Name_________________________
TeamName______________________
CHM112Lab–IodineClockReactionPart1–GradingRubric
Criteria
Pointspossible
Pointsearned
LabPerformance
Printedlabhandoutandrubricwasbroughttolab
3
Initialconcentrationscompletedbeforecomingtolab.
2
Safetyandproperwastedisposalproceduresobserved
2
Followedprocedurecorrectlywithoutdependingtoomuchon
instructororlabpartner
3
Workspaceandglasswarewascleanedup
1
LabReport
Calculationsforchangeinmolarity(workshown)
1
Ratelawcalculations
2
Rateconstantcalculations
2
Question1
0.5
Question2
1
Question3
1.5
Question4
1
Total
20
Subjecttoadditionalpenaltiesatthediscretionoftheinstructor.
IodineClockReactionPart1
Introduction
InthisexperimentyouwilldeterminetheRateLawforthefollowingoxidation-reductionreaction:
(1)
2H+(aq)+2I—(aq)+H2O2(aq)→ I2(aq)+2H2O(l)
Therateorspeedofthereactionisdependentontheconcentrationsofiodideion(I-)andhydrogenperoxide,
H2O2.(Thespectatorionsareleftoffthereaction.)Therefore,wecanwritetheRateLaw(concentrationdependence)
forthereactionas:
(2)
Rate=k[I ]x[H2O2]y
Where:xistheorderofthereactioninI-,yistheorderofthereactioninH2O2,andkistherateconstant.
Thetemperaturedependenceoftherateisseenink–thatis,thereisaseparatevalueofkforeach
temperatureatwhichthereactiontakesplace.Thetemperaturemustthereforebeheldconstanttoaccurately
calculatex,yandk.SincetheRateLawisempirical,wehavetogotothelabtomakemeasurementsthatwillenable
thesevaluestobecalculated.Theratewillbemeasuredforthereactionneartime=0,sothatfewproductsbeen
formedandtherewillbenoreversereaction.Theconcentrationsofiodideandhydrogenperoxidewillbevariedand
theratescomparedtofindeachorder(i.e.,theexponentsxandy).ThisistheMethodofInitialRatesanditwillbeused
tofindx,yandk.
Aswithalotofkinetics,theconcentrationofreactantsorproductsatanyinstantisdifficulttomeasuredirectly,
sointhislabtheratewillbedeterminedindirectly.Wehaveaveryhandytestforthepresenceofoneoftheproducts,
iodine(I2),namelystarch.Starchreactswithiodinetoformablue/blackcoloredcomplex.Unfortunatelyassoonasany
iodineisproduceditwillreacttomakethecomplexandthesolutionwillturnblue/blackinstantaneously.Thus,using
starchasanindicatorbyitselfwouldnotbeofmuchhelp.ItconfirmsthatsomeamountofI2isbeingformed,butittells
usnothingaboutwhatwearetryingtomeasure-therate(howlongittakestoproduceagivenquantityofI2.)
TogetaroundthisproblemwewillintroduceasidereactionthatwillremovetheinitialI2thatisproducedby
ourmainreaction.Thiswillpreventthesolutionfromturningblacklongenoughsothatwecanmakesometime
measurements.Wewillusethefollowingsidereaction:
(3)
I2(aq)+2S2O32—(aq)→ 2I—(aq)+S4O62—(aq)
S2O32—,thiosulfateion,reactswithI2whichpreventsthesolutionfromturningblue/black.Howwillthishelp?Since
wehavecarefullymeasuredtheamountofthiosulfate(asmallamountthatwillrunoutfairlyquickly),weknowexactly
howmuchiodineitwilltaketoreactwiththisthiosulfate.Oncethesmallamountthiosulfatehascompletelyreacted,I2
willstarttobuildupinthesolution.Assoonasthethiosulfaterunsout,I2willreactwiththestarchandthesolutionwill
turnblue/black.Byputtinginthis"timedelay",wecannowcalculatetherateatwhichI2isbeingformed.
Therateofreactionisequaltothechangeinconcentrationdividedbythechangeintime.Thechangeintime
willbethetimeittakesforthesolutiontoturndark.WewillcalculatethechangeinconcentrationinI2basedonthe
amountofthiosulfateadded.Usingtheknownvolumeandmolarity,wecancalcuatemolesofthiosulfate(S2O32—).
Basedonstoichiometry,wecancalculatethemolesofI2:accordingtoequation(3),2molesofthiosulfatereactwith
every1moleofI2,Thisgivesusthechangeinmoles,howeverfortherateformulaweneedchangeinconcentration.
DividethemolesofI2reactedbythetotalvolumetofindthechangeinmolarity.
(4)
Rate=[ΔI2]/Δt
−
Equipment
Three125or250mLErlenmeyerflasks
Three100or150mLbeakers
10mland5mlPipettes
Stopwatchorothertimekeeper
Onebinofchemicalspergroupthatwillcontain:
0.050MKI
0.050MKCl0.010MNa2S2O30.050MH2O2
1.0MH2SO4
1%starchsolution
Disposal:AllmixturesSpill/DisposalB1(downthesink)
Procedure:
1. Beforecomingtolab,completetheinitialconcentrationtablebelowshowingallwork.Initialconcentrationsarenot
the concentration of the solutions you start with (i.e. not 0.050M for I- and H2O2). Initial concentrations are the
concentrationofspeciesinthefinaltotalvolumeofcontentsoftheErlenmeyerplusbeakerforeachrun(seetables
insteps4&6).DetermineinitialconcentrationsusingtheformulaM1V1=M2V2.M1andV1arethemolarityand
volumeofthesolutionsadded,V2isthetotalvolumeoftheErlenmeyerplusbeaker.Copytheseinitialvaluesonto
thetableinthedatasheet.
