reates of reactin.docx
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reates of reactin.docx
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reatesofreactin
1.Whatdowemeanby'Rate'andhowisitmeasured?
∙WHATDOWEMEANBYSPEEDORRATEINTHECONTEXTOFACHEMICALREACTION?
∙ISITTOFASTORTOSLOWTOMEASURETHESPEED?
∙WHATSORTOFWAYSCANWEMEASURETHESPEEDOFACHEMICALREACTION?
∙Thephrase‘rateofreaction’means‘howfastorhowslowisthereaction’or'thespeedofthereaction'.Itcanbemeasuredasthe'rateofformationofproduct'(e.g.collectingagaseousproductinasyringe)orthe'rateofremovalofreactant'.Thespeedsofreactionsvarygreatly,withsomeprettyextremesituationsofbothslowandfast!
o
The chemicalweatheringofrocksisanextremelyveryslowreaction andlikefossilformationbymineralisationprocesses,itmighttakehundredsofthousandsormillionsofyears!
o
Rustingisavery‘slow’reaction,youhardlyseeanychangelookingatit,butafterafewweeksormonthsyounoticethedifferenceinapieceofexposedironorsteel!
However,itsalotslowerwithaluminium,soyourgreenhouseshouldlastafewyears!
oCorrosionofstonework,especiallylimestonebuildings,byacidrainisslow,butoverthelastfewhundredyearsoftheindustrialrevolutionmanyamedievalbuildinghassufferedfromitseffects!
oThe fermentationofsugartoalcoholisquiteslow butyoucanseethecarbondioxidebubblesforminginthe'froth'inalaboratoryexperimentorbeermakinginindustry!
o
A fasterreactionexampleismagnesiumreactingwithhydrochloricacidtoformmagnesiumchlorideandhydrogen orthe evenfasterreaction between sodiumandwatertoformsodiumhydroxideandhydrogen. Thesereactionstakeafewminutes.
oA'useofwords'revisionnote:
Reactingand/ordissolving?
Chemicalorphysicalchange?
▪Ifyoutakethesolidsmagnesiumchlorideorsodiumhydroxideandmixthemwithwaterthey dissolve toformasolution,butnochemicalreactiontoformnewsubstancestakesplacei.e. dissolvingonitsownisbasicallyaphysicalchange.
▪However,thetwosubstancesmentionedaboveareformedina chemicalreactionchange,wheretheword'dissolving'onitsownisinadequate.Thephrases reactionwith... or reactionbetween... aremuchmoreappropriate,butthereisnodenyingthatthesodiumdissolveinwaterormagnesiumdissolvesinacid, BUTonlybecausetheyhaveformedawatersolublecompound e.g.sodiumhydroxideormagnesiumchloride.
o
Amuchfasterwouldbemagnesiumburninginair,overinseconds!
oCombustionreactionse.g.whenafuelburnsinairoroxygen,areveryfastreactions.
o
Explosivereactionswouldbedescribedas‘veryfast’,infacttheyaresomeofthefastestreactionsknowne.g.thepopofahydrogen-airmixtureonapplyingalitsplintortheproductionofagastoinflatetheairbagssafetyfeatureofmanycars.
▪
Explosionsarethefastestchemicalreactionsyouwillencounterandareoverinafractionofsecond,butabunsenburnerflamesisasortof'controlledexplosion'andthechemicalreactionsofthefuelcombustionareveryrapidindeed!
buttherapidlyburnedfueliscontinuouslyreplacedbythegasflow.
▪ExplosivematerialslikedynamiteorTNTproducepowerfulexplosionsinafractionofasecond.
∙The importanceof"RatesofReactionknowledge":
oTimeismoneyinindustry,thefasterthereactioncanbedone,themoreeconomicitis.
▪Youneedtoknowhowlongreactionsarelikelytotaketomakeasmuchproductinagiventime.
▪Hencethegreat importanceofcatalysts e.g.transitionmetalsorenzymeswhichreducetimeandsavemoney.
▪Evenjustraisingthetemperaturecanhaveadramaticeffectonthespeedofareaction,butraisingthetemperaturecostsmoney.
▪Rateofreactionknowledgeisessentialtooperateachemicalplantsafelyandefficientlysosometimesacompromisemustbereachedi.e.economicoptimumconditions.
▪The HaberSynthesisofammonia isagoodexampletostudyintermsofvariouseconomicfactors.
oHealthandSafetyIssues:
▪Thefasterareaction,themorelikelytheremightbehealthandsafetyissuestocontendwith.
▪Thephysicalstateorconcentrationofamaterialcanbeanimportantfactor.
▪Finepowdersinflourmillscanbeeasilyignitedandcanexplodeasiftheflourwasagas.
▪Similarly,finecoaldustinamineposesthesamethreat.
▪Mixturesofflammablegasesinairpresentanexplosionhazard(gasreactionslikethisareamongstthefastestreactionsknown).Iftheflammable/explosivegasisinlowconcentration,theremaybenorisk,butyouneedtoknowthesafelimits!
▪e.g.Methanegasinmines,petrolvapouretc.areallpotentiallydangeroussituationssoknowledgeof'explosion/ignitionthresholdconcentrations',ignitiontemperaturesandactivationenergiesareallimportantknowledgetohelpdesignsystemsofoperationtominimiserisks.
