Lab Manual Softcopy

EXPERIMENT1 THERMOCHEMISTRY:DETERMININGTHEHEATOFREACTION Objectives 1 Todeterminethevalueofheatcapacityofacalorimeter. 2 TodeterminetheheatofneutralisationofHClandNaOH. Introduction Chemicalreactionscanreleaseorabsorbheat,whichcanbemeasuredbyusinga calorimeter.Acalorimeterisacontainerthatisthermallyisolatedfrom theenvironment. Heatreleased bythe chemicalreaction (-qr )isabsorbed bythe solution and the calorimeter. -qr = qs + qc (1) where qs = heatabsorbedbysolution qc = heatabsorbedbycalorimeter Theheatabsorbedisproportionaltothechangeintemperature.Theproportionality constant,Ccisknownastheheatcapacityofasample.Heatcapacityisdefinedasthe amountofheatrequiredtoincreasethetemperatureby1oC. q = CcT (2) Forahomogenoussamplesuchassolutions,theheatabsorbedisproportionaltothe massofthesampleandtheincreaseintemperature.Proportionalityconstant,cs is knownasspecificheatcapacityofsolutionperunitmass. Thespecificheatcapacityforwateris4.184JK-1g-1.Inmostsituations,wherethesolute (ieHCl)isdilute,thespecificheatcapacityofthesolvent(water)willonlybeaffectedto aminimaldegree. Thus,thespecificheatcapacityofadilutesolutionofHClforexample isveryclosetothespecificheatcapacityofpurewaterandthisvaluewillbeused throughoutthewholeexperiment. Inthisexperiment,welabelthereactingmixture(water,NaOH,HCl)thesystem andthe restoftheuniversethesurroundings.Becauseenergycannotbecreatedordestroyed, anyenergylostbythesystem mustbegainedbythesurroundings. Anexothermicreactionisanyprocessthattransfersthermalenergytothesurroundings forexamplethecombustionofethanolmakingthebeakercontainingethanolbecomes hot.Anendothermicprocesshappenswhentheheathastobesuppliedtothesystem bythesurroundingsforexamplethedecompositionofmercury(II)oxide(HgO)which makesthetesttubecontainingHgObecomescold. Heatthatisbeing released orabsorbed (qr)can be determined bymeasuring the temperaturebeforeandafterthereaction. -qr= CcT + mcsT (3) where T = finaltemperatureofsystem –initialtemperatureofsystem m = massofsolution 1

Cc = heatcapacityofcalorimeter cs = specificheatcapacityofsolution Forthisexperimentthedensityofthesolutionisassumedtobethesameasthedensity ofwater(1gcm-3).Thereforethemassofthesolutioncanbecalculated. Apparatus ChemicalReagents coffeecupcalorimeterwithlid NaOH1.0M 100mLbeaker HCl1.0M measuringcylinder distilledwater conicalflask thermometer waterbath/hotplate Method A Determinationoftheheatcapacityofacalorimeter 1. Measurethetemperatureofanemptycalorimeterbyputtingathermometer insidethecalorimeter,T1. 2. Pour50mldistilledwaterintoa100mlbeaker. 3. Heatthebeakertoatemperaturebetween50oC-60oC. 4. Pourthehotwaterintothecalorimeter.Immediatelymeasuretheinitial temperatureofthehotwater,T2. 5. Observethedecreaseintemperatureuntilitstaysconstantfor3minutes.Record thistemperature,T3. Results: = Temperatureofcalorimeter = Initialtemperatureofthehotwaterused = Constanttemperatureofwater = Massofwater B DeterminationoftheheatofneutralisationofHCl1.0M andNaOH1.0M 1.Usingameasuringcylinder,measure25mLNaOH1.0M and25mLHCl1.0M. 2. PourtheNaOHsolutionintothecalorimeterandtheHClsolutionintoaconical flask.Recordtheinitialtemperatureofeachsolution. 3. Withoutremovingthethermometer,liftthecoverslightlyandquicklypourallthe HClsolutionintothecalorimeter.Quicklyreplacethecoverofthecalorimeter. 4. Swirlthesolutionandbegintorecordthetemperatureevery15secondsforat least2minutes.Recordthemaximum temperature. 5. Repeatthisexperiment. Note:Closethecalorimeterwithitslidallthetimeduringstep1to6. Results: = InitialtemperatureofNaOH = InitialtemperatureofHCl = Averageinitialtemperatureofsolution = Maximum temperatureofsolution = ∆T 2

