Ch 15. Chemical Equilibrium Chem 30A
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.
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.
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
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