▪Flammablefinedustpowderscanbeeasilyignitede.g.coaldustinmines,flourinmills,custardpowderproductionlines!
▪Finepowdershavealarge surfacearea whichgreatlyincreasesthereactionratecausinganexplosion.Anysparkfromfrictionisenoughtoinitiatethereaction!
∙Areactionwillcontinueuntiloneofthereactantsisusedup,ifareactantisinexcess,thensomeofitwillbeleftover.
∙Tomeasurethe‘speedofachemicalreaction’ orthe ‘rate’ofareaction dependsonwhatthereactionis,andwhatisformedthatcanbemeasuredasthereactionproceeds.
oHowdowedefinetherateorspeedofachemicalreaction?
oRateofreaction=amountofproductformed/timeintervalinvolved
oOR
oRateofreaction=amountofreactantusedup/timeintervalinvolved
oThissortofdefinitionwillbecomeprettyobviouswhenanalysinggraphsofdatee.g.volumeofgasformedatvarioustimeintervals,sotheratemightbedefinedas..
▪rateofreaction=cm3 gasformed/timetakenforgastoform
∙EXAMPLESofexperimentalset-upstofollowtherateofachemicalreactionaredescribedbelow.
oExamplesofresultsdata,graphsandtheirinterpretationarealsogiven.
∙MEASURINGGASVOLUMES -followingtheformationofagaseousproductofareactionbycollectingthegas
∙When agasisformed fromasolidreactingwithasolution,itcanbecollectedina gassyringe (seethe apparatusdiagram belowandthe graphofresults).
oUsinge.g.agassyringesystem(illustratedbelow),youmeasurethe volumeofgasgivenoffatregulartimeintervals.
▪Allresultsshouldbeneatlytabulated.Seesome 'concentrationeffect'results.
oFromtheneattableofresults,youthenplottheresultsongraphpaper(orwithasoftwarepackagelikeexcel)making timethehorizontalx-axis andthe volumegasofgastheverticaly-axis.
oThe initialgradient ofthegraphe.g.in cm3/min (speedorrate)givesanaccuratemeasureofhowfastagaseousproductisbeingformede.g.in ametalcarbonate-acidreactiongivingcarbondioxide,ormetal-acidreactiongivinghydrogengas.
oItisbesttomeasuretheinitialratebecausethereactionmixtureisbecomingdepletedinreactantssochangingtheinitialconcentrationsorsurfaceareaifthereisasolidreactantdissolving.
oYoucanmeasurethegasformedeverye.g.30or60seconds(0.5or1.0minutesistypical)andplotthegraphandmeasuretheinitialgradientine.g.cm3/minorcm3/sec.
oThemostaccuratemeasurementsaremadeearlyoninthereactionwhenthegasvolumeversustimeisalmostlinear.
▪Youcantakeaseriesofmeasurementsanddrawthegraph(origin 0,0)togettheratefromthegradient(e.g.cm3/min),
▪ormeasurethetimetomakeafixedvolumeofgas (*seebelow).
oIfthereactionisallowedtogoon,youcanmeasurethefinalmaximumvolumeofgasandthetimeatwhichthereactionstops,thoughthisaverypoormeasureofrate,becausethereactionjustgoesslowerandslowerasthereactantamounts/concentrationsaredecreasing-so don't usethisasamethodofmeasuringreactionspeed.
o
Results-usinggraphpaper
oThisgraphofvolumeofgasformedversustimerepresentsatypicalsetofresults,thisparticulargraphisdiscussedinmoredetailsin Moreexamplesofgraphsandtheirinterpretation.
▪Thesteepertheinitialslope/gradient,thefastertheinitialreaction -itsthatsimple!
▪Thegradientofthegraphbecomelesssteepbecausethereactionwillnaturallyslowdownduetothecumulativelossofreactantmolecules.
▪Whenthe gradientfirstbecomeszero,i.e.thegraphlinebecomeshorizontal,oneormoreofthereactantshasbeenusedupandthe reactionhasthenstopped.
▪e.g.inthecaseofexperimentrunE,at4mins,atamaximumvolumeof20cm3 ofgasformed.
▪Intermsofinitialrate,the'speedorder'isW>X>E>Y>Z,withdecreasinginitialgraphgradient.
▪GraphsX,E,YandZcanbetheresultofdecreasingconcentrationfromXtoZ.
▪ThesamepatternmightbeseenbydecreasingthetemperaturefromXtoZ.
▪GraphWmightbetheresultofdoublingtheamountorconcentrationofoneofthereactantscomparedtoE,steeperinitialgradient(faster)andtwicetheamountofgaseousproduct.
▪GraphsXandEcouldrepresenttheeffectofaddingacatalysttoX.
▪GraphsZtoXcouldbeproducedbyprogressivelycrushingthesameamountofsolidreactante.g.lumps ▪Note: Complyingwith'afairtest'experimentdesignisdiscussedindetailfortheexperimentsineachofthesectionsfortheeffectof3a.concentration,3c surfacearea,3d temperature and3e. catalyst. ▪Apartfromtheinitialgradient,youcanuseasameasureoftherateofreaction,howmuchgasisformedforaconstanttimee.g.productgasvolumeafter2minutes. o(*) Thereciprocalofthereactiontime, 1/time,canalsobeusedasameasureofthespeedofareaction. ▪Thetimecanrepresenthowlongitt
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