CALCULATION: 1. Determinetheheatcapacityofyourcalorimeter. 2. Calculatethevalueoftheheatofneutralisationoftheabovereaction DISCUSSION: 1. Definetheheatofneutralisation. 2. Thetheoreticalvalueoftheheatneutralisationoftheabovereactionis-58kJmol-1. Comparetheheatofneutralisationobtainedinthisexperimentwiththetheoretical value.Giveyourcomments. 3. Suggestthreeprecautionsshouldbetakenduringthisexperiment. CONCLUSION: 1. 2. EXPERIMENT2 NERNSTEQUATION:DETERMINATIONOFELECTRODEPOTENTIALSOFZINCAT VARIOUSCONCENTRATIONSOFELECTROLYTESOLUTION Objectives 1. Todeterminetheeffectofconcentrationofelectrolytesolutionsonthereduction potentialofZn2+/Znhalfcell. 2. TocomparereductionpotentialvaluesofZn2+/Znhalfcellobtainedfrom the Nernstequation. Introduction Reactionsinelectrochemicalvoltaic/galvaniccell Voltaiccellisa type ofelectrochemicalcellthatproduceselectricity.A voltaiccell consistsofanode(oxidation)andcathode(reduction)half-cells,connectedbyawireto conductelectronsandasaltbridgetomaintainchargeneutralityasthecelloperates. Theredoxreactions(oxidationandreduction)occurspontaneouslyandgenerateelectric flow. Inthereactionsofthecell,electronsmovefrom anodetocathode,whilecationsmove from thesaltbridgeintothecathodehalf-cellandanionsmovefrom thesaltbridgeinto theanodehalf-cell.Oxidationwilloccuratanodewhilereductionwilloccuratcathode. Thepurposeofavoltaiccellistoconvertthefreeenergychangeofaspontaneous reactionintothekineticenergyofelectronsmovingthroughanexternalcircuit(electrical energy).Thiselectricalenergyisproportionaltothedifferenceinelectricalpotential betweenthetwoelectrodes,whichiscalledthecellpotential(Ecell),alsothevoltageof thecellortheelectromotiveforce(emf). ThemorepositiveEcellis,thespontaneousthereactionoccurswhileanegativecell potentialindicatesthenonspontaneouscellreaction.IfcellEcell=0,thereactionisat equilibrium. Reduction potentialthatis obtained from a standard condition where solution 3

concentrationis0.1M,gaspartialpressureis1atm andtemperatureis25Cisknownas standardreductionpotential. Thesestandardreductionpotentialvaluesarearrangedinacertainorderandthislistis knownastheStandardReductionPotentialTable(Appendix1).Inthistable,species withmorepositivevaluesaremoreeasilyreducedwhilespecieswithmorenegative valueswillundergooxidation. Cellpotentialatconditionsthatarenotstandardisgivenbythisequation: Ecell=Ecathode-Eanode@ Ecell=Ereduction-Eoxidation (1) where Ecathodeand Eanodearetheelectrodepotentialsorhalfcellpotentials. At25CtheelectrodepotentialisgivenbytheNernstequation: Ecell=Ecell– 0.0592 logQ (2) n where, Ecell = reductionpotentialofelectrode Ecell = standardreductionpotentialofelectrode n = no.ofmolesofelectroninvolvedinthehalfreaction TheQintheNernstequationaboveindicatesthereactionsquotientforareaction: Q= [product]x (3) [reactant]y wherethexandyisthestoichiometriccoefficients. ExpressionofQcontainsonlythosespecieswithconcentrations(and/orpressures)that canvary;thussolidsdonotappear,evenwhentheyaretheelectrodes. Forexample,theQ inthereactionbetweencadmium andsilverion,theCdandAg electrodesdonotappearintheexpressionforQ: Cd(s)+2Ag+(aq)→ Cd2+(aq)+Ag(s), Q=[Cd2+] (4) [Ag+]2 The above Nernstequation shows clearly thatelectrode potentialvalues and consecutivecellpotentialvaluesdependontheconcentrationofionsinvolved. Experimentalaspectsthathavetobeobserved: 1. Rinseelectrodewithdistilledwatereverytimebeforeuse. 2. Cleanelectrodewithsandpapereverytimebeforeusetogetridofrustanddirt. 3. Ensurethatthesaltbridgeisstillingoodcondition. 4. Ensurethattheopenpartofthesaltbridgeisfilledwiththeelectrolytesolution, i.e.therearenoairbubbleswithin. Apparatus ChemicalReagents 4

