Entropy-and-Free-Energy---Senays-Classes熵与自由能--Sen

1EntropyandFreeEnergy2SpontaneousPhysicalandChemicalProcesses•Alumpofsugardissolvesinacupofcoffeespontaneousnonspontaneous18.2•Awaterfallrunsdownhill3•At1atm,waterfreezesbelow00Candicemeltsabove00CiceliquidwaterSpontaneousforT0°CSpontaneousforT0°C4Ironexposedtooxygenandwaterformsrustbutwilltherustspontaneouslyformironandoxygen?5•Heatflowsfromahotterobjecttoacolderobject6spontaneousnonspontaneousAgasexpandsinanevacuatedbulb7•Allthesearespontaneousprocesses.Aspontaneousprocess,HasadefinitedirectioninwhichitoccursOccurswithoutanyongoingoutsideintervention.canbefastorslow8•Anonspontaneousprocessisonethatcannottakeplaceinasystemlefttoitself.•Ifaprocessisspontaneous,thereverseprocessisnonspontaneous,andviceversa.•However,“spontaneous”signifiesnothingabouthowfastaprocessoccurs.9Whatfactorsmakeaprocessspontaneous?Enthalpyisafactorinwhetherareactionisspontaneous;butisnottheonlyfactor.10Enthalpyisafactorinwhetherareactionisspontaneous;butisnottheonlyfactor.CH4(g)+2O2(g)CO2(g)+2H2O(l)DH0=-890.4kJH+(aq)+OH-(aq)H2O(l)DH0=-56.2kJSpontaneousreactions18.211Anexothermicreaction(-DH)favorsaspontaneousreactionsinceproductsareatalowerpotentialenergy.PotentialEnergyReactantsProducts-DH12Enthalpyisafactorinwhetherareactionisspontaneous;butisnottheonlyfactor.H2O(s)H2O(l)DH0=+6.01kJNH4NO3(s)NH4+(aq)+NO3-(aq)DH0=+25kJH2OSpontaneousreactionsOnthebasisofenthalpy,bothoftheseprocessesareunfavorable.Buteachsharessomethingincommon:theproductsaremorerandomanddisorderedthanthereactants.13•Processesinwhichthedisorderofthesystemincreasestendtooccurspontaneously.•Thechangeindisorderalongwiththechangeinenthalpyaffectsthespontaneityofaprocess.14•Therearetwonaturaltendenciesbehindspontaneousprocesses:thetendencytoachievealowerenergystateandthetendencytowardamoredisorderedstate15Entropy,SThedisorderisexpressedbyathermodynamicquantitycalledENTROPY(S)(however,entropyisnot“disorder”).Themoredisorderedorrandomasystem,thelargeritsentropy.Entropyisastatefunction,doesn’tdependonthepathwaybywhichthesystemchanges.ΔS=Sfinal-Sinitial16Entropy(S)isameasureoftherandomnessordisorderofasystem.orderSdisorderSDS=Sf-SiIfthechangefrominitialtofinalresultsinanincreaseinrandomnessSfSiDS018.2+DS;favorsaspontaneousreactionforachemicalreaction17•ΔSis(+)•ΔSis(-)Theproductsaremoredisorderedthanthereactants18GuidelinesforDeterminingifDSis+or-.1.Foranysubstance,thesolidstateismoreorderedthantheliquidstateandtheliquidstateismoreorderedthangasstateSsolidSliquidSgasH2O(s)H2O(l)DS019S(gases)S(liquids)S(solids)So(J/K•mol)H2O(liq)69.91H2O(gas)188.8GuidelinesforDeterminingifDSis+or-.20Theentropyofliquidwaterisgreaterthantheentropyofsolidwater(ice)at0˚C.GuidelinesforDeterminingifDSis+or-21IncreaseinEntropyintheVaporizationofWaterEvaporationisspontaneousbecauseoftheincreaseinentropy.GuidelinesforDeterminingifDSis+or-.22GuidelinesforDeterminingifDSis+or-.2.Entropyoftenincreaseswhenonematerialdissolvesinanother.23Entropyusuallyincreaseswhenapureliquidorsoliddissolvesinasolvent.GuidelinesforDeterminingifDSis+or-.24EOSGuidelinesforDeterminingifDSis+or-.253.Entropyofasubstanceincreaseswithtemperature.Molecularmotionsofheptaneatdifferenttemps.GuidelinesforDeterminingifDSis+or-.264.IncreaseinmolecularcomplexitygenerallyleadstoincreaseinS.So(J/K•mol)CH4248.2C2H6336.1C3H8419.4GuidelinesforDeterminingifDSis+or-.275.Entropiesofionicsolidsdependoncoulombic(electrostatic)attractions.So(J/K•mol)MgO26.9NaF51.5GuidelinesforDeterminingifDSis+or-.28GuidelinesforDeterminingifDSis+or-.18.36.Whengasesareproduced(orconsumed)•Ifareactionproducesmoregasmoleculesthanitconsumes,DS00.•Ifthetotalnumberofgasmoleculesofthereactantsdiminishes,DS00.•Ifthereisnonetchangeinthetotalnumberofgasmolecules,thenDS0maybepositiveornegativeBUTDS0willbeasmallnumber.Whatisthesignoftheentropychangeforthefollowingreaction?2Zn(s)+O2(g)2ZnO(s)Thetotalnumberofgasmoleculesgoesdown,DSisnegative.29GuidelinesforDeterminingifDSis+or-.7.Entropyincreasesasnumberofmolesofgasesincreasesduringchemicalreaction.N2(g)+3H2(g)-2NH3(g)4molesgas2molesgas-DS30Howdoestheentropyofasystemchangeforeachofthefollowingprocesses?(a)CondensingwatervaporRandomnessdecreasesEntropydecreases(DS0)(b)FormingsucrosecrystalsfromasupersaturatedsolutionRandomnessdecreasesEntropydecreases(DS0)(c)Heatinghydrogengasfrom600Cto800CRandomnessincreasesEntropyincreases(DS0)(d)SublimingdryiceRandomnessincreasesEntropyincreases(DS0)18.231ExamplePredictwhethereachofthefollowingleadstoanincreaseordecreaseintheentropyofasystem.Ifindoubt,explainwhy.(a)Thesynthesisofammonia:N2(g)+3H2(g)2NH3(g)(b)Preparationofasucrosesolution:C12H22O11(s)C12H22O11(aq)(c)Evaporationtodrynessofasolutionofurea,CO(NH2)2,inwater:CO(NH2)2(aq)CO(NH2)2(s)H2O(l)32Entropy,S•Thechangeinentropyisrelatedtotheheattransferredduringtheprocess.33EntropyandTemperatureSincreasesslightlywithTSincreasesalargeamountwithphasechanges34TheSecondLawofThermodynamics•Thereisaninherentdirectioninwhichprocessesoccuriscalledthesecondlawofthermodynamics.•It’susuallyexpressedintermsofentropy.DSuniv=DSsys+DSsurr