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Transcript of System – what we care about Surroundings– everything else ... · PDF file•...

  • AChemistsViewoftheUniverse

    System whatwecareabout Surroundings everythingelse Boundary separatessystemfromsurroundings

  • Somebasicdefinitions

    Work motionagainstanopposing(external)force Heat energychangeassociatedwithachangeintemperature Exothermic releasesheattothesurroundings Endothermic absorbsheatfromthesurroundings

  • InternalEnergy(UorE)

    Totalenergyofthesystem(kineticandpotential) Internalenergyisastatefunction,whichmeanswecandefineachangeinitasDU=Uf Ui.

  • Enthalpy(H)

    Totalpotentialenergyofthesystem Enthalpyisastatefunction,whichmeanswecandefineachangeinitasDH=Hf Hi.

  • 1st LawofThermodynamics

    Theinternalenergyofanisolated systemisconstant. Theonlywaystochangetheinternalenergyofasystemareheatandwork

    DU=Q+W Thechangeininternalenergyforasystemisequalandoppositetothechangeininternalenergyforthesurroundings

    DUsys +DUsurr =DUtot =0

  • 1st LawofThermodynamics

    Greedyconvention(Ch andChEs) Heatabsorbed bysystem Q>0 Heatreleased bysystem Q0Workdoneby systemW

  • HesssLaw

    Directapplicationofthepropertiesofstatefunctions. Thestandardenthalpyofanoverallreactionisthesumofthestandardenthalpiesoftheindividualreactionsintowhichareactionmaybe

    divided.(whethertheyarerealornot)

    ThiscanbeextendedtoANYthermodynamicvariable

  • PhaseChanges(Transitions)

    Anychangeofphasewillhaveacorrespondingchangeinenthalpy Becauseenthalpyisastatefunction,severalusefulpropertiesemerge. Considerachangeofphase(orstate)fromAtoBwithachangeofenthalpy=DH.

    Forthereverseprocess(goingfromBtoA),thechangeinenthalpywillbeDH. ConsiderachangefromAtoC.Wecanconsiderthisashappeningintwosteps:first

    fromAtoBandthenfromBtoC.ThusDHAC=DHAB+DHBC

  • Enthalpiesofreaction

    Asimilartypeofanalysiscanbeperformedforachemicalreaction.Wecandefinethestandardreactionenthalpyasthechangeinenthalpy

    betweentheproductsandthereactants,inthestandardstate:

    whereni referstothestoichiometriccoefficientofspeciesi,andHm,ireferstothemolarenthalpyofspeciesi(akaenthalpyofformation).

    Theo indicatesstandardstate.

  • The2nd LawofThermodynamics

    Usedtopredictspontaneity(tendencyforaprocesstohappennaturally)

    The1st Lawonlytalksaboutconservationofenergy,itsaysnothing aboutthedirection thataprocesswilltendtogoin!

    Whydontballsleavethegroundandbounceup? Whydoesntshatteredglassreform? Whydoesntgreenpigmentseparateintoblueandyellow

    pigments?

  • The2nd LawofThermodynamics

    The2nd Lawdescribeshowspontaneityisrelatedtothedistribution ofenergy,not tothetotal energy.

    Energytendstoflowinadirectionwhereitwillbemorespreadout,ordispersed.

  • SoWhatsEntropy?

    Thisleadstoanotherviewofthe2nd law:Theentropyofanisolatedsystemincreasesinthecourseofaspontaneouschange

    DStot>0 Relatedtochaos,randomness,disorder NoticethatQisnotastatefunctionbutSis!

  • Phasetransitions

    Previouslywesawthatforaphasetransitionoccurringataconstantpressure,Q=DHtr

    ThismeansthatwecanalsocalculateDStr:whereTtr isthetemperatureatwhichthetransitionoccurs

    Thusexothermicprocesses(freezing,condensing)have(-)changesin

    entropy,whileendothermicprocesses(melting,boiling)have(+)

    changesinentropy

  • Howisentropymeasured?

