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5.1ReactionRates

LearningTarget:Explaintherelationshipbetweentherateofachemicalreactionandexperimentalparameters.

Factoraffectingrate

•  Manyfactorscaninfluencetherateofachemicalreaction

•  Eachfactormusthaveaneffectonthemicroscopiccollisionsthatleadtoachemicalreaction

Concentration

•  Increasingthereactantsconcentrationbyputtingmorereactantsintothesamespaceincreasesthecollisionfrequencyoftheparticles,leadingtoafasterrateofreaction

•  Asimilareffectisobservedwhenincreasingthepressure(reducingthevolume)inagaseousreaction

Temperature

•  Ariseintemperaturewillresultinanincreasedrateofreaction.

•  Asaroughguide,inmanyreactionsa10Crisewillresultinanapproximatedoublingoftherate

Temperature–Maxwell-Boltzmanndistribution

•  Showshowthespeed(orenergy)ofthemoleculesineffectedbytemperatureandotherfactors.

Solidparticlesize

•  Whenasolidreacts,onlytheparticlesonthesurfaceofthesolidareavailableforreaction.

•  Ifthesolidisbrokenupintosmallerpiecesitssurfaceareagetslargerandmoreparticlesareavailableforcollision,thereforethereactionrateincreases.

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Catalysts

•  Catalystsaffecttherateofareactionbyloweringtheactivationenergyorprovidinganalternatepathway

•  Catalystsarenotusedupduringthechemicalreaction

TypesofCatalysts

1.  Acid/Base–reactantwilleitherloseofgainH+,forminganewintermediateandasaresultthereactionrateischanged

2.  Surface–oftenametal,workinginagaseousreactantenvironment

3.  Enzyme–complexproteinmoleculesthatactasbiologicalcatalysts

Task5.1

Task5.21.  Whatcanbededucedaboutthetotalmassofthechips,relative

tooneanother,usedineachofthetwoexperiments?Explain.2.  Howlongdideachexperimenttaketogotocompletion?Howcan

youtellfromthedata?3.  Whatarepossibleunitsofrateofreactionintheseexperiments?4.  Writeachemicalreactionforthisprocess.5.  Whyisgasproducedatdifferentratesinthetwodifferent

experiments?6.  Explainhowtheapparatuslistedbelowcouldbeusedgenerate

thedatainthetableErlenmeyerflask,deliverytubeandrubbertubing,gassyringe,electronicbalance,graduatedcylinder

7.  Explainhowtheexperimentcouldbemodifiedtoinvestigatetheroleoftemperaturesand/orconcentrationinratesofreaction.

SetUp

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5.2IntroductiontoRateLaw

LearningTarget:Representexperimentaldatawithaconsistentratelawexpression.

RateLaws

•  Thesequenceofelementarystepsthatmakeupacomplex,chemicalreactionisknownasthemechanism

•  Eachstepwilleitherberelativelyfast,orrelativelyslow,buttheoverallrateofthecomplex,chemicalreactionisonlydependentupontheslowestelementarystep.

•  Forthisreason,thesloweststepisknownastheratedeterminingstep(RDS)

RateLaws&ReactionMechanisms

•  Inordertostudyreactionratesweneedtoconvertqualitativeelementarystepsintoquantitativeentities

•  Usingtherateequationorratelaw

•  k=therateconstantandx,y,andzaretheorderswithrespecttotheconcentrationofthereactantsA,B,andC

Order

•  Theorderwithrespecttoagivenreactantisthepowertowhichtheconcentrationofthatreactantisraisedtotherateequation

•  Theoverallorderofthecomplexchemicalreactionisthesumoftheindividualorders

•  Sinceonlythereactantsthatappearintherate-determiningstepareonesthataffecttherate,itisonlythesereactantsthateverappearintherateequation,andvice-versa

ImportantinformationaboutRateLaws

1.  Ordersmustbedeterminedexperimentallyorfromexperimentaldata,notfromstoichiometryofthecompletecomplex

•  However,givenjusttheslowest(ratedeterminingstep)stoichiometrycanbeused– Thestoichiometricnumberofareactantisthepowerthattheconcentrationofthatsubstanceisraisedtointherateequation

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ImportantinformationaboutRateLaws

6.Intermediatesareformedinoneelementarystepduringtheoverallreaction,butarethenusedupinasubsequentelementarystep.7.Ifasubstanceispresentedatthebeginningofareactionandpresentinthesameformattheendofthereaction,itcanbeidentifiedasacatalyst.

•  Forexample,themechanismforthecatalyzeddecompositionofhydrogenperoxidetoyieldwaterandoxygenisshownbelow.I-isacatalystandIO-isanintermediate.

•  Catalystscanappearinrateequationssincetheirconcentrationsareoftenmoreeasilydeterminedthanintermediates

ImportantinformationaboutRateLaws Task5.3

Identifythecatalystandtheintermediateinthemechanismbelow.

