Chemical-Equilibrium(化学平衡)

CHAPTER14ChemicalEquilibrium14.4.WhatDoestheEquilibriumConstantTellUs?14.5.FactorsThatAffectChemicalEquilibriumIngeneral,theequilibriumconstanthelpsustopredictthedirectioninwhichareactionmixturewillproceedtoachieveequilibriumandtocalculatetheconcentrationsofreactantsandproductsonceequilibriumhasbeenreached.14.4.WhatDoestheEquilibriumConstantTellUs?14.4.1PredictingtheDirectionofaReaction14.4.2CalculatingEquilibriumConcentration14.5FactorsThatAffectChemicalEquilibriumChemicalequilibriumrepresentsabalancebetweenforwardandreversereactions.Inmostcases,thisbalanceisquitedelicate.Changesinexperimentalconditionsmaydisturbthebalanceandshifttheequilibriumpositionsothatmoreorlessofthedesiredproductisformed.14.5.1LeChatelier’sPrincipleTherule,knownasLeChatelier’sprinciple,statesthat:Ifanexternalstressisappliedtoasystematequilibrium,thesystemadjustsinsuchawaythatthestressispartiallyoffsetasthesystemreachesanewequilibriumposition.ExternalstressTheword“stress”heremeansachangeinconcentration,pressure,volume,ortemperaturethatremovesthesystemfromtheequilibriumstate.14.5.2ChangesinConcentration14.5.3ChangesinVolumeandpressureIngeneral,anincreaseinpressure(decreaseinvolume)favorsthenetreactionthatdecreasesthetotalnumberofmolesofgases(thereversereaction,inthiscase),andadecreaseinpressure(increaseinvolume)favorsthenetreactionthatincreasesthetotalnumberofmolesofgases(here,theforwardreaction).Forreactionsinwhichthereisnochangeinthenumberofmolesofgases,apressure(orvolume)changehasnoeffectonthepositionofequilibrium.14.5.3ChangesinTemperatureColdwaterhotwater14.5.4TheEffectofaCatalystWeknowthatacatalystenhancestherateofareactionbyloweringthereaction’sactivationenergy.acatalystlowerstheactivationenergyoftheforwardreactionandthereversereactiontothesameextent.Wecanthereforeconcludethatthepresenceofacatalystdoesnotaltertheequilibriumconstant,nordoesitshiftthepositionofanequilibriumsystem.Addingacatalysttoareactionmixturethatisnotatequilibriumwillsimplycausethemixturetoreachequilibriumsooner.SummaryofFactorsThatMayAffecttheEquilibriumPosition