2 chemical energetics Alevel考试化学课件.pptxVIP

bondmaking,bondbreakingandenthalpychange

Bondbreaking(endothermic)Bondmaking(exothermic)A·+B·→A-BA-B→A·+B·

CH4(g)+2O2(g)=CO2(g)+2H2O(l);△H=-890.3kJmol-1Drawingthemoleculestoshowthebonds:HcHHHO=OO=OC=O=COHHOHHforthisreactiontooccur,somebondsmustbreakandothersform:bondsbreaking:4×C-Hand2×O=Obondsforming:2×C=Oand4×H-O

Bonddissociationenergiesareusedtocalculatetheenthalpychange(?H0)inareactioninwhichseveralbondsarebrokenandformed.

bondenthalpy(E)Bondenthalpyistheenergyneededtobreakacovalentbond,andthisindicatesthestrengthofthebond.forexample:O2(g)→2O(g);△H=+498kJmol-1

typicalvaluesofbondenthalpiestheyareallpositive,asthechangesduringbreakingofbondsareendothermictheyareaveragevalues.forexample,theC-Cbondhasslightlydifferentstrenghinethane(C2H6)andinpropane(C3H8).thebondenthalpychangequotedindatabookforC-Cisanaverageofthevaluesfrommanydifferentmolecules.theyarecomparedforbondsingaseouscompoundsonly.theyareverydifficulttomeasuredirectly.theyareusuallycalculatedusingdatafrommeasurementsofenthalpychangeofcombustionofseveralcompounds.

SomeimportantlimitationsinbonddissociationenergiesBondenthalpiespresentoverallenergychangesonly.Theyrevealnothingaboutthereactionmechanismorhowfastareactionproceeds.Bondenthalpiesaredeterminedforreactionsinthegasphase,whereasmostorganicreactionsoccurinaliquidsolventwheresolvationenergycontributestotheoverallenthalpyofareaction.Bondenthalpiesareimperfectindicatorsofenergychangesinareaction.However,usingbonddissociationenergiestocalculate?H°givesausefulapproximationoftheenergychangesthatoccurwhenbondsarebrokenandformedinareaction.

Howcanwecalculatetheenthalpychangeforareaction?Answer:UtilizeBond