Example1

•  Thechemicalreaction,W+YàZ,hasthefollowingsequencesofelementarysteps(mechanism)

•  Here,therateonlydependsoftheconcentrationofW

•  Rate=k[W]

Example2

•  Thechemicalreaction,A+BàC+D,hasthefollowingsequenceofelementarysteps(mechanism)

•  Here,theslowstepcontainsQandBandwouldleadtoarateequationthus;

Rate=k[Q][B]

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Example2

•  However,sinceQisanintermediateandintermediatescanbedifficulttoisolateanditmaynotbepossibletodeterminetheconcentrationofQ

•  Asaresult,itispreferabletosubstituteforitintherateequation,withsomethingthatwecandetermineconcentrationof–  sincetheformationofQisdependentonA,wecanreplaceQintherateequation

Rate=k[A][B]

Example3•  Averycommonreactionforinvestigatingratesisthereactionbetween

bromate(V)ionsandbromideions,inacidsolution

Example3

•  ComparingtheresultsfrommixturesA&B;Doublingthe[BrO3

-]doublestherate.(Notetheotherconcentrationsarekeptconstanttoensureafairtest).Thereforetherate[BrO3

-]andtherateissaidtobefirstorderwithrespectto[BrO3

-]

Example3

•  ComparingtheresultsfrommixturesB&C;Doublingthe[Br-]doublestherate.(Notetheotherconcentrationsarekeptconstanttoensureafairtest).Thereforetherate[Br-]andtherateissaidtobefirstorderwithrespectto[Br-]

Example3•  ComparingtheresultsfrommixturesC&D;Doublingthe[H+]quadruplestherate.(Notetheotherconcentrationsarekeptconstanttoensureafairtest).Thereforetherate[H+]2andtherateissaidtobesecondorderwithrespectto[H+].

•  Combiningtheseresultsgives,

Rate=k[BrO3-][Br-][H+]2

Task5.4

Deducetheordersofreactionwithrespecttoeachreactant,theoverallorder,therateequation,avaluefortherateconstantandtheunitsoftherateconstant.

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Task5.5

Deducetheordersofreactionwithrespecttoeachreactant,theoverallorder,therateequation,avaluefortherateconstantandtheunitsoftherateconstant.

Mathematically

•  Thisallowsthesimplificationoftheexpressiontoread;

Mathematically

5.3ConcentrationChangesoverTime(Graphs)

LearningTarget:Identifytheratelawexpressionofachemicalreactionusingdatathatshowhowtheconcentrationsofreactionspecieschangeovertime.

Plotting[Reactant]againsttime

•  Onewayofdefiningtherateistousetheexpressionbelowandthereforethegradient(slope)ofsuchagraphwillequaltherate.

ZeroOrder:Astraight-lineshowszeroorderwithrespecttothatreactant.

•  Theslopeofthegraphisconstant,i.e.therateisconstantsochangingtheconcentrationofthereactanthasnoeffectontherate.

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FirstOrder:Aconstanthalf-lifegraphshowsthefirstorderwithrespecttothereactant.

•  Thegr

FirstOrder

•  Ifareactionisfirstorderwithrespecttoareactantbeingmonitored,aplotofthenaturallog(ln)ofthereactantconcentrationasafunctionoftimewillbelinear.

SecondOrder:Agraphthatlooksverysimilartothefirstordergraph,BUToncloserinspection

thehalf-lifeisnotconstantSecondOrder

•  Ifareactantissecondorderwithrespecttoareactantbeingmonitored,aplotofthereciprocal(1/[A])oftheconcentrationofthatreactantversustimewillbelinear.

Plottinginitialratesagainstconcentrationofthereactants

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Task5.6•  DataconcerningthechangeinconcentrationofasinglereactantA,in

particularchemicalreaction,arecollectedandtabulatedbelow.1.  WhatistheorderofthereactionwithrespecttoA?Justifyyouranswer.2.  Useyourgraphtocalculatethehalf-lifeforthisreaction.3.  Calculatetherateconstantforthereaction.

Half-lifeofareaction,t1/2:thetimerequiredfora reactant’s[]todropto ½ofitsorig.value

For1storderrxns:

For1storderreactions:

--t1/2isindependentofinitialconcentration

--the[]ofreactantsiscutinhalf...everyhalf-life

kt1/2=0.693=ln2

Ernest Rutherford is given credit for discovering the concept of radioactive half-life. It was later

shown that the same math applies to the kinetics of first-order chemical reactions.

Arrheniusequation

•  TheArrheniusequationrelatesrateconstants,activationenergyandtemperature.

•  Threeformatsareshownbelow(notonyourequationsheet–conceptual,notcalculations)

•  Ea=activationenergy•  R=gasconstant(J/molK)•  E=thebaseofthenaturallog•  T=temperatureinKelvin•  A=collisionfrequencyfactor

CollisionFrequencyFactor

•  Consideredconstantforaparticularreactionoveralargetemperaturerange,butAisaffectbythemolecularityofthereaction.

