Lab22:Catalysts
Conceptstoexplore:
x Evaluatedifferentcatalyststodeterminewhichoneisthebestchoice
x Illustrate the differences between using heterogeneous and homogeͲ
neouscatalysts
x Demonstratehowvaryingquantitiesofacatalystaffectthereaction
Introduction
Why do bubbles form when you put hydrogen
peroxideonawound?
Ouch!Youjustscrapedyourknee.Afteryouhobbletoasink
towashitoff,youapplysomehydrogenperoxide.SmallbubͲ
bles start forming almost immediately. Why? The reason is
becausebloodandtissuecontainacertainenzymethataccelͲ
erates the decomposition reaction of hydrogen peroxide
forming oxygen gas and water. The enzyme is a biological
catalyst.Whenthereisaneedforspeedinareaction,usinga
catalystisoftenthebestmethod.
Figure1:Addingacatalystcandrasticallyspeeduptherate
Manychemicalsthehumanbodyneedsaremadewithin the ofachemicalreaction.CatalystsplayadirectroleintheenviͲ
cells. This means the human body has a need for superͲfast ronmentandinbiology,andareoftenusedinindustrialapͲ
chemical reactions. A high reaction temperature or a large plicationsforfoodprocessingandchemicalrefinement.
concentrationofthereactantswilloftensufficientlyspeedup
areactioninthelaboratory.Butourcellscannotrapidlyincreasetheirtemperatureorsuddenlyincreasetheavailabilityof
certainchemicals.Instead,thehumanbodyusescatalysts.Acatalystisasubstancethatspeedsupareaction,butisnot
consumed during the reaction. Biological catalysts are called enzymes. There are many different types of enzymes, and
eachtypespeedsupacertainreactionyourbodyneedstohavehappenrightthen!Withoutcatalystsyourbodycouldnot
doeventhesimplesttask.
Reactionshaveaminimumamountofenergyrequiredtooccur.Thisiscalledtheactivationenergy.Acatalystwilllower
the activation energy by requiring less energy for the reaction to occur. Chemists cannot always speed up a reaction by
changingtheusualvariables,andreactionsthattakealongtimeareseldomveryuseful.Forthisreason,chemistsoften
addcatalyststospeedupreactions.
Afterachemistdecidestouseacatalyst,thereareseveralthingsthathavetobestudied.Oneofthebiggestchallengesis
findingtheidealcatalystforaparticularreaction.Frequentlyseveralcatalystsarefoundthatwillwork,andtheyarecomͲ
paredtodeterminewhichisbest.Someofthefactorsthatachemistwilltakeintoconsiderationincludethedesiredspeed
ofthereaction,thecostofthecatalyst,howlongthecatalystwillwork,andifitistoxicorharmfultotheenvironment.
Anothercharacteristictoconsiderisacatalyst’’sphase.Aheterogeneouscatalystisinadifferentstateofmatter(phase)
thanthereactantswhenitisapplied,whileahomogeneouscatalystisappliedinthesamephase.Generally,homogeneous
catalystswillreactfaster,butheterogeneouscatalystsareeasiertoseparatefromtheproducts.
Inadditiontotheenzymesinbloodandtissue,thereareseveralothercatalyststhatcanbeusedtoacceleratethereaction
todecomposehydrogenperoxideintowaterandoxygengas.Manganesedioxide,manyfruitsandvegetables,household
bleach,andevensoilcanallbeusedtocatalyzethisreaction.Apieceofacarrotputintoasolutionofhydrogenperoxideis
anexampleofaheterogeneouscatalyst,asthecarrotisinasolidphaseandthehydrogenperoxideisinaliquidphase.If
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Lab22:Catalysts
insteadthecarrotismadeintoajuiceandaddedtothehydrogenperoxidesolution,itisahomogeneouscatalystsinceitis
thenintheliquidphase.
Chemistsalsohavetodeterminetheoptimalamountofacatalystpresentinthereaction.Ifacatalystisveryexpensive,
toxic,orhardtoremovefromtheproduct,theymayusetheleastamountofcatalystthatwillwork.Ifinsteadthereisa
needtohavethereactionhappenmorerapidly,achemistmaychoosetoaddmorecatalyst.Thereisapointhowever,
whereaddingmorecatalystwillnotincreasethereactionrate.Thisisbecausethereisasmuchormorecatalystthanthe
limitingreactant.
PreͲlabQuestions
1. Whatisacatalyst?
2. Ifyoucontinuetoaddmorecatalystwillthespeedofareactionalwayscontinuetoincrease?Explainyour
answer.
