10Introduction toElectrochemistry
Objectives
The objectives of this laboratory are to:
•Understand what is meant by oxidation, reduction, voltaic cells, and electrolytic cells.
•Developanelectrochemicalseriesbasedonpotentialdifferencesbetweenhalf-cells.
•Develop a working understanding of the Nernstequation.
•Investigate the chemical reactions occurring during the electrolysis ofwater.
Background
Electrochemistryisabranchofchemistrythataffectsyoueveryday.Batterieswhich startyourcar,runcalculatorsandwristwatches,andsupplyemergencypoweralloper- ateonelectrochemicalprinciples.Corrosionofpipes,buildings,bridges,boats,cars, andplanescanbepreventedwithknowledgeofbasicelectrochemicalprinciples.Nerve impulsesaretheresultofelectrochemicalreactionsinourbody.Mostgeneralchemistry textbookshaveachapteronelectrochemistry.Itwouldbeagoodideatoreviewthis chapter in your book before performing thislaboratory.
Galvanic Cells and the Electrochemical Series
DuringPartIofthisexperiment,youwillbebuildinggalvaniccells(alsoknownasvoltaic cells).Galvaniccellsactasbatteries,takingadvantageofthefactthatsubstancescan spontaneouslyloseorgainelectrons.Theresultofelectronsbeingtransferredbetween substancesiselectricity.Oneofthefirstcommercialbatteries,knownasaDaniellcell, wasdevelopedin1836.TheDaniellcelllookedsimilartoFigure1,butusedacopper rather than a leadelectrode.
Althoughthefiguremaylookcomplicated,itisreallyfairlysimple.Eachcompartment, orside,ofthegalvaniccellisknownasahalf-cell.Themetalstripsintheirrespective solutionsareknownaselectrodes.Theionicsolutionjunctionbetweenthetwohalf-cells iscalledasaltbridge.Thegalvaniccellmakesuseofthefactthatdifferentsubstances(in thiscase,zincandlead)havedifferenttendenciestogainandloseelectrons.Thelead electrodewantstogainelectronsmorethanthezincelectrodedoes.Anotherwayof statingthisisthatthezincelectrodewantstoloseelectronsmorethantheleadelectrode. Inthisgalvaniccell,theleadisreduced,orgainselectronsandthezincisoxidized,or loseselectrons.Let’slookattheconceptofoxidationandreductionabitmoreclosely.
Acknowledgement: Several experimental techniques were adapted from S. Novicki, “Electrochem Lab I,” U.S. Air Force Academy, Spring 1988.
Figure 1. A zinc/lead galvanic cell.
Thezincelectrodeislosingelectrons,whichareflowingtotheleadelectrode.Since theelectronsareflowingawayfromthezincelectrode,thezincisconsideredtobethe negativepoleandisknownastheanode.Theelectronsareacceptedattheleadelec- trode,makingtheleadthepositivepole(knownasthecathode).Sincethezincelectrode islosingelectrons,thezincmetalisbecomingzincionsandgoingintosolution.This can be represented by the followingequation:
Eq1.Zn0(s) → Zn21(aq)12e–oxidation
Theabovereactioniscalledanoxidationhalfreaction.Conversely,intheotherhalf-cell, theleadisgainingelectrons,withleadcomingoutofsolutiontoformleadmetal.This isknownasareductionhalfreactionandcanberepresentedbyEquation2:
Eq2.Pb21(aq) 1 2e–→Pb0(s)reduction
Thus,thezincelectrodewilleventuallydissolveasitisconvertedtozincions,while theleadinsolutionwillplateoutasmetallicleadontotheleadelectrode.Equation3, thecombinationofthetwohalfreactions,resultsintheflowofelectronsfromthezinc electrode to the lead electrode ... abattery!
Eq3.Zn0(s) 1 Pb21(aq) → Zn21(aq) 1Pb0(s)
ThesaltbridgeshowninFigure1allowselectricalcontactbetweenthetwosolutions inthehalf-cells.Thesaltbridgealsopreventsmixingofelectrodesolutionsanden- sureselectricallyneutralsolutionsbyprovidingabalancingflowofpositiveandnega- tive ions.
