Ch 15. Chemical Equilibrium

Chem 30A

Reaction Rates: Kinetics

Reaction Rate

•  Reac%onrate:theamountofaproductthatformsinagivenperiodof%me

•  Reac%onrateisexplainedbyCollisionTheory:Chemicalreac%onsoccurthroughcollisionsbetweenmoleculesoratoms.

Reaction Rate

Reac%onratedependson:1)  thefrequencyofcollisionsinthecorrect

orienta%on,and2)  the“forcefulness”ofthecollisions:whether

collisionoccurswithenoughenergy(togooverenergybarrier)

Eg.O3+NOàO2+NO2

Reaction Rates and Activation Energy

•  Ac%va%onenergy:theamountofenergynecessaryforreactantstosurmounttheenergybarriertoreac%on

*Atagiventemp:highertheac%va%onenergy,theslowerthereac%onrate.

Effects of Three Conditions on Reaction Rates

1.  Concentra%on2.  Temperature3.  Catalysts

Effect of Concentration on Reaction Rates

Reac%onrategenerallyincreaseswithincreasingreactantconcentra%on.

of reactants

Effect of Temperature on Reaction Rates

Reac%onrategenerallyincreaseswithincreasingtemperatureofreac%onmixture.

Effect of Catalyst on Reaction Rates

•  Catalyst:Asubstancethatincreasestherateofachemicalreac%onbutisnotconsumedbythereac%on.

Catalyst

•  Acatalystincreasesreac%onratebyallowingthereac%ontotakeplacebyanalterna%vechemicalpathwaywithalowerenergybarrier.

Examples of Catalysts

•  Enzymecatalysts

•  Non-enzyma%ccatalysts

ExProbs

Summary: Effects of Conc, Temp, Catalyst on Rxn Rates

Chemical Equilibrium

Reversible Reactions

•  ReversibleReac%on:areac%onthatcangoineitherdirec%on,fromproductstoreactantsorreactantstoproducts

•  Bothreactantsandproductshaveapproximatelyequalstability.

Forward

Reverse

Chemical Equilibrium

•  Chemicalequilibrium:Astateinwhichtheforwardrateandreverserateofthereac%onarethesame,sotheconcentra%onsofreactantsandproductsremainconstant

Equilibrium

•  Equilibriumischaracterizedbysamenessandconstancy(“stablestate”).

•  Equilibriumisimportantinexplainingmanynaturalphenomena.

•  Livingthingsarenotinequilibriumwiththeirsurroundings.

Dynamic Equilibrium

•  Chemicalequilibriumisadynamicstate:Eachsubstanceisbeingcon%nuouslymadeandbrokendown(althoughatthesameratesoitsconcentra%onremainconstant).

Equilibrium

•  Equilibriumdoesnotmeanthattheconcentra%onsofreactantsandproductsareequalatequilibrium(onlythattheydon’tchange).

Equilibrium Constant and Equilibrium Constant Expression

Coefficientsinchemicalequa%onbecomeexponentsinequilibriumexpression.

aA+bB⇌cC+dD

Concentrations

•  Theconcentra%onsinanequilibriumexpressionshouldalwaysbeinunitsofmolarity(M),buttheunitsthemselvesaredropped.

•  Concentra%onsofpuresolidsandliquidsareomiXedfromequilibriumconstantexpression.

Why is Keq Useful?

•  IfK>>1,therearemoreproductsthanreactantsatequilibriumàForwardrxnisclosetocomple%on(Forwardrxnisfavored).

•  IfK<<1,therearemorereactantsthanproductsatequilibriumàForwardrxnhardlyproceeds(Reverserxnisfavored).

•  IfK~1,significantamountsofbothreactantsandproductsarepresentatequilibrium(Neitherdirec%onisfavored.)

KTellsExtentofaReac%onatEquilibrium.

Ex Problem: Equilibrium

Thefollowingpicturesrepresenttwodifferentreac%onsthathavereachedequilibrium,followedbythereac%onequa%onforeachreac%on.

•  Foreachreac%on,writetheexpressionforKandcalculatethevalueofK.

ExProbs

Equilibrium: Le Chatlier’s Principle

Le Chatlier’s Principle

•  Whathappenswhenachangeinreac%oncondi%onsisimposedontheequilibrium?

•  LeChatlier’sPrinciple:Whenasystematequilibriumisdisturbed,thesystemshibsinadirec%ontocounteracttheeffectofdisturbance.

•  Disturbance:changeinconcentra%on,pressure/volume,temperature

•  Equilibriumshib:Concentra%onsofproductsandreactantschangetonew,constantvalues(anewequilibriumstateisreached).

Effect of Concentration Change on Equilibrium

1.  Allowthefollowingreac%ontoreachequilibrium:CO(g)+2H2(g)⇌CH3OH(g)

2.  ThenaddmoreCO(reactant).Whathappens?LeChatlier’sprinciple:Reac%ongetsridoftheCObyusingitup.Reac%onshibstoright(concofPé).•  Newequilibriumstateisreached(newconcs):greater[CO],smaller[H2],greater[CH3OH],butsameK

Effect of Concentration Change on Equilibrium

1.  Allowthefollowingreac%ontoreachequilibrium:CO(g)+2H2(g)⇌CH3OH(g)2.  ThenaddmoreCH3OH.Effect?LeChatlier’sprinciple:Reac%ongetsridofCH3OHbyusingitup.Reac%onshibstoleb(concofRé).•  Newequilibriumstateisreached(newconcs):greater[CH3OH],greater[H2],greater[CO],butsameK

Effect of Concentration Change on Equilibrium

1.  Allowthereac%onbelowtoreachequilibrium.2.  Thencon%nuallyremoveethylacetateproduct.

Effect?.LeChatlier’sprinciple:Reac%ontriestomakemoreethylacetate.Reac%ontriestoshibtoright.

Effect of Pressure/Volume Change on Equilibrium 1.  Allowrxnbelowtocometoequilibrium.2.  Thenincreasepressureofreac%on(decrease

volume).LeChatlier’sPrinciple:Reac%ondecreasespressurebymovingindirec%onthatproducesfewermolesofgas.Equilibriumshibstoright.•  Remember,pressureinfluencesequilibriumonlyifgasesareinvolved.

•  S%llsameK.

Effect of Temperature Change on Equilibrium

1.  Allowtheexothermicrxnbelowtocometoequilibrium.

2.  Raisetempofreac%on.LeChatlier’sPrinciple:Rxngetsridofheatbyusingitup.Reac%onshibstoleb.

•  Iflowertempofexothermicreac%on?

•  ChangeintemperaturechangesK.

EquilàR

Effect of Temperature Change on Equilibrium

1.  Allowtheendothermicrxnbelowtocometoequilibrium.

2.  Raisetempofreac%on.LeChatlier’sPrinciple:Rxngetsridofheatbyusingitup.Reac%onshibstoright.

•  Iflowertempofendothermicreac%on? EquilàL

Effect of Catalyst on Equilibrium: NONE

•  Catalystsdonotaffectequilibriumconcentra%ons.

•  Catalystsonlyspeedupthereac%on.

ExProbs

Activation Energy and Free Energy

•  Ac%va%onenergy:Determinesrateofreac%on.•  Freeenergychange(ΔG):Determineswhetherreac%on

occursornotwithoutaddi%onofenergy(spontaneously)

•  Enzymeonlychangesac%va%onenergy,notfreeenergychange.