Chemistry12

ReactionMechanismsWorksheetName:Date:Block:

1. Itisknownthatcompoundscalledchlorofluorocarbons(C.F.C.s)(eg.CFCl3)will

breakupinthepresenceofultravioletradiation,suchasfoundintheupperatmosphere,formingsinglechlorineatoms:

CFCl3à CFCl2+Cl TheClatomsthenreactwithOzone(O3)asoutlinedinthefollowingmechanism. Step1:Cl+O3à ClO+O2 Step2:ClO+Oà Cl+O2(single"O"atomsoccurnaturallyintheatmosphere.) a)Writetheequationfortheoverallreaction.(Usingsteps1and2) b)Whatisthecatalystinthisreaction? c)Identifyanintermediateinthisreaction d)Explainhowasmallamountofchlorofluorocarbonscandestroyalarge amountof

ozone. e)WhatbreaksthebondintheCFCl3andreleasesthefreeClatom? 2. Giventhefollowingmechanism,answerthequestionsbelow: Step1:O3+NOà NO2+O2(slow) Step2:NO2+Oà NO+O2(fast) a)Givetheequationfortheoverallreaction. b)Whatcouldthecatalystbeinthismechanism? c)Whatisanintermediateinthismechanism?

03 t O 202ClCIO

Fcb produces a Cl atom which can beregenerated as a catalyst

Ultraviolet radiation

Oz to 202NONOz

d)Giventhattheuncatalyzedoverallreactionisaslowexothermicreaction,drawa

potentialenergygraphwhichshowsthepossibleshapeofthecurvefortheuncatalyzedreaction.Onthesamegraph,showapossiblecurveforthecatalyzedreaction.

Potential Energy

ProgressofReaction3. Considerthefollowingmechanism: Step1:H2O2+I-à H2O+IO-(slow) Step2:H2O2+IO-à H2O+O2+I-(fast) a)Givetheequationfortheoverallreaction. b)Whatactsasacatalystinthismechanism? c)Whatactsasanintermediateinthismechanism? 4. Whatismeantbytheratedeterminingstepinareactionmechanism? 5. Whatismeantbyareactionmechanism?

guncatalyzed rxn

Ate

2HzOz 2h20 t 02IIo

theslowest step

a series of elementary rxus bywhicha rxn takesplace

6. Howarereactionmechanismsdetermined? 7. GiventhefollowingPotentialEnergyDiagramfora3stepreaction,answerthequestions

belowit:

a)Whicharrowindicatestheactivationenergyforthefirststepofthereversereaction? b)Whicharrowindicatestheactivationenergyforthefirststepoftheforwardreaction? c)Whicharrowindicatestheactivationenergyforthesecondstepoftheforwardreaction? d)Whicharrowindicatestheenthalpychange(ΔH)or"enthalpychange"fortheoverall forwardreaction? e)Whicharrowindicatestheenthalpychange(ΔH)or"enthalpychange"fortheoverall reversereaction?

1

2

34

5

POTENTIAL ENERGY

PROGRESS OF REACTION

6

research

Mined

5

I

2

6

6

f)Whicharrowindicatestheactivationenergyfortheoverallforwardreaction? g)Whichstepwouldbetheratedeterminingstepintheforwardreaction? h)Inadashedlineoranothercoloursketchapossiblecurvethatwouldrepresenttheroute

fortheuncatalyzedoverallreaction.Labelthisonthegraph.8. Giventhereaction: 4HBr+O2à 2H2O+2Br2

a) Wouldyouexpectthisreactiontotakeplaceinasinglestep?Whyorwhynot? b)Thisreactionisthoughttotakeplacebymeansofthefollowingmechanism: Step1:HBr+O2à HOOBr(slow) Step2:HBr+HOOBrà 2HOBr(fast)Step3:2HBr+2HOBrà 2H2O+2Br2(fast) c)Identifythetwointermediates d)AcatalystisdiscoveredwhichincreasestherateofStep3.Howwillthisaffecttherateof

theoverallreaction?Explainyouranswer. e)AcatalystisdiscoveredwhichincreasestherateofStep1.Howwillthisaffecttherateof

theoverallreaction?Explainyouranswer. f)Whichstephasthegreatestactivationenergy?

