Chapter 5 Chemical equilibrium參考資料說明

物理化學(xué)電子教案—第5章ChemicalEquilibriumChapter5Chemicalequilibrium5.1Theconditionsofchemicalequilibrium

andaffinityofchemicalreaction

5.2Theequilibriumconstantofareactionandisothermalequation

5.3Heterogeneouschemicalequilibrium

5.4Determinationofequilibriumconstants

5.5ThestandardGibbsfunctionofformationContents5.8

Couplingreaction5.7

Chemicalequilibriumofsimultaneousreaction5.6

TheresponseofreactionstotheconditionsExpressmathematicallyhowtheequilibriumconstantdependsonthetemperature,andderiveandusethevan’tHoffequation.ChemicalreactionsWehaveseenthatthenaturaldirectionofallchangeatconstanttemperatureandpressureistowardsminimumGibbsfunction.Theideaisentirelygeneral,andinthischapterweuseitasabasisforthediscussionofchemicaltransformations.ConsiderthereactionA+BC+DIsthenaturaltendencyforthemixtureA+BtogotothemixtureC+D,ortheopposite?ThiscanbeansweredbycalculatingtheGibbsfunctionofeachmixture.IfthesumoftheGibbsfunctionsofA+BishigherthanthatofC+D,thenthereactionhasatendencytorollfromlefttoright.ChemicalreactionsystemSupposeareaction：Basedonextentofreaction：Or:FundamentalequationAtconst.TandP， ：

Thereactionspontaneouslyproceedsfromlefttoright.Thereactionspontaneouslyproceedsfromrighttoleft.Thereactionisatequilibrium.DirectionandlimitofachemicalreactionUsetodetermine:FromlefttorightFromrighttoleftAtequilibriumDirectionandlimitofachemicalreactionistheslopeofGwithrecpecttotheextentofreaction.DirectionandlimitofachemicalreactionVan’tHoffequilibriumboxAffinityofchemicalreaction

1922年，比利時(shí)熱力學(xué)專家德唐德（Dedonder）首先引進(jìn)了化學(xué)反應(yīng)親和勢的概念。他定義化學(xué)親和勢A為：或

A是狀態(tài)函數(shù)，體系的強(qiáng)度性質(zhì)。用A判斷化學(xué)反應(yīng)的方向具有“勢”的性質(zhì)，即：A>0

反應(yīng)正向進(jìn)行

A<0

反應(yīng)逆向進(jìn)行A=0

反應(yīng)達(dá)平衡5.2Forpgreactions

aA﹦bBThesimplestassumptionistosupposethatthespeciesareperfectgases,inwhichcasetheirchemicalpotentialsaredeterminedbytheirpartialpressures:Isothermalequationofchemicalreaction＜-Reactionproceeds＝-ReactionisatequilibriumThechemicalpotentialofarealgases：

isthestandardmolarGibbsfunctionofthereactionfBisthefugacityofBSubstitutingthisexpression,gives

Reaction:IsothermalequationofchemicalreactionWhereQisthereactionquotient.Whenthereactionisatequilibrium，

isthethermodynamicequilibriumconstant.Thethermodynamicequilibriumconstant

IvidesawayofmeasuringthestandardGibbsfunctionsofreactions.PredictthedirectionofreactionUsethevan’tHoffequation：Forpgreaction:fromlefttorightfromrighttoleftatequilibrium.Subscriptm

denotingtheextentofreactionisunityi.e.1mol.RelationofconstantandequationExample：（1）（2）isreferredtoEmpiricalequilibriumconstantsDistinguishthethermodynamicequilibriumconstantfrompressureandcompositionequilibriumconstants.Example：1.Pressureequilibriumconstantwhen(Dimensionisunity)2.molefractionequilibriumconstantApgobeysDoltonpartialpressurelaw，Empiricalequilibriumconstants3．Concentrationequilibriumconstantpg，Empiricalequilibriumconstants ，4．ActivityequilibriumconstantEmpiricalequilibriumconstantsDifferentFormsofEquilibriumConstantsHeterogeneouschemicalreactionis

dissociationpressure

ConsiderareactionEquilibriumbetweenidealgasandpurecondensedphases:Ifthesubstancestakingpartinthechemicalequilibriumisofmorethanonephase,dissociationpressureexample：Itissaidearlierthatchemicalpotentialsofcondensedphasesareinsensitivetothepressurechange,unlessthepressureisverylarge.

ThestandardGibbsfunctionofformation△Gconcentratesourattentiononthesystem,andletsusavoidhavingtothinkexplicitlyabouttheentropyofthesurroundings.Inanisothermalchange△GcanberelatedtotheenthalpyandentropychangesofthesystembyUsing

△rGm,calculatethethermodynamicequilibriumconstant:ThestandardGibbsfunctionofreaction(1)-(2)得（3）2.計(jì)算實(shí)驗(yàn)不易測定的平衡常數(shù)例如，求的平衡常數(shù)ThestandardGibbsfunctionofreactionThestandardGibbsfunctionofreaction3．近似估計(jì)反應(yīng)的可能性只能用 判斷反應(yīng)的方向。但是，當(dāng)?shù)慕^對值很大時(shí)，基本上決定了的值，所以可以用來近似地估計(jì)反應(yīng)的可能性。（B,,T）ThestandardGibbsfunctionofformationDefinetheGibbsfunctionofreactionandcalculateitfromtablesofGibbsfunctionofformation.CalculatetheequilibriumconstantsforreactionsonthebasisoftheGibbsfunctioninformationgiveninTableX.X.