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Lab22:Catalysts
3. In this lab you will produce oxygen and water from hydrogen peroxide (H2O2). Write a balanced reaction
equationforthisreaction.
4. Whatcausesthebubblestoforminthisreaction?
5. Theexhaustgasfromcarenginespassthroughcatalyticconvertersthatcontainverysmallamountsofsolid
platinum,palladium,andrhodiumcatalysts.Arethesemetalshomogeneousorheterogeneouscatalysts?
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Lab22:Catalysts
Experiment:ReactionswithCatalysts
Inthislaboratoryexercise,youwillevaluatecarrots,tomatoes,yeast,andsoilascatalyststodecomposehydrogenperoxide.
Youwillalsoobservethedifferencesbetweenusingapieceofcarrotorcarrotjuiceasacatalystforthisreaction.Finally,you
willobservetheeffectsofaddingdifferentamountsofcarrotjuiceonthereactionrate.
Materials
SafetyEquipment:Safetygoggles,gloves
11testtubes
10mLgraduatedcylinder
Hydrogenperoxide(H2O2)
Yeast 3Droppers(pipettes) 2Testtuberacks
250mLbeaker
Stirrod
Soil
Warmwater(45϶C)*
Smallpieceofacarrot*
Smallpieceofatomato*
*Youmustprovide
Procedure
Part1:ComparisonofDifferentCatalysts
1. Place11testtubesinthetesttuberacks.
2. Usethepermanentmarkertolabel5ofthetesttubesC,T,D,andY,symbolizingcarrot,tomato,soil,and
yeast.HINT:It’’sbesttoclearlylabelglasswaretopreventcrosscontamination.
3. Mix100mLwarmwater(45϶C)withonepacketofyeastina250mLbeaker.StirwiththestirroduntildisͲ
solved.
4. Usea10mLgraduatedcylinderandapipettetoadd3mLofhydrogenperoxidetoEACHofthe5labeled
testtubes.
5. Carefullyaddasmallpiece(approximately1cm2)ofcarrottothetesttubelabeledC,andasmallpieceof
tomatotothetesttubelabeledT.AddasimilarsmallamountofsoiltothetesttubelabeledD.
6. Measure3mLofyeastsolutionintoaclean10mLgraduatedcylinder.Slowlyaddtheyeastsolutiontothe
testtubelabeledY.Swirluntilallbubblingandfoamingstops.ThiswillindicatethecompletionofthereacͲ
tion.
7. RecordobservationsforeachofthereactionsintheInitialObservationscolumninTable1intheDatasecͲ
tion.Letthereactionscontinueuntiltheendofthenextpartoftheprocedure.
Part2:CatalystQuantityComparison
7. Useapermanentmarkertolabelthe6remainingtesttubes,1,5,10,1A,5A,and10A.
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Lab22:Catalysts
8. Filltesttube1A,5A,and10Awith1mL,5mL,and10mLofyeastsolutionrespectively.
9. Usea10mLgraduatedcylinderandpipettetoadd1mLofhydrogenperoxidetothetesttubeslabeled1,5,
and10.
10. Simultaneously(orasclosetothesametimeaspossible)pourtheyeastsolutionfromtesttubes1A,5A,
and10Aintothecorrespondingtesttubes1,5,and10containingthehydrogenperoxide.
11. RecordyourobservationsofthethreereactionsinTable2oftheDatasection.HINT:BesuretonoteobserͲ
vationsbothofsimilaritiesanddifferencesinthebubblingandfoamingamongthereactions.
12. ReturntothetesttubesinPart1andmakefinalobservations.RecordyourobservationsintheFinalObserͲ
vationsColumninTable1oftheDatasection.
Data
Table1:Catalystcomparisonobservations
Typeof
Catalyst
InitialObservations
FinalObservations
Carrot
Tomato
Soil
Yeast
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Lab22:Catalysts
Table2:Catalystquantityobservations
Amountof
yeast
Observations
1mL
5mL
10mL
PostͲlabQuestions
1. Classifyeachcatalystyouusedashomogeneousorheterogeneous.
Homogeneouscatalysts:
Heterogeneouscatalysts:
2. Whichcatalystmadethereactiongothefastest?Isitahomogeneousorheterogeneouscatalyst?
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Lab22:Catalysts
3. Whichcatalystwouldbetheeasiesttoremovefromthewaterthatwasformed?IsitahomogeneousorhetͲ
erogeneouscatalyst?
4. Whichamountofyeastsolutionyoutestedwouldbethebesttouse?Explainyouranswer.
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