H2Cl4(g)C2H2Cl2,Hf =4.27kJ/molC2H2Cl4,Hf=155.6kJ/molProducts=-155.6=-155.6kJReactants=0+4.27kJProducts-Reactants=-155.6- 4.27=-160kJ

CompoundN2O4(g)

H2O(l) HNO3(aq)NO(g)

Hof (kJ/mol) +11.1 285.8 207.4 +91.3

Given the standard heats of formation in the table below, what is Ho for the following reaction: 3 N2O4(g) + 2 H2O(l) 4 HNO3(aq) + 2 NO(g)

-108.7kJ