Theforcepushingtheelectronawayfromthezincelectrodeandtheattractiveforce exertedontheelectronasitapproachestheleadelectrodeprovideanexcellentmeasure ofthetendencyforoxidationandreductiontotakeplaceinthisreaction.Inthecase ofzincandlead,thesumoftheforcesmovingtheelectronfromthezinctotheleadis theoreticallyabout0.63V.Theexactvoltageofsuchacelldependsuponthematerials usedfortheelectrodes,theconcentrationofthesolutions,andthetemperature.Your textbook discusses the theoretical prediction of cellvoltages.
InPartIofthisexperiment,youwilltryothermetal/ionhalf-cellcombinationswith thePb0/Pb21(aq)half-cell.Fromthisdata,youwilldevelopatablelistingthevarious elementsandionsinorderoftheirtendencytogainorloseelectrons.
The Nernst Equation
Theoreticalpredictionsoftendencytogainelectronsareusedtopredictthevoltage difference between two electrodes. The voltage difference between electrodes, the cell voltage,isalsocalledtheelectromotiveforce,oremf(EorEcell).Understandard-state conditions(25°C,1Msolutionconcentration,1atmpressureforgases),thesetheoreti- callypredictedvoltagesareknownasstandardemfs(E°orE°cell).
Inreality,standard-stateconditionsareoftendifficultifnotimpossibletoobtain.The Nernstequationallowscellvoltagestobepredictedwhentheconditionsarenotstandard. WalterNernstdevelopedthefollowingequationinthelate1800swhilestudyingthe thermodynamics of electrolytesolutions:
Eq.4Ecell 5 E°cell – (2.303 RT logQ)/nF
InEquation4,Risthegasconstant(8.314Jmole-1K-1),Tisthetemperature(Kelvins), FisFaraday’sconstant(96,485coulombs/mole),nisthenumberofelectronstrans- ferredinthebalancedoxidation/reductionreaction,andQisthereactionquotient,or ([products]/[reactants]). This expression for Q is not completely general, and you will seeitwrittenalittledifferentlyinlatercourses.Ifthereactionsarecarriedoutatroom temperature (25°C), the Nernst equationbecomes
Eq.5Ecell 5 E°cell – (0.0591 logQ)/n
Note: in Equations 4 and 5, if the reaction quotient is equal to 1, Ecell 5 E°cell.
InPartIIofthisexperiment,youwillmeasurevoltagesatvarioussolutionconcentra- tions for the copper/zinc galvanic cell and compare your results with thosecalculated using the Nernstequation.
The Electrolysis of Water
Electrolyticcellscanbethoughtofasbeingtheoppositeofgalvaniccells.Inanelectro- lyticcell,orelectrolysiscell,electricalenergyisusedtoforceachemicalreactiontooccur which,undernormalconditions,wouldnotoccurspontaneously.Galvaniccellsareoften usedtopowerelectrolysisreactions.Theprocessofelectrolysisisveryimportantinthe
plating,purifying,andrefiningofmetals.Perhapsthebestknownexampleofpractical electrolysisisintheproductionofaluminumfromitsore,bauxite.Theproduction ofaluminumviaelectrolysisisextremelyenergyintensive.Themajorcostsavingsin aluminumrecyclingisnotinthemetalsaved,butintheelectricalenergyrequiredto produce it from the ore.
Inthenot-so-distantfuture,oilwillbecomeanincreasinglyscarcecommodity,amplify- ingtheneedtofindreplacementsforgasoline(aportablefuel)topowercars,planes, andothermeansoftransportation.Hydrogenisseenasapossiblealternativetooil.Currently,theonlybiguseofhydrogenasafuelisintherocketindustry,wherehy- drogenfuelsthemainenginesofthespaceshuttle.Thelargequantitiesofhydrogen neededforthespaceshuttleareproducedfromnaturalgas(whichisalsoadepleting resource).However,themuchlargerquantitiesrequiredtopoweranationcouldonly beproducedpracticallybyelectrolysis.This,ofcourse,wouldrequirearelativelycheap sourceofelectricity,presumablysolarornuclearpower.(Itshouldbereadilyapparent fromeventhisshortparagraphthattherearenoeasyanswerstoenergyproblems!)
