普通化學(xué) 第七章 總結(jié)

第第頁普通化學(xué)第七章總結(jié)EnthalpyasaStateFunction___

Hisastatefunction.ΔH=Hfinal-Hinitialindependentofpath:

Standard-StateEnthalpiesofReaction____

Forareactioncarriedoutatapressureof1bar,ΔH=ΔH.ΔHiscalledthestandard-stateenthalpyofreaction.1barispartofthedefinitionofstandardconditions.MosttabulatedenthalpiesofreactionareΔH.

CalculatingEnthalpiesofReaction__

EasiertocalculateΔHthantomeasureit.Thesearecalculations,notestimates.Breakallthereactantbonds.Formalltheproductbonds.ΔH=thedifferenceinenergybetweenthesetwoprocesses.

EnthalpiesofAtomCombination__

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N(g)+3H(g)→NH3(g)Noticethesereactantsarenotintheirdiatomicelementalforms.If1moleofNatomscombinewith3molesofHatomstoproduceonemoleofNH3molecules,1171.76kJofenergywillbereleased.

EnthalpiesofAtomCombination______

N(g)+3H(g)→NH3(g)Reactioniscalledatomcombination.Notmeanttoreflectactualmechanism.Theenthalpyassociatedwithitiscalledtheenthalpyofatomcombination,ΔHac.Intheabovee*ample,ΔHac=-1171.76kJ.Whyisitnegative?

EnthalpiesofAtomCombinationThereversereactionNH3(g)→N(g)+3H(g)iscalledatomization.Theenthalpychangeiscalledtheenthalpyofatomizationandinthise*ampleequals+1171.76kJ.

EnthalpiesofAtomCombination__

Canbeusedtostudychemicalprocesses2H2(g)+O2(g)→2H2O(g)

ΔH=2ΔHac(H2O(g))-2ΔHac(H2(g))-1ΔHac(O2(g))FromAppendi*B.13ΔH=2(-926.29kJ)-2(-435.30kJ)-1(-498.340kJ)ΔH=-483.64kJ

EnthalpiesofAtomCombination____

Canbeusedtostudychemicalprocesses.2H2(g)+O2(g)→2H2O(g)Establishedthisase*othermic.Determinedthat483.64kJofenergywillbereleasedforeachmoleofO2(g)consumed.

UsingEnthalpiesofAtomCombinationtoProbeChemicalReactionsIsomersHO2CCHCHHCO2HHO2CCCCO2HH

UsingEnthalpiesofAtomCombinationtoProbeChemicalReactions_

KnowingΔHacforeachisomerallowsfor_thecalculationoftheenthalpychangeassociatedwiththetransformationHO2CCHCHHCO2HHO2CCCCO2HH

UsingEnthalpiesofAtomCombinationtoProbeChemicalReactions__

KnowingΔHacgivesinsightintoaveragebondstrengths.TheseprovideamicroscopicinterpretationofoverallΔHforareaction.endothermicore*othermiccharactermagnitudeofΔH

UsingEnthalpiesofAtomCombinationtoProbeChemicalReactions4HF(g)+SiO2(g)→SiF4(g)+2H2O(g)ΔH=-103.4kJ4HCl(g)+SiO2(g)→SiCl4(g)+2H2O(g)ΔH=+139.6kJ

Whythedifferenceinsign?

UsingEnthalpiesofAtomCombinationtoProbeChemicalReactions4HF(g)+SiO2(g)→SiF4(g)+2H2O(g)ΔH=-103.4kJ4HCl(g)+SiO2(g)→SiCl4(g)+2H2O(g)ΔH=+139.6kJ

Whythedifferenceinsign?TheSi-FbondisstrongerthantheSi-ClbondbyanamountgreaterthanthedifferenceinbondstrengthbetweenH-

FandH-Clshownonthene*tslide.

BondLengthandtheEnthalpyofAtomCombinationTable7.5

BondLengthandtheEnthalpyofAtomCombination__

Longerbondstendtobeweakerbonds.Multiplebondstendtobestrongerthansinglebonds.

Hess'sLaw___

AnalternativemethodforcalculatingΔHDoesnotuseΔHac.TakesadvantageofHbeingastatefunction.

Hess'sLaw_

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Desiredreactionsareconstructedfromknownreactions.ΔHfromknownreactionscombinedinsamewaytocalculateΔHforthedesiredreaction.Trialanderrormethod!

Hess'sLawC(s)+H2O(g)→CO(g)+H2(g)ΔH=?fromC(s)+1/2O2(g)→CO(g)H2(g)+1/2O2(g)→H2O(g)

ΔH=-110.53kJΔH=-241.82kJ

EnthalpiesofFormation_

Enthalpyoffo