3

2

No smallpossibility of a simultaneous collisionof 4HBRand 1oz particles

HOOBr HoBr

Thiswillhave no effect since step3 is not the ratedeterminingstep

This will increasethe rate sincestep1 is the ratedeterminingstep

Step 1 because it is the sloweststep

g)Howmany"bumps"willthepotentialenergydiagramforthereactionmechanismhave? h)Whichstepiscalledtheratedeterminingstepinthismechanism? i)Inordertohavesuccessfulcollisions,thecollidingparticlesmusthaveboththeproper amountofenergyandtheproper_____________________. j)Onthesetofaxesbelow,drawtheshapeofthecurveyoumightexpectforthereaction

inthisquestion.Theoverallreactionisexothermic!Makesureyougetthe"bumps"thecorrectrelativesizes.

Potential Energy

Progress of Reaction 9. Theequationforanoverallreactionis: I-+OCl-à IO-+Cl- a)Thefollowingisaproposedmechanismforthisreaction.Oneofthespecieshasbeenleft

out.Determinewhatthatspeciesisandwriteitinthebox.Makesurethechargeiscorrectifithasone!

Step1:OCl-+H2Oà HOCl+OH-(fast) Step2:I-+HOClà IOH+Cl-(slow) Step3:IOH+OH-à +H2O(fast) b)Whichspeciesinthemechanismaboveactsasacatalyst? c)Whichthreespeciesinthemechanismaboveareintermediates? d)Step______________istheratedeterminingstep.

3

I

m

Io

H2O

2HOUOH IOH

e)Onthesetofaxesbelow,drawtheshapeofthecurveyoumightexpectforthe

reactioninthisquestion.Theoverallreactionisendothermic!Makesureyougetthe"bumps"thecorrectrelativesizes.

Potential Energy

Progress of Reaction 10. Giventhefollowingstepsforamechanism: Step1:Br2à 2Br(fast) Step2:Br+OCl2à BrOCl+Cl(slow) Step3:Br+Clà BrCl(fast) a)Writetheequationfortheoverallreaction. b)Asubstanceisaddedthatdecreasestheactivationenergyforstep1.Willthisspeed

up,slowdown,orhavenoeffectontherateoftheoverallreaction?Giveareasonforyour

answer. c)Isthereacatalystinthismechanism?__________.Ifso,whatisit? d)Isthereanintermediateinthismechanism?________.Ifso,whatisit? e)Whichstepistheratedeterminingstep?

m

s

l

Bre t 042 BroCI t Bru

No effectStep 1 is not the RDSNoYes Brand Cl

step 2

11. Thefollowingpotentialenergydiagramreferstoaveryslowone-stepreactionofozone(O3)

andoxygenatomsintheupperatmosphere. Ontheaxisbelow,drawapotentialenergydiagramwhichcouldrepresentthecatalyzed

mechanismforthereaction: Step1:O3+NOà NO2+O2(slow) Step2:NO2+Oà NO+O2(fast)

0

50

100

150

200

250

Progress of Reaction

PE (kJ)

0

0

50

100

150

200

250

Progress of Reaction

PE (kJ)

0

O + O

2O

3

2

n

12. Acertainchemicalcanprovideareactionwithanalternatemechanismhavingagreater

activationenergy.Whatwillhappentotherateofthereactionwhenthischemicalisadded? Explainyouranswer. 13. Thefollowingoverallreactionisfastatroomtemperature: H++I-+H2O2à H2O+HOI Astudentproposesthefollowingtwo-stepmechanismfortheabovereaction: Step1:H++H++H2O2à H4O22+ Step2:H4O22++I-à H2O+HOI+H+

Wouldyouagreeordisagreewiththisproposedmechanism?Explainyouranswer 14. Considerthefollowingoverallreaction: CO+NO2à CO2+NO a)Thefirststepineachoftwoproposedreactionmechanismsfortheabovereactionislisted

below.Ifeachproposedreactionmechanismconsistsofonlytwosteps,determinethesecondstepforeachmechanism.

ProposedMechanismOne:

Step1:2NO2à NO3+NO(slow) Step2:__________________________________________ (fast)

ProposedMechanismTwo: Step1:2NO2à N2O4(fast) Step2:__________________________________________(slow) b)Experimentaldatashowthattherateofthereactionisnotaffectedbyachangeinthe[CO].

Whichofthesetwomechanismswouldbeconsistentwiththesedata?Explainyouranswer.

No change The rain will continue to take placewith the lower Ea

Disagree Step 1 involves the collision of 3particleswhichwould beslow Theraincould not be fast asobserved

Nost co Cozt Nae

Lo tNoy NOCozNO

If the rain rate