isveryimportantAtT(1)Howequilibriumrespondstotemperature.IftheequilibriumconstantisknownatatemperatureT*,itsvalueatanothertemperatureTcanbefoundasfollows.IneachcaseHowequilibriumrespondstotemperatureSubtractingtheserelatesthetwoequilibriumconstants:Nowwrite

△G=△H-T△S,andmaketheapproximationthatneither△Hnor△Schangessignificantlyoverthetemperaturerangeofinterest:Theisobaricequationofchemicalreaction.Thelastequationisveryremarkablebecauseitpredictstheshiftinequilibriumwhenthetemperatureischanged;allweneedtoknownisthestandardmolarenthalpyofreaction.DeducetheisobaricequationTheTaffectsthechemicalequilibriumTheinfinitesimalexpressionofvan’tHoffrelation對吸熱反應(yīng)， ，升高溫度， 增加，對正反應(yīng)有利。對放熱反應(yīng)， ，升高溫度， 降低，對正反應(yīng)不利。TheTaffectsthechemicalequilibriumIfweassurethatdoesnotdependonthetemperaturethisequationintegratesto

TheTaffectsthechemicalequilibrium當(dāng)理想氣體用濃度表示時(shí)，因?yàn)?，可以得到這個公式在氣體反應(yīng)動力學(xué)中有用處。Thevan’tHoffequationProvidesawayofmeasuringtheenthalpyofareactionwithoutusingacalorimeter,butinpracticeitisinaccurate.Equilibriumcompositionsaremeasuredoverarangeoftemperatures(atconstantpressure),andlnKisplottedagainst1/T.Itfollowsfromeq.thattheslopeis-△Hm/R.ThePaffectsthechemicalequilibrium

Lechatelier原理，增加壓力，反應(yīng)向體積減小的方向進(jìn)行。這里可以用壓力對平衡常數(shù)的影響從本質(zhì)上對原理加以說明。=0ThePaffectsthechemicalequilibriumaA=bBTheinertgasaffectsthechemicalequi.對于分子數(shù)增加的反應(yīng)，加入水氣或氮?dú)?，會使反?yīng)物轉(zhuǎn)化率提高，使產(chǎn)物的含量增加。5.9

simultaneousequilibrium在一個反應(yīng)體系中，如果同時(shí)發(fā)生幾個反應(yīng)，當(dāng)?shù)竭_(dá)平衡態(tài)時(shí)，這種情況稱為同時(shí)平衡。在處理同時(shí)平衡的問題時(shí)，要考慮每個物質(zhì)的數(shù)量在各個反應(yīng)中的變化，并在各個平衡方程式中同一物質(zhì)的數(shù)量應(yīng)保持一致。simultaneousequilibrium例題：600K時(shí)， 與 發(fā)生反應(yīng)生成 ，繼而又生成 ，同時(shí)存在兩個平衡：已知在該溫度下， 。今以等量的 和 開始，求 的平衡轉(zhuǎn)化率。simultaneousequilibrium解：設(shè)開始時(shí) 和 的量為1.0，到達(dá)平衡時(shí)，生成HCl的量為x，

生成 的量為y，則在平衡時(shí)各物的量為：simultaneousequilibrium因?yàn)閮蓚€反應(yīng)的 都等于零，所以將兩個方程聯(lián)立，解得 。 的轉(zhuǎn)化率為0.048或4.8。5.10

CoupledReactions

Itcanbeseenquiteoftenthatcertainreactionsarenotspontaneousi.e.△G＞0.Butifthesamereactionsarecoupledtootherreactions,whicharespontaneous,theoriginalreactionsbecomefeasible.ThiscanonlybepossibleifthetotalfreeenergychangeI.e.,sumofthefreeenergiesofthesecondandtheoriginalreactionisnegative.Applicationof

CoupledReactions例如：在298.15K時(shí)：反應(yīng)(1)、(2)耦合，使反應(yīng)(3)得以順利進(jìn)行。ThermodynamicdatafromKThestandardGibbsfreeenergyfortheforwardreactionisrelatedtotheequilibriumconstantbytheexpression:TheHcanbeobtainedfromthevariationoflnKwithtemperature:TheSisthencalculableas…5.11ApproximatelyCalculation1． 的估算當(dāng) 不大，或不要作精確計(jì)算時(shí)，設(shè)，則： 這里實(shí)際上設(shè)焓和熵變化值與溫度無關(guān)，從298.15K的表值求出任意溫度時(shí)的 值。PhysicaltransitionsForanequilibriumbetweenaliquidAandagasA(thevaporizationandcondensationofA):AtP,T;ApproximatelyCalculation2．估計(jì)反應(yīng)的有利溫度通常焓變與熵變在化學(xué)反應(yīng)中的符號是相同的。要使反應(yīng)順利進(jìn)行，則 越小越好。提高溫度對反應(yīng)有利。降低溫度對反應(yīng)有利。TheendJACOBUSHENRICUSVAN’THOFFJACOBUSHENRICUSVAN’THOFF(1852-1911) Dutchphysicalchemist,receivedthefirstNobelPrizeinchemistryin1901for“thediscoveryofthelawsofchemicaldynamicsandofosmoticpressure.”Van’tHoffwasoneoftheearlydevelopersofthelawsofchemicalkinetics,developingmehtodsfordeterminingtheorderofareaction;hededucedtherelationbetweentemperatureandtheequilbriumconstantofachemicalreaction.JACOBUSHENRICUSVAN’THOFFIn1874,van’tHoff(andalsoJ.A.LeBel,independently)proposedwhatmustbeconsideredoneofthemostimportantideasinthehistoryofchemistry,namelythetetrahedral