Toelectrolyzewater,thewatermustbemoreconductivethanpurewater(remember, the [H3O1] and [OH–] of pure water are only 1 3 10–7 M). To make water more con- ductiveforelectrolysis,asaltoracidisusuallyadded.Forthisexperiment,youwilluse asolutionof0.10MKNO3torepresent“conductivewater.”A9-voltbatterywillbe usedtoprovidetheelectricalpower.Graphiteelectrodesattachedtothebatteryleads
willprovideanonreactivesurfacefortheelectrolysisreactions.AccordingtoWhitten etal.,thefollowingreactionswilloccurduringtheelectrolysisofwater:
Eq. 6 / cathode / 2 3 (2 H2O 1 2e– → H2(g) 1 2 OH–) / (reduction)Eq. 7 / anode / 2 H2O → O2(g) 1 4 H1 1 4e– / (oxidation)
The overall reaction (Equation 6 1 Equation 7) is as follows:
Eq.86H2O→2H2(g)1O2(g)14H114OH–
giving the net reaction of
Eq.92 H2O → 2 H2(g) 1O2(g)
Thus,thenetreactionoftheelectrolysisofwaterishydrogenandoxygengas.Hydrogen isproducedatthecathode,withthesolutionbecomingbasicaroundthatelectrode. Oxygenisproducedattheanode,withthesolutionbecomingacidicaroundthatelec- trode.TheK1andNO3–fromthepotassiumnitratearemuchlessreadilyreducedand oxidizedthantheH2Oandthus,takenopartinthereaction.TheKNO3servesonly to conduct current through thesolution.
InPartIIIofthisexperiment,youwillobservetheelectrolysisofwaterandtheacid/base properties of the solution around theelectrodes.
Safety Precautions
Wear eye protection at all times and immediately clean up all spills!
Materials
Equipment
•MicroLAB Voltage Probe, wire leads with Cu wire probes, 9-volt battery with con- nectingwiresandgraphite(pencillead)electrodes(checkoutfromstockroom).Do not let the battery wires or electrodes touch eachother!
•Plastic document protector, envelope containing metal pieces and sandpaper (sup- plied in lab)
•50 mL beaker (from yourlocker)
Supplies
•Multi-EChem half-cellmodule
Reagents
•0.10 M Pb(NO3)2, 0.10 M Ni(NO3)2, and 0.1 MKNO3
•0.00010 M, 0.0010 M, 0.010 M, 0.10 M, and 1.0 MCu(NO3)2
•0.10 M and 1.0 MZn(NO3)2
•Yamadaindicator
Experimental Procedures
Part I. Galvanic Cells and the Electrochemical Series
Asimpleandeasy-to-useelectrochemicalcellcanbebuiltwiththeMicroLABMulti- EChem Half-cell Module, illustrated in Figures 2 and3.
Thisunithaseightsmallwells,eachconnectedtoacentralsaltbridgethroughasmall channel.Thesaltbridgeismadeofanaqueoussolutionofpotassiumnitrate,andcon- tactsthehalf-cellsthroughawater-soakedporouscylinder.Ionscanmovethroughthis porousbarrierwithoutallowingmuchmixingofthesolutions.Thecenterofthecylinder isfilledwithpotassiumnitratetokeepthecylinderwetandsoakedwithmobileions.
Differentmetal/ionhalf-cellpairscanbesetupineachoftheouterwells,andthevolt- ageproducedbythetwohalf-cellsmeasuredwithavoltmeterasillustratedinFigures 2 and3.
Voltmeter
Figure2.Onecanexperimentallydeveloptheelectrochemicalseriesbycomparingaseriesofmetal/ion pair half-cells against one “reference”metal/ion half-cell.