    Inananalogousfashiontoenthalpy,wecandefinethereactionentropychangeas:

    NotethatunlikeHmo,whichcan=0forsubstancesintheirstandardstate,Sm

    o is0(unlessT=0)

  • Gibbsfreeenergy

    letsdefinetheGibbsfreeenergyasG=H-TS. Foramacroscopicchange,DG=DH TDS ThisalsoleadstothefamiliarconclusionthatforaspontaneousprocessDG0

    DGalsorepresentsthemaximumnon-PVwork thatcanbedonebyasystem

  • HowcanDGbemeasured?

    Asfortheotherthermodynamicquantitieswehaveencountered,wecandefinethestandardfreeenergychangeforareactionas:

    Aswithenthalpyofformation,DGfo foranelement(oranaturallyoccurringdiatomicmolecule)=0.

    ExperimentallyDGisoftenobtainedbydeterminingDHandDSseparately.

  • Gibbsenergyandtheequilibriumconstant

    WhatdoesDGrxno represent?Itisthedifferenceinthe(molar)Gibbsenergiesofproductsandreactants,intheirstandardstate.

    isthecruciallinkbetweenthermodynamics(energy)andchemicalequilibrium

    NoticethatifK>1thentheproductsarefavoredatequilibrium,whileifK

  • Calculate H298 for the process Sb(s) + 5/2 Cl2(g) SbCl5(g) from the following information:Sb(s) + 3/2Cl2(g)SbCl3(g) H298 = 314 kJSbCl3(s) + Cl2(g)SbCl5(g) H298 = 80 kJ

  • Calorimetry itsdabomb! Experimentsmaybedoneatconstantvolume(bomb)orconstantpressure(coffee-cup)

    Entiresystemisadiabatic thereisnoheatlostbetweenthesystem(sample)andthesurroundings(waterbathandmetalcasing)

    Thechangeintemperatureofthecalorimeterwillbeproportionaltotheheatthatitabsorbs:QT,orQ=CT,whereCistheheatcapacityofthecalorimeter.

  • Example

    Inapreliminaryexperiment,theheatcapacityofabombcalorimeterassemblyisfoundtobe5.15kJ/oC.Inasecondexperiment,a0.480gsampleofgraphite(carbon)isplacedinthebombwithanexcessofoxygen.Thewater,bomb,andothercontentsofthecalorimeterareinthermalequilibriumat25.00oC.Thegraphiteisignitedandburned,andthewatertemperaturerisesto28.05oC.CalculateHforthereaction:C(graphite)+O2(g) CO2(g)

  • Solution

    Thekeytothisproblemistorealizethatalltheheatabsorbedbythecalorimetermusthavecomefromthecombustionreaction.

    FirstcalculatetheheatabsorbedbythecalorimeterusingQ=CT:

    Theheatgivenupbythereactionmustbeequalandoppositetothis:Qrxn =-15.7kJ

  • WecanthenequatethistoUforthecombustionof1molofgraphite:

    Finally,recallthatH=U+RTnforanisothermalprocess.Sincethetemperaturechangeisprettysmall(3.05oC)wecanassumethatitisisothermal.n=(1-1)=0soHU.ThusH=-393kJ/mol

  • Calorimetryrevisited thatsonefancycoffeecup!

    Forisobaricmeasurements,useathermallyinsulatedvesselthatisopentotheatmosphere

    MoresophisticatedcalorimeterscanbeusedAdiabaticflamecombustionDifferentialscanningIsothermaltitration Forsolidsandliquids,H Usincetheirvolumeisnegligible(atleastcomparedtogases)

  • Example

    A15.5gsampleofametalalloyisheatedto98.9oCandthendroppedinto25.0gofwaterinacalorimeter.Thetemperatureofthewaterrisesfrom22.5to25.7oC.Calculatethespecificheatofthealloy.

  • Solution Thekeytosolvingthisproblemistorealizethatalltheheatlostbythehotsolidmustbegainedbythewaterinthecup.