Voltmeter 0.1M CuSO4 saltbridge 0.1M ZnSO4 sandpaper zincelectrode pipettefiller copperelectrode crocodileclips 50mLbeaker 25mLpipette 25mLvolumetricflask 50mLvolumetricflask 5mLgraduatedpipette 1mLgraduatedpipette ApparatusSetup Zn SaltBridge Cu (s) (s) ZnSO4 CuSO4 (aq) (aq) Figure1:Theapparatussetupfordeterminingtheelectrodepotentialsofzinc. Method A. MeasurementofCellPotentialbetween0.1M ZnSO4and0.1M CuSO4 1.Pipette 25 mL 0.1 M ZnSO4 solution and 0.1 M CuSO4 solution previously preparedintotwo separate50mLbeakers. (Usetheappropriatepipettefillertopipettethesolutions.DO NOTEVER use yourmouthasthepipettefiller!) 2. ConnectbothsolutionswithasaltbridgeasseeninFigure1forapparatussetup. 3.Byusinga2Vrangefrom thevoltmeter,recordthecellpotential,Ecell(uptotwo decimalpoints). Note: Donotdiscardthe0.1M CuSO4solution.ItcanbereusedinpartBandC. Result:Ecell= B. MeasurementofCellPotentialbetween0.01M ZnSO4and0.1M CuSO4. 5

1.Preparationof0.01M ZnSO4solution. i. Byusingagraduatedpipette,pipetteanascertainedvolume(needstobe calculated)of0.1M ZnSO4intoa25mLvolumetricflask. ii. Adddistilledwateruntilthemark,coverthemouthofthetesttubewitha stopperandshaketheflaskuntilyougetahomogeneoussolution. 2.Pourallthe0.01M ZnSO4solutionintoa50mLbeaker. 3.SetuptheapparatusasinPartAbyreplacingthe0.1M ZnSO4withthe0.01M ZnSO4solution. 4. RecordthecellpotentialvalueasinPartA. Results: 1.Calculatethevolumeof0.1M ZnSO4shouldbeaddedtoprepare0.01M ZnSO4. 2. Ecell= C. MeasurementofCellPotentialbetween0.001M ZnSO4and0.1M CuSO4 1.Preparationof0.001M ZnSO4solution. i. Byusingagraduatedpipette,pipetteanascertainedvolume(needstobe calculated)of0.1M ZnSO4intoa25mLvolumetricflask. ii. Adddistilledwateruntilthemark,closethetesttubewithastopperand shaketheflaskuntilyougetahomogeneoussolution. 2.Pourallthe0.001M ZnSO4intoa50mLbeaker. 3.SetuptheapparatusasinPartAbyreplacingthe0.01M ZnSO4withthe0.001M ZnSO4solution. 4. RecordthecellpotentialvalueasinPartA. Results: 1.Calculatethevolumeof0.1M ZnSO4shouldbeaddedtoprepare0.001M ZnSO4. 2. Ecell= CALCULATION: Writethehalfcellreactionequations Atanode : Atcathode: Writethefullcellreactionequations. Fillinthedatatablebelowandshowallthecalculationsforeveryconcentrationofzinc solution. Concentrationof CellPotential,Ecell. EZn2+/Zn ZnSO4 (ExperimentalValue) Experiment NernstEquation 0.10M 0.010M 0.0010M Calculationofhalfcellpotentials,EZn2+/Znfrom: a)experiment:Ecell= ECu2+/Cu-EZn2+/Znwhere ECu2+/Cu=+0.34V b) Nernstequation: 6