Figure3.TheMulti-EChemhalf-cellmodulecanholdeightdifferentmetal/ionhalf-cells.Ifyouuse oneasareference,youcanusethesignandmagnitudeofthevoltageobservedtoordertheelementsin termsoftheirabilitytogainorloseelectrons,ascomparedtothereferencemetal/metalionhalf-cell.
SetupyourMicroLABtomeasurevoltage.SelecttheVoltagemeasurement,thedual bananajackvoltageinput,the2.5voltrange,andacceptthefactorycalibration.
Eachofthewellswillholdadifferention/metalhalf-cell.Forexample,inthisfigure, theleftcellhasacopperIInitratesolutionandacopperwire.
YouhaveblackandredalligatorclipsconnectingtotheMicroLABvoltageinput.If yougetapositivevoltagereading,electronsarerunningfromthehalf-cellmoduleinto theblackalligatorlead,ontotheMicroLABvoltmeter,andbackouttheredleadto the solution.
Inthisexample(Figures2and3),thevoltagereadspositive,indicatingthatthezinc/zinc ionhalf-cellontherightisproducingelectrons.Thereaction(seeFigure2)isgoingto theleft.Itisanoxidationreaction.Zn2++2electrons←Zn0.Thecopper/copperion half-cellontheleftinFigures2and3isacceptingelectrons.Itisareductionreaction. This reaction is going to the right: Cu2+ + 2 electrons →Cu0.
Byconvention,oxidation–reductionreactionsarealwayswrittenfromlefttorightasreduction reactions.Alongerarrowcanbeusedtoindicatethepredominantdirectionofthereaction.
Half-Cells and Ion Path
Thelowerchannelfromeachhalf-cellwelltothecentralsaltbridgeareaprovidesa non-drying,lowresistanceaqueouspathforionsuptotheporoussaltbridgemembrane. TheporousmembraneisconditionedbeforeusebysoakingtheunitwithKNO3solu- tion.Thisfillstheporesinthemembraneandpreventssolutionsfromleakingfrom onehalf-celltoanother.Theaqueoussaltbridgeinsidethecircularporousmembrane contacts all half-cellsequally.
Half-Cell Overflow Volume
Themilledoverflowvolumeaboveeachchanneltothesaltbridgeistheretocatch overflowasyoufillthehalf-cellwell.Itpreventscross-contaminationofhalf-cellsolu- tionsduringfilling.Ifyouuseaplasticpipettetodelivertheionsolutiontothewell,it is easy to stop filling when the lower channel isfull.
Experimental Development of the Electrochemical Series
StartingwiththePb/Cucombination,measurethevoltagesproducedbythefilterpa- pergalvaniccells.Usingthevoltageprobe,touchthecopperwirestothemetalstrips oneithersideofthesamefilterpaper,asshowninFigure2.Donotallowtheprobesor alligatorclipstocomeincontactwiththesolutions.Positiontheprobessothatyoureada positivevoltageforthePb/CusystemandnotewhichcolorprobeisonthePb.The readingsmayfluctuatesomewhat;ifso,selectanaveragevaluetorecord.Youmustpress hardonthecopperwireprobestogetagoodcontactwiththemetalpieces.
MeasurethevoltagesfromthePb/ZnsystemandthePb/Nisystem,alwaysusing the same color probe on the Pb as you did for the Pb/Cu combination. Record thesevoltagesontheDataSummarysheetattheendofthischapter.Makesureyou indicatetherelativepositionofthereductionreactionsyouobservedwithrespectto theleadhalfreaction.Yourtablemayresemblethefollowing,withthereactionfor yourmostpositivevoltageatthetop,andthereactionforyourleastpositive(ormost negative) voltage on thebottom.
Example electrochemical series table
Reaction / Cell Voltage Compared toPb0/Pb21 Half-cell
Cu21 1 2e– → Cu0 / ?
Zn21 1 2e– → Zn0 / ?
Pb21 1 2e– → Pb0 / 0.00 mV
Ni21 1 2e– → Ni0 / ?
Note: This is not necessarily the order you will have; this is simply an example.