Run
[I—](initial)
[H2O2](initial)
1(Flask1+beaker1)
(15.0mL)(0.050M)=(42.5ml)(M2)
SolvingforM2,weget[I—](initial)=0.018M
2(Flask2+beaker2)
3(Flask3+beaker3)
2. CleanandmostlydrythreeErlenmeyerflasks.Labelthem1,2and3.
3. Obtainabinofchemicalsforyourgroup.Useonlythesechemicalsforallofyourruns.Usefreshpipettesforeach
solution.Rinsepipettetwicewiththesolutionthatyouwillbemeasuringandkeepthispreparedpipettewiththe
correspondingsolution.
4. Addtheamountsofthesolutionsbelowtoprepareeachflask.Thechemicalsmustbeaddedintheorderlisted(top
tobottom).
Flask#1
Flask#2
Flask#3
0.050MKI
15.0mL
15.0mL
7.5mL
1%Starch
5.0mL
5.0mL
5.0mL
0.010MNa2S2O3
2.5mL
2.5mL
2.5mL
1MH2SO4
5.0mL
5.0mL
5.0mL
0.050MKCl
0
0
7.5mL
Note:TheKClsolutionisaddedsothattheionicstrengthofandvolumeofeachsolutionisthesame.
5. Rinseandmostlydry3beakersandlabelas1,2and3.
6. Preparethefollowingsolutionsincleanbeakers:
Beaker#1
Beaker#2
Beaker#3
0.050MH2O2
15.0mL
7.5mL
15.0mL
DeionizedH2O
0
7.5mL
0
Note:deionizedwaterareaddedsothatthetotalvolumeofeachrunisthesame.(42.5mL)
7. Getyourtimerready.Addthecontentsofbeaker1toflask1.
8. Startthestopwatchassoonasyoumixthesolutions.Swirltheflasktomixandnotethetimeittakesforthecolor
tochange.ThisisRun1.Recordthetemperatureofthemixture.
9. Addthecontentsofbeaker2toflask2.Repeatprocedureinstep8.ThisisRun2.
10. Addthecontentsofbeaker3toflask3.Repeatprocedureinstep8.ThisisRun3.
Theshadeofcoloryouobserveshouldbethesameineachofthethreeruns.Ifitisnot,consultyourinstructor.
Disposal:
Allcontentsofthereactionflasksmaybedisposedofintothesink.
Calculations:
Reviewthemethodofinitialratesinyourtext.Youwillneedthevaluesfortheinitialconcentrationsofeachreactantas
wellastherate.
Hypotheticaldataforreaction: A+2B→C+2D
Run
[A]0
[B]0
InitialΔ[C]/ Δt (Molarity/sec)
1
0.150M
2
0.150M
30.300M
0.150M
0.300M
0.150M
8.00
16.0
32.0
Rate=k[A]x[B]y.Weneedtocalculatex,yandk
Togetx,wehavetohold[B]constantandjustseehow[A]affectstherate.TaketheratioofRun3/Run1
Run3=InitialΔ[C]/ Δt
=k[A]x[B]y=32.0M/sec= k(0.300M)x(0.150M)y
Run1 InitialΔ[C]/ Δt
k[A]x[B]y 8.0M/sec k(0.150M)x(0.150M)y
Simplifying:4.0=(0.300M)x/(0.150M)x
or 4.0=(0.300/0.150)xor4.0=2xOrx=2
DothesameforB(holding[A]constant)anddiscoverthaty=1.
Youcancalculatekaswell.Usingrun1:
8.00M/sec=k(0.300M)1(0.150M)1 andsolvingfork,wegetk=178M-1s-1.
‘k’canbecalculatedusinganyofthethreerunsfromPartI
Foryourdata:
Findxandyasexplainedabove.Calculatekforeachrunandaveragethethreevaluesattheend.Ifyourvaluesforx
andydonotturnouttobewholenumbers,roundtothenearestwholenumber.WritetheRateLawusingyourvalues
forx,yandk:
Rate=k[I ]x[H2O2]y
−
IodineClockReactionPart1:DataSheetName_________________________
ShowyourcalculationsformolesofI2reacted:(findthroughmolesofS2O32-)
ShowyourcalculationformolarityofI2reacted[ΔI2]:(dividemolesreactedbytotalreactionvolume)
Run
Initial[I-]M
1
2
3
Initial[H2O2]M
ReactionTime(ins)
Δt
ReactionRate(inMs-1)
=[ΔI2]/Δt
Showcalculationsforxandy:(Includeappropriateunits,attachaseparatesheetifnecessary.)
x=______
y=_______(roundtothenearestwholenumber)RateLaw:__________________________
Showcalculationsfork:
k(run1)=__________ k(run2)=__________
k(run3)=__________Averagek=__________
ReportPage1of2
IodineClockReactionPart1:PostLabName_________________________
1. Whatistheoverallorderofthereactioninthislab?
2. UseyourRateLawandaveragektocalculatethereactionratewhen[I-]=0.025Mand[H2O2]=0.032M.
3. ConsiderthereactionA+B→Products.
ThisreactionwasexperimentallydeterminedtobefirstorderinAandsecondorderinB.
a. Writedowntheratelawforthisreaction.
b. Whatwouldbetheunitofrateconstant‘k’fortheabovereaction?
c. IftheconcentrationofAisdoubledwhilekeepingtheconcentrationofBconstant,byhowmuchwilltherateof
thereactionchange?
4.ThedecompositinoofacertainchemicalABisfoundtobesecondorderwithresepectto[AB]andhavearate
constantof3.5x10-2M-1s-1.IftheinitialconcentrationofABis1.54M,whatwillbetheconcentrationafter73seconds?
ReportPage2of2