DISCUSSION: a. Basedonyourresults,statetheeffectofelectrolyteconcentrationchangesoncell potentialvaluesandvaluesofelectrodepotentialEZn2+/Zn i.e.whetherthevalues havebecomemorepositiveormorenegativewhentheconcentrationisdecreased. b.Explainthetrendbycomparingthevaluesofhalfcellpotentialsobtainedfrom the experimentandalsofrom theNernstequation. c. State4factorsthatcancontributetotheexperimentalerrors. CONCLUSION: 1. EXPERIMENT3 REACTIONRATES Objective Toostudytheeffectofconcentration,temperatureandcatalystontherateofreaction. Introduction Chemicalkineticsistheareaofchemistryconcernedwiththespeedsorratesatwhicha chemicalreactionoccurs.Therateofareactionisthechangeinconcentrationofthe reactantsorproductsperunittime. Amongthefactorswhichinfluencetherateofa reactionaretheconcentrationofthereactants,temperatureandcatalyst. Therateofareactioncanbestudiedbyobservingthechangeinthechemicalproperties orthechangeinphysicalpropertiesofspeciesinvolvedinthereaction.Therateofa reactionaffectsthetimefactor,i.e.thefasterthereactionoccurs,theshorterthetimefor thereactiontocomplete. Therelationshipbetweentheconcentrationofthechosenreactantsandtimecanbe usedtomeasuretherateofreaction.Therateofreactioncanalsobeinfluencedby changesintemperatureandthepresenceofacatalyst. Weknowthatanyreactioncanberepresentedbythegeneralequation reactants→ products (1) Thisequationstellsusthatduringthecourseofareaction,reactantsareconsumed whileproductsareformed.Tomeasureareactionrate,weusuallyobserveeithera productorareactantforitschange.Anyphysicalcharacteristicrelatedtothequantityor concentrationofaproductorreactantcanbemonitored. Ingeneral,itismoreconvenienttoexpressthereactionrateintermsofthechangein concentrationwithtime.Thuswecanexpresstherateas rate=-Δ[reactants]=Δ[products] (2) 7

Δt Δt whereΔ[reactants]andΔ[products]arethechangesinconcentration(molarity)overa timeperiodΔt. Someofthecharacteristicstobemonitoredarechangeinpressure,changeincolour (spectroscopicmeasurement),temperatureforexothermicorendothermicreactionand presenceofcertainkeysubstance. Thechangecanbeplottedonagraph,andfrom thegraph,wecangettheaveragerate ortheinstantaneousratebyeithergraphicalmethodorusingcomputerforthedata analysis. Apparatus ChemicalReagents boilingtubes 0.10M HCl steam bath 10% MnSO4 stop-watch 0.2M KMnO4 thermometer 2.00M H2SO4 Bunsenburner 0.10M Na2S2O3 10mLpipette 0.25M H2C2O4 50mLburette 100/250mLconicalflask glassrod Method A Determinationoftheeffectofconcentrationontherateofreaction 1.Place50mL of0.10M sodium thiosulphate,Na2S2O3 intoaconicalflask.Put theconicalflaskonapieceofwhitepaperonwhichacrosshasbeendrawn. Drawthecrossusingpencilonly. 2.Add10mLof0.10M HClintotheconicalflaskandstartthestop-watch.Shake theErlenmeyerflaskgentlyuntilthecrossonthepaperisnotvisible. 3. Notethetimerequiredforthecrosstodisappear. 4.Repeatsteps1-3withtheadditionofdistilledwatertothesodium thiosulphateas instructedinTable3.1. Table3.1:Concentrationofreactants Volumeof Volumeof Concentration Volumeof Timeforcross 0.1M Na2S2O3 distilled of 0.10M HCl todisappear water Na2S2O3 solution(mL) solution added(mL) (seconds) (mL) 50.00 0.00 10.00 40.00 10.00 10.00 30.00 20.00 10.00 20.00 30.00 10.00 10.00 40.00 10.00 B Determinationoftheeffectoftemperatureandcatalystontherateof 8

reaction 1.Place10mLof0.25M oxalicacid,H2C2O4solutionintotwoboilingtubeslabeled A1andA2. 2.FillboilingtubelabeledasB1with5mLof0.2M KMnO4solutionand10mLof 2.0M H2SO4solution.Stirwithaglassrod. 3.FillanotherboilingtubelabeledasB2with5mL0.2M KMnO4and10mL2.0M H2SO4 solutionsasinboilingtubeB1,andthenadd5dropsof10% MnSO4 solution. 4. PlaceboilingtubesA1,B1,A2andB2inahotwaterbathwithpresettemperature. 5.i. MixthesolutionsinboilingtubesA1andB1. Notethetimetakenforthemixturetobecomecolourless. ii. MixthesolutionsinboilingtubesA2andB2. Notethetimetakenforthemixturetobecomecolourless. 6. Repeatsteps1-5fortheothertemperaturesasinTable3.2. Table3.2:Effectoftemperatureandcatalystonreactionrate Temperature Timetakenforthesolutiontobecomecolourless(seconds) (oC) WithoutcatalystMnSO4 WithcatalystMnSO4 30 (A1+B1) (A2+B2) 40 50 9

CALCULATION: A Effectofconcentrationontherateofreaction. ConcentrationofNa2S2O3afterdilutionand1/tvalues Volumeof Concentrationof 10-3(s-1) 0.10M Na2S2O3 (mL) Na2S2O3(M) 50.00 40.00 30.00 20.00 10.00 ShowanexampleofNa2S2O3(aq)concentrationdilutioncalculation. 1.Plotalineargraph(usinggraphpaper)oftheconcentrationofNa2S2O3(aq) after thedilutionagainstthevalueof 2.DeterminetherelationshipbetweentheconcentrationofNa2S2O3(aq) withtime andtherateofreaction. B Effectoftemperatureandcatalystontherateofreaction Temperature 10–2 (s–1) (C) 30 WithoutcatalystMnSO4 WithcatalystMnSO4 40 (A1+B1) (A2+B2) 50 1. Plotalineargraph(usinggraphpaper)of againstthetemperatureofthe reactionsforthemixturesofA1+B1solutionsandA2+B2onthesamegraph. 2. Basedonyourgraph,deducetherelationshipbetween i. temperatureandtherateofreaction ii. catalystandtherateofreaction. DISCUSSION: 1. WritetheequationsforthereactionsthatoccurinPartsAandB. 2. WhatisthefunctionofthecatalystusedinPartB? 3.State 2 possible errors thatmay occurwhile you are conducting the above experiment. 4.Giveconclusionfrom thegraphsthatyouhaveplotted. 10

EXPERIMENT4 REACTIONSOFALIPHATICANDAROMATICHYDROCARBONS Objective Tocarryoutchemicaltesttodifferentiateanalkanefrom analkeneandanarene. Introduction Hydrocarbonsarethesimplestofallorganiccompounds,containingonlycarbonand hydrogen.Hydrocarbonsareclassifiedintotwogroups: a) Saturatedhydrocarbonsmoleculesaremadeentirelyofcarbon-carbonsinglebonds. Saturatedhydrocarbonscontainonlysinglecovalentbonds. b) Unsaturatedhydrocarbonsmoleculescontainatleastonecarbon-carbondoubleor triplebond.Unsaturatedhydrocarbonscontainmultiplebonds. A hydrocarbonthatcontainsonlysinglebondsisanalkane(generalformulaCnH2n+2, wherenisapositiveinteger).Inalkanes,eachC issp3 hybridised.BecauseeachC is bondedtothemaximum numberofotheratoms(C orH),alkanesarereferredtoas saturatedhydrocarbons. Forexample: Hexane Cyclohexane Thelowreactivityofalkanestowardmanyreagentsexplainswhyalkaneswereoriginally calledparaffins.Alkanesundergofreeradicalsubstitutionreaction.Theyareunreactive towardspolarorionicreagentsbutcanreactwithnon-polarreagentssuchasoxygen andbromine. Alkanesreactwithhalogensuchasbrominetoproducehaloalkanesinthepresenceof lightortemperaturegreaterthan100oC.Buttheexperimentcannotbedoneinlaboratory becausebrominehascorrosivepropertiesandtheodourisverydangeroustobeinhale byhuman. uv R–H + X2 R–X + HX (1) AhydrocarbonthatcontainsatleastoneC=Cbondiscalledanalkene(generalformula CnH2n). Thedouble-bondedC atomsaresp2 hybridised.Becausetheircarbonatoms aretofewerthanthemaximum offouratomseach,alkenesareconsideredunsaturated hydrocarbons. Forexample: 11

