Phreeqc教程2--热力学数据库

Calorimetry•Measurementofheatflow(throughtemperature)associatedwithareaction•BecausedH=q/dT,measuringTemperaturechangeatconstantPyieldsenthalpyProblemWhen50.mLof1.0MHCland50.mLof1.0MNaOHaremixedinacalorimeter,thetemperatureoftheresultantsolutionincreasesfrom21.0oCto27.5oC.CalculatetheenthalpychangepermoleofHClforthereactioncarriedoutatconstantpressure,assumingthatthecalorimeterabsorbsonlyanegligiblequantityofheat,thetotalvolumeofthesolutionis100.mL,thedensityofthesolutionis1.0g/mLanditsspecificheatis4.18J/g-K.qrxn=-(cssolutionJ/g-K)(massofsolutiong)(DTK)=-(4.18J/g-K)[(1.0g/mL)(100mL)](6.5K)=-2700Jor2.7kJDH=2.7kJEnthalpychangepermoleofHCl=(-2.7kJ)/(0.050mol)=-54kJ/molHess’sLawKnownvaluesofDHforreactionscanbeusedtodetermineDH’sforotherreactions.DHisastatefunction,andhencedependsonlyontheamountofmatterundergoingachangeandontheinitialstateofthereactantsandfinalstateoftheproducts.Ifareactioncanbecarriedoutinasinglestepormultiplesteps,theDHofthereactionwillbethesameregardlessofthedetailsoftheprocess(singlevsmulti-step).CH4(g)+O2(g)--CO2(g)+2H2O(l)DH=-890kJIfthesamereactionwascarriedoutintwosteps:CH4(g)+O2(g)--CO2(g)+2H2O(g)DH=-802kJ2H2O(g)--2H2O(l)DH=-88kJCH4(g)+O2(g)--CO2(g)+2H2O(l)DH=-890kJNetequationHess’slaw:ifareactioniscarriedoutinaseriesofsteps,DHforthereactionwillbeequaltothesumoftheenthalpychangefortheindividualsteps.DeterminingEntropy•AsforH,dS=q/dTcanbemeasuredasheatenergy(q)•Anotherwaytothinkofentropicenergy–foranyreaction,energyis‘dispersed’to/fromthesurroundings–measuredfrom0K(actuallyjustclosetoit),whereS0=0forANYsubstance(at0K,atomsdonotMOVE!)•S0forwater=69.9J/molEntropydetemrination•S0forwater=69.9J/mol–0Kto298Kwhathappenstowater?–Heatsup,changesphase(ice-iceliquid)–69.9joules/molisaverysmallpartofthatenergy!•Howtoevaluatethatsmallheatchange–CAREFULLYdetermineCpoverthisrangeinincrementalstepstosubtractHcomponentTheoreticalestimations•Innaturalsystems,therearemanyspecies,minerals,gasesthatareverydifficulttoimpossibletodeterminewithanyaccuracybyexperiment•Correlationmethodsbasedonisostructural-isovalentanalogues,electrosaticmodels,ligandfieldmodelsexist,butarebasedonempiricalevidenceandhavelittlegroundingintheory–thustheseoftensufferfrominnaccuracy(ifthatisevenknown!)Theoreticaldeterminations•Abinitio(firstprinciples)calculationsbasedonelectronenergy(complicatedrulesforESTIMATINGthis)canbeusedtodetermineenthalpy,entropy,GibbsenergyfromamolecularbasisDeterminingK-Titrations•Especiallyimportantinacid-baseequilibriumconstantszzaMHMOHK]][[1][]][[AHHAKxxGregWedOct0620040510152025303540455023456789101112NaOHreacted(mmoles)pHCarbonatetitrationVoltammetrictitrations•Canusevoltammetrytomeausureacid-basereactionsforelectroactivespecies–HS-+Hg=HgS+H++2e-–H2S+Hg=HgS+2H++2e-–WhereEisEpfromtheanalyticalpeak,E0istheformalpotential,nis#e-’s,FisFaraday’sconstant–PlotofEpvs.pHgivesaslopeproportionaltoH+complexedtosulfideHHSnFRTEEln0Voltammetryforcomplexes•DeHumeandFordformalism–nM+L[Mn(L)]•Stabilityconstantscanbefitfromtherelation:–F0(X)=SBn[X]n=B0+B1[X]+B2[X]2+…+Bn[X]nwhereF0(X)isapolynomialfunctionrepresentingBn=overallstabilityconstantofthenthcomplex,[X]istheaddedspecies(suchasM)Wherecisthecomplexedionandsisthefreeion,Ipisthepeakcurrent,DEp=(Ep)s-(Ep)c,n=#e-sinrxnLMLMnnnDcpsppIIERTnFantiXF)()log(][434.0log)(0Voltammetrictitrations•Canalsotitratesulfideormetalintoanelectrochemicalcellandmeasurethechangesinfreespeciesassociatedwithcomplexation•Competetivecoomplexationapproach–whereonespeciesisdisplacedfromaweakercomplexasatitrantisadded•Moleratioapproach-Errorinthermodynamicdata•Therecanbesignificanterrorinthethermodynamicdatausedindifferentdatabases.•Forexample,DG0Fe2+datawasevaluatedat-78.8KJ/mlforalongtime,recentlyre-evaluatedat-89.9KJ/mol…•Oneprogram,PHREEQC,hasafunctionbuiltintoevaluateequilibriumvaluesformineralsusing+/-10%onthethermodynamicdataused(KNOBS…)Calculatinguncertainty•Becausesomuchofwhatweuseinthermodynamicdatabasesisadditive,thegeneralaccumulationoferrorisestimated:•σx=(a2σx2++b2σx2++…)1/2•BUT–thatassumesnoneofthevaluesaredirectlyrelated,whichreducestheerror(i.e.if2equationssharethesamedatatheerrorisnotadditiveforthesamespecies…)ThermodynamicDatabaseConsistency1.Dataconsistentwiththermodynamicrelationships(appropriatebasiclawsandconsequences)2.CommonscalesusedforT,energy,mass,physicalconstants3.Conflictsbetweendifferentreportsforsamedataareresolved4.Samemathematicalmodelusedtofitdatafromdifferentsets5.Samechemicalmodelisusedtofitdatafromdifferentsets6.Appropriatestandardstatesareused,andconsistentlyappliedthroughoutFromNordstromandMunoz(1994;p.370)logKeq•CaCO3(calcite)=Ca2++CO32--8.48•CO2(g)+H2O=H2CO30-1.47•H2CO30=H++HCO3--6.35•H++CO32-=HCO3-+10.33CaCO3(calcite)+CO2(g)+H2O=Ca2++2HCO3--5.97AnotherwaytodothisistosimplycombinetheKeqdataalgebraically:212KKKKKCOcalciteeqStillanotherwayisrecomputetheDG0RforthereactionofinterestandcalculateKeqWhatdoesadatabaselooklike?2.0000H2O+1.0000O2+1.0000Mn++=MnO4--+4.0000H+-llnl_gamma4.0log_k-32