Accordingtoyourtable,whichhalf-cellreactionhasthegreatesttendencytoward reduction;thatis,togainelectrons?Whichhalf-cellhasthegreatesttendencytoward oxidation?Basedonyourelectrochemicalseriestable,whatwouldyoupredictforthevoltageofacopper/zinccell?Buildacopper/zinccellandmeasurethevoltage.How does this compare with yourprediction?
Part II. The Nernst Equation
Inthispartoftheexperiment,youwillbeexaminingtheeffectofsolutionconcentra- tions on the cell voltage for thereaction
Eq.10Cu21(aq)1Zn0(s)→Cu0(s)1Zn21(aq)
TheNernstequationallowsyoutocalculateEcellasafunctionoftheconcentrations ofreactantandproduct.Fortheabovereactionat25°C,theNernstequationcanbe rewritten as Equation 11.
Eq.11Ecell 5 E°cell – (0.0591 log ([Zn21] /[Cu21]))/2
Remember,solidsandpureliquidsarenotincludedintheQexpression.Theoretically, E°cellfortheabovereactionis1.10V.Thus,avalueforEcellcanbecalculated,knowing [Zn21] and[Cu21].
Setupfivezinc/coppercellsusingthefollowingZn21andCu21solutionconcentrations. ThecellswilllookliketheoneshowninFigure2,exceptyou’llusecopperinsteadof lead.
Cell # / [Cu21] / [Zn21]1 / 1.0 / 1.0
2 / 0.10 / 1.0
3 / 0.010 / 1.0
4 / 0.0010 / 1.0
5 / 0.00010 / 1.0
Measurethevoltage(Ecell)fromeachoftheabovehalf-cellcombinations.Recordthis voltageontheDataSummarysheetattheendofthisexperiment.AssumingE°cell51.10 Vandthetemperatureoftheroomyouareworkinginis25°C,calculateEcellforeach oftheabovehalf-cellcombinations.Recordthiscalculatedvoltageonthedatasheet.
HowdoyourmeasuredandcalculatedvaluesforEcellcompare?Iftheydiffersignifi- cantly,canyouofferanyexplanationforthedifferences?Payattentiontotheunitsyouare comparing.
Part III. The Electrolysis of Water
Youwillbuildanelectrolyticcellusinga9-voltbatteryattachedtotwographite(pencil lead)electrodes.Donotletthebatterywiresorelectrodestoucheachother.PlaceseveralmL of0.1MKNO3ina50mLbeaker,andaddabout8dropsofYamadaindicator.Then, placethegraphiteelectrodes,connectedtothebattery,inthesolution.Donotallow thealligatorclipstotouchthesolutions;onlythegraphiteelectrodesshouldtouchthesolution. Figure4showstheproperlyconfiguredcellfortheelectrolysisofwater.
Graphite electrode
9-volt battery
0.10 M KNO3
©Hayden-McNeil, LLC
Figure 4. Cell configuration for the electrolysis of water.
Youwillobservegasbeingevolvedateachelectrode.Youwillalsonoticethatthecolor oftheYamadaindicatorisdifferentaroundeachelectrode.Inacidsolution,Yamada givesanorange-to-redcolor;inbase,thecolorispurpleorblue.Basedontherelative amountofgasbeingproducedateachelectrodeandthecoloroftheYamadaaround eachelectrode,whichelectrodeistheanodeandwhichelectrodeisthecathode?(Hint: Yourobservationsshouldsupportthehalfreactionwhichyoubelieveisoccurringat each electrode. Refer to Equations 6 and7.)
Note:PartsIVandVdonotrequireanyadditionallaboratorymeasurements.Thesepartsuse datayouhavealreadyobtainedandsimplyrequireadditionalcalculations.
Part IV. Comparison of Calculated and Measured EMFs
Usingatableofstandardreductionpotentials(e.g.,fromyourtextbook)andtheNernst equation,calculateEcellforthethreegalvaniccellsyoubuiltinPartI.Compareyour calculatedvaluesofEcellwiththosemeasuredinPartI.Ifthevaluesaresignificantly different,canