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APChemistrySpring2020Mr.Cameron

1stLawofThermodynamics

Spontaneous?

•  Considerthefollowingphysicalandchemicalprocesses– Awaterfallrunsdownhill,butneveruphill,spontaneously

– Alumpofsugarspontaneouslydissolvesinacupofcoffee,butsugardoesnotspontaneouslyreappearinitsoriginalform

– Waterfreezesspontaneouslybelow0˚C,andicemeltsspontaneouslyabove0˚C(at1atm).

Spontaneous?

•  Considerthefollowingphysicalandchemicalprocesses– HeatflowsfromahoIerobjecttoacolderone,butthereverseneverhappensspontaneously.

– Theexpansionofagasinanevacuatedbulbisaspontaneousprocess.Thegatheringofallthemoleculesintoonebulbisnotspontaneous.

Spontaneous?

•  Considerthefollowingphysicalandchemicalprocesses– Apieceofsodiummetalreactsviolentlywithwatertoformsodiumhydroxideandhydrogengas.However,hydrogengasdoesnotreactwithsodiumhydroxidetoforwaterandsodium.

–  Ironexposedtowaterandoxygenformsrust,butrustdoesnotspontaneouslychangebacktoiron.

Spontaneous

•  AreacNonthatdoesoccurunderthegivensetofcondiNonsiscalledspontaneous.

•  ProcessesthatoccurspontaneouslyinonedirecNoncannot,underthesamecondi:on,alsotakeplacespontaneouslyintheoppositedirecNon.

•  Spontaneousdoesnotmeanfast.Itdoesmeanthatitiscertaintooccur.

Entropy-S

•  Adirectmeasureofrandomnessordisorderofasystem.

•  Adrivingforceforaspontaneousprocessisanincreaseintheentropyoftheuniverse.

Entropy-S

•  Entropyiscloselyrelatedtoprobability•  ThemorewaysaparNcularstatecanbeachieved,thegreateristhelikelihoodoffindingthatstate.

•  NaturespontaneouslyproceedstowardthestatesthathavethehighestprobabiliNesofexisNng.

PosiNonalEntropy

Processesthatleadtoanincreaseinentropy(ΔS>0)

2ndLawofThermodynamics

•  Inanyspontaneousprocessthereisalwaysanincreaseintheentropyoftheuniverse.

•  Theentropyoftheuniverseincreasesinaspontaneousprocessandremainsunchangedinanequilibriumprocess.

ASpontaneousProcess:ΔSuniv = ΔSsys +ΔSsurr >0AEquilibriumProcess:ΔSuniv = ΔSsys +ΔSsurr =0

EffectsofTemperature

•  LookingatΔSsysandΔSsurr,consider:

•  WhatisthesignofΔSsys

•  WhatisthesignofΔSsurr

H2O(l)⎯→⎯ H2O(g)

EffectsofTemperature-CharacterisNcs

•  SignofΔSsurr:dependsonthedirecNonoftheflowofheat– Exothermicprocessesincreasetheentropyofthesurroundings

– Endothermicprocessesdecreasetheentropyofthesurroundings

EffectsofTemperature-CharacterisNcs

•  MagnitudeofΔSsurr:dependsonthetemperature

•  “Howmuchwillthetransferofheataffectthesurroundings?”– DependsdirectlyonthequanNtyofheattransferred

– Dependsinverselyonthetemperature

CalculaNngΔSsurr

•  Constanttemperature:K•  Constantpressure•  NegaNvesignbecauseΔHisdeterminedwithrespecttothesystem.

ΔSsurr = −

ΔHT

PracNceCalculaNngΔSsurr

InthemetallurgyofanNmony,thepuremetalisrecoveredviadifferentreacNons,dependingonthecomposiNonoftheore.Forexample,ironisusedtorecoveranNmonyinsulfideoresandcarbonisusedforoxideores.CalculateΔSsurrforeachofthesereacNonsat25˚Cand1atm.

Sb2S3(s)+3Fe(s)⎯→⎯ 2Sb(s)+3FeS(s)ΔH = −125kJSb4O6(s)+6C(s)⎯→⎯ 4Sb(s)+6CO(g)ΔH =778kJ

GibbsFreeEnergy(FreeEnergy)

•  IfΔG>0–NotSpontaneous

•  IfΔG=0–SystematEquilibrium

•  IfΔG<0–SpontaneousJosiahWillardGibb

ΔG = ΔH −TΔS

FreeEnergy–Spontaneous?Case SignofΔHsys SignofΔSsys SignofΔG Spontaneous?1 - +2 + -3 - -4 + +

3rdLawofThermodynamicsTheentropyofapurecrystallinesubstanceatabsolute

zeroisS=0at0K.

CalculaNngΔHo,ΔSoandΔGo

ΔHreactiono = npΔHf

o(products)−∑ nrΔHfo(reactants)∑

ΔSreactiono = npΔS

o(products)−∑ nrΔSo(reactants)∑

ΔGo = ΔHo −TΔSo

PracNceCalculaNngΔHo,ΔSoandΔGo

ConsiderthereacNon

carriedoutat25˚Cand1atm.CalculateΔHo,ΔSo,andΔGo.

2SO2(g)+O2(g)⎯→⎯ 2SO3(g)

CoupleofThoughtsonΔGo

•  StateFuncNon–CanuseHess’sLaw•  CanuseΔGo

f:

ΔGreactiono = npΔGf

o(products)−∑ nrΔGfo(reactants)∑

APChemistrySpring2020Mr.Cameron