•  Highmolecularityreactionsarelesslikelytoproduceeffectivecollisions,asAissmaller

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5.5CollisionModel

LearningTarget:Explaintherelationshipbetweentherateofanelementaryreactionandthefrequency,energy,andorientationofmolecularcollisions.

CollisionTheory

•  Chemicalreactionscanoccuratdrasticallydifferentrates– Rustingisveryslowandmaytakemanyyears– Explosionstakejustafractionofasecond

•  Therateofthereactionisdeterminedbymonitoringthedecreaseintheconcentrationofthereactants,ortheincreaseintheconcentrationoftheproductsovertime

CollisionTheory

•  Collisiontheorytellsthatareactionwillonlytakeplace(willonlybesuccessful)ifthreeconditionsaremet;1.  Thereactantscollide2.  Thecollisionsoccurwithacertainminimum

energy,calledtheactivationenergyEact3.  Thecollisionsmusthavethecorrectmolecular

orientation.Aspecificpartofthereactingspeciesmustcontactaspecificpartoftheotherreactantspecies

CollisionTheory

•  Ifthepreviousrequirementsarenotmet(collide,energy,orientation)therewillbenoreaction

•  Describedasunsuccessfulastheydonotleadtoachemicalreactionandthereactantsremainunchanged

5.4ElementaryReactions

LearningTarget:Representanelementaryreactionasaratelawexpressionusingstoichiometry.

ElementarySteps•  Allchemicalreactionstakeplaceviaaseriesofelementarysteps

•  Anelementarystepisareactionthatformsproductsinasinglestep,withonlyonetransitionstateandnointermediates.

•  Anenergyprofilecanbeusedtoshowtheprogressofareactionfromreactants,throughatransitionstateandthenontoproducts

•  Reactantswithenergiesclosertothetransitionstateatthebeginningwillhaveloweractivationenergies,andthereforefasterratesofreaction

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Reactionprogressandelementarysteps

•  Elementarystepscanfallintooneorthreecategories,orthree‘molecularities’,butallsuccessfulcollisionsarestillsubjecttothecriteriaofsufficientenergyandcorrectorientation.1.  Unimolecular:Asinglespeciesreactstoform

productswhenarearrangementoccurs,activatingareactantmolecule.Ex.O3àO2+O

2.  Bimolecular:Twospeciescollideandreacttoformproducts.Ex.NO+O3àNO2+O2

3.  Trimolecular(ortermolecular):Threespeciescollideandreacttoformproducts.Ex.2NO+O2à2NO2

Reactionprogressandelementarysteps

Reactionprogressandelementarysteps

•  Thefewerthemoleculesinvolvedintheelementaryreaction,themorelikelyitisthatoneofthecollisionswillbeinthecorrectorientation.Meaning,thatwithincreasedmolecularity,thechancesofcorrectlyorientedcollisionsgoesdown

•  Trimolecularreactionsarerelativelyrareandreactionswith4ormorereactantsarealmostneverseen

5.6ReactionEnergyProfile

LearningTarget:Representtheactivationenergyandoverallenergychangeinanelementaryreactionusingareactionenergyprofile.

PotentialEnergyDiagrams PotentialEnergyDiagrams

•  Ifwestringtogetheralloftheelementarysteps,wegetamorecomplexseries

•  Thisexampleshowsanintermediateandtwotransitionstates.

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5.7IntroductiontoReactionMechanisms

LearningTarget:Identifythecomponentsofareactionmechanism.

ReactionMechanism

•  Theprocessesbywhichreactionsoccur•  i.e.,whathappensingettingfromreactant(R)toproduct(P)

•  Sometimes,mechanismsaredependentontemperature

Formultistepmechanisms,sequencesofelementarystepsareneededtogofromRtoP.

e.g.,Forthereaction

NO2+CO NO+CO2NO2 + NO2 NO + NO3 NO3 + CO NO2 + CO2

Elem.Step1:

Elem.Step2:

Above,NO3isanintermediate.Allmultistepmechanismshavethem.

NO2+CO NO+CO2thesemustaddtogivetheoverallreaction

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5.8ReactionMechanismandRateLaw

LearningTarget:Identifytheratelawforareactionfromamechanisminwhichthefirststepisratelimiting.

5.9Steady-StateApproximation

LearningTarget:Identifytheratelawforareactionfromamechanisminwhichthefirststepisnotratelimiting.

5.10MultistepReactionEnergyProfile

LearningTarget:Representtheactivationenergyandoverallenergychangeinamultistepreactionwithareactionenergyprofile.

5.11Catalysis

LearningTarget:Explaintherelationshipbetweentheeffectofacatalystonareactionandchangesinthereactionmechanism.

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