1-pentene 2-pentene Cyclopentene Alkenesaremorereactivecomparedtoalkanes.Alkaneshavecarbon-carbonsingle bonds(σbonds)whilealkeneshavecarbon-carbondoublebonds(πbonds).Thedouble bondisasiteofhighelectrondensity(nucleophilic).Therefore,mostalkenesreactions areelectrophilicadditionsreactions. Forexample,alkenesundergohydrogenationreactiontoform alkanes.Theyalsoreact withhalogenslikechlorineandbrominetoform dihalides. Alkenes undergo a numberofreactions in which the C=C is oxidised potassium permanganateasfollows: a) Basic(OH-),cold,dilute ThisreactioniscalledBaeyer’stest.Itcanserveasatestforthepresenceofcarbon- carbondoublebondswherethepurplecolouroftheKMnO4 decolourised,andbrown precipitateofMnO2 isformed. Example: 12

b) Acidic(H+),hot,concentrated Thisreactionisusedtoprepareketone,whilealdehydeformedwillundergoesfurther oxidationprocesstoform carboxylicacid.Itcanserveasatestforthepresenceof carbon-carbondoublebondswherethepurplecolouroftheKMnO4decolourised. Example: Unlikethecycloalkanes,aromatichydrocarbonsareplanarmolecules,usuallywithone ormoreringsofsixC atoms,andareoftendrawnwithalternatingsingleanddouble bonds.Arenesarearomatichydrocarbonswithstablemolecularstructures.Theyhavea veryhighdegreeofunsaturationandarerelativelyinerttowardsalladditionreactions. Example: CH3 Benzene Toluene Naphtalene Anthrance Areneswillonlyundergoelectrophilicsubstitutionreactionsinthepresenceofcatalyst. Theymayundergoadditionreactiononlyinverystrongcondition. 13

Apparatus ChemicalReagents evaporatingdish cyclohexane woodensplinter cyclohexene dropper toluene testtubes basicKMnO4 tile/A4whitepaper Method A Combustiontest 1.Place a few dropsofcyclohexane in an evaporating dish.Ignite a wooden splinterandplaceitatthecyclohexaneintheevaporatingdish.Observethe colouroftheflame.Recordyourobservation. 2. Repeattheabovetestwithcyclohexene. Note:This testis notcarried outwith toluene because ofits carcinogenic properties. B Oxidationtestwithbasic,coldKMnO4 1. Labelthreecleananddrytesttubes. 2. Put2mLcyclohexane,cyclohexeneandtolueneintoeachtesttuberespectively. 3. Add10fewdropsofbasicKMnO4intoeachtesttube. 4. Recordtheinitialcolourandthefinalcolourasyourobservation. 14

RESULTS: Observation ChemicalEquation A CombustionTest Observation ChemicalEquation Compound Cyclohexane Compound Cyclohexene B BromineTest Compound Test Observation ChemicalEquation Tube ChemicalEquation Cyclohexane A(dark) B(uv) Cyclohexene C(dark) D(uv) Compound Test Observation Toluene Tube E(dark) F(uv) C OxidationTestwithKMnO4 Test Compound Observation ChemicalEquation Tube 1 Cyclohexane 2 Cyclohexene 3 Toluene DISCUSSION: 1. Givethemechanism forthereactionofcyclohexanewithbromine. 2. Explainthefunctionofsunlightinthebrominetest. CONCLUSION: 15