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1、4.Thirdlawofthermodynamicsandstandardentropy1.ThirdlawofthermodynamicsIn1906,W.H.Nernstproposed,thenMaxPlanchandG.N.Lewismodified,asfollow:Theentropy(S)ofacompletelyorderedcrystalofpurematteris0at0K.S*(completelyorderedcrystal,0K)=0(1)ThirdlawofthermodynamicsThereisonlyonearrangementofatomsormolecul
2、esinanycompleteorderedcrystalat0K,correspondingly,theentropyofthisarrangementis0.(2)StandardentropyEntropyof1molematterinitsstandardstateisdefinedasabsoluteentropy.Symbol：SmUnit：Jmol-1K-14.Thirdlawofthermodynamicsandthestandardentropy2.StandardmolarentropyThedefinedentropy:Theentropyvalueofasubstanc
3、efromabsolutezerotoaspecifiedtemperature.S =ST -S 0=STST:definedentropy(absoluteentropy)Standardmolarentropy:Atadesiredtemperature(T)andstandardpressure,thespecifiedentropyofamoleofpuresubstanceBisdefinedasthestandardmolarentropyofthissubstanceB.Symbol:Sm(B,Phase,T)Unit:Jmol-1K-1Sm(element,Phase,298
4、.15K)0SomeexperiencesofstandardmolarentropyForthesamesubstances，Forsimilarstructurewithdifferentrelativemolecularmass,itmustincreasewiththeincreaseofmolecularmass,Forthosewithsimilarmolecularweight,theentropyofacomplexstructuremustbegreaterthanthatofasimpleone.The entropy at high temperature must be
5、higherthanthatatlowtemperature.ST1ST2T1T2Thementioned-belowaregeneralphenomenaofentropychange,asfollow:Itpresentsadditivepropertyduetoitsstatefunction.Forthesamesubstances,itsentropyincreaseswithincreasingtemperature.Forthesamesubstances，Sm(g)Sm(l)Sm(s)；Foradispersedsystematthesametemperature,theent
6、ropyofsolutionisalwaysgreaterthanthatofpuresolventorpuresolution.Forthesametypeofsubstance,thegreaterthemolarmass,thegreatertheentropy.Pressurehaslittleeffectonsolidstate,but,forgas5.Secondlawofthermodynamicsandtheentropychangeofchemicalreaction1.ThecalculationoftheentropyforchemicalreactionThestand
7、ardmolarentropychangeforchemicalreactionisexpressedusingrSmReaction：aA+bB=gG+dDrSm(298.15K)=BSmB(298.15K)=S(298.15K)products-S(298.15K)reactants=gS(G，298.15K)+dS(D，298.15K)-aS(A，298.15K)+bS(B，298.15K)2.SecondlawofthermodynamicsForanyspontaneousprocess,thesumofentropychangesofsystemandenvironmentalwa
8、ysincreases,whichisgreaterthanzero.Stotal=(Ssystem+Senvironment)0Stotal 0:spontaneousprocessStotal 0:non-spontaneousprocessG 0:spontaneousprocessG=0:anequilibriumstateH0,S0,G0Aspontaneousprocessoccursatanytemperature.S0,H0Aspontaneousprocessdontoccuratanytemperature.S0,H0Itoccursspontaneouslyatlowte
9、mperatures,butcannotathightemperature.S0,H0Itoccursspontaneouslyathightemperatures,butcannotatlowtemperature.EffectoftemperatureonG.EffectoftemperatureonGintheisobaricsystemType1.1/2H2(g)+1/2F2(g)=HF(g)H(298K)=-271kJmol-1S(298K)=8JK-1mol-1HSGevaluationType1-+0Aspontaneousprocessdontoccuratanytempera
10、tureType3-0athighTItoccursspontaneouslyatlowtemperatures,but cannot at hightemperatureType4+0atlowT0athighTIt occurs spontaneously at hightemperatures,but cannot at lowtemperatureType2.CO(g)=C(S)+1/2O2(g)H(298.15K)=110kJmol-1S(298.15K)=-88.8JK-1mol-1Type3.HCl(g)+NH3(g)=NH4Cl(S)H(298.15K)=-176.9kJmol
11、-1S(298.15K)=-284.3JK-1mol-1Atlowtemperature:G0Howtojudgethedirectionofspontaneousreaction?rGm0,itisntspontaneousrGm0,ithasreachedequilibriumstatewhichinvalueisequaltothemaximumnon-volumeworkthatthesystemcando.rGm WmaxThephysicalmeaningofGibbsfreeenergychangeEstimationofthetransitiontemperatureofreactionIftheeffectsoftemperatureandpressureonGareignored,WhenG=0,Ttrans=Example:Tdecomp=