    Firstwewillfindtheheatabsorbedbythewater:QH2O =mcTso

  • Thismustbeequalandoppositetotheheatlostbythealloy(rememberthesignconvention!)soQalloy =-334J

    Finallycalculatethespecificheatofthealloy:

  • Example

    A50.0mLsampleof0.250MHClat19.50oCisaddedto50.0mLof0.250MNaOH,alsoat19.50oC,inacalorimeter.Aftermixing,thesolutiontemperaturerisesto21.21oC.Calculatetheheatofthisreaction.

  • Solution Firstrecognizethereactionthatistakingplace:HCl(aq)+NaOH(aq) NaCl(aq)+H2O

    Nowletsmakeafewassumptions/simplifications:Takesolutionvolumestobeadditivesothetotalvolumeofsolutionis50.0+50.0=100.0mLConsidertheNaCl(aq)solutiontobesufficientlydilutethatthedensityandspecificheatarethesameasthoseforpurewater(1.00g/mLand4.184J/goC)Thesystemisperfectlyinsulated,sonoheatescapesfromthecalorimeterTheheatrequiredtowarmanypartofthecalorimeter(otherthantheNaClsolution)isnegligible)

  • Findtheheatretainedinthecalorimeter:

    Finallyfindtheheatofreaction:Qrxn =Qp =-Qcal =-715J

  • ChemicalKinetics

    Studyoftheratesofchemicalreactions Howquicklyaprocesscantakeplace

    Understandingofthemechanismofareaction How(onamolecularlevel)aprocesscantakeplace

    Becausethesemeasurementsarechangingwithrespecttotimeandaresensitivetomanyvariables,theyarenotoriouslydifficultexperimentstocarryout!

  • Factorsaffectingtherateofachemicalreaction

    Concentration Pressure(gasesonly) Temperature Presenceofacatalyst Understandingthedependenceofareactiononthesefactorscanaidouroptimizationofachemicalprocess

  • Measuringtherateofareaction

    Wecandefinetherateintermsofthelossofareactantortheformationofaproduct:

    Noticethatthismeanstherateisrelatedtotheslope(tangentline)tothecurve

    Howeverthisdoesnttakeintoaccountthestoichiometryofthereaction(i.e.ifthereactionisR2Ptherateofformationofaproductwillbetwiceasgreatasthelossofreactant).

    Intermsofagivencomponenti, whereni isthestoichiometric number.

  • Inpictures

    http://textbook.s-anand.net/ncert/class-xii/chemistry/4-chemical-kinetics

  • Ratelaws Itisfoundexperimentallythattherateofareactionisusuallyproportionaltothe

    concentrationofeachreactant,raisedtoacertainpower:wherexandyaretheorders ofthereactionwithrespecttoAandB,respectively.Theordersmaybeanyrealnumber(including0andfractions).Theoverallorderisgivenbyx+y.

    IngeneraltheordersmustbedeterminedexperimentallyandareNOTnecessarilythestoichiometriccoefficientsofthereaction(unlessthereactioniselementary).

  • Determinationoftheratelawforareaction

    Isolationmethod systematicallyvarytheconcentrationsofthereactantssothatallareinalargeexcessexceptforone.Thisallowsthedeterminationoftheorderofthatonespecies.

    Forexample,ifthegeneralratelawisandBispresentinalargeexcess,thenitsconcentrationcanbeassumedtobeconstantasthereactionproceeds,whichmeansthatd[B]/dt0.Thuswecanwritetheratelawas whereandwecanfindthevaluesofkandxbycurve-fitting.

  • Determinationoftheratelawforareaction

    Typicallytheinitial ratesaremeasured.ForexampleifBisinexcessthentheinitialratecanbewrittenas

    Thuswecanplotvo vs.[A]o andgetkandxbyfittingthedatanonlinearly(powerlaw),orwecanlinearizetheequationbytakingthelogofbothsides:

    WecanrepeatthisprocesswhereAisheldinexcess,anddetermineanewpseudo-rateconstantk( )andy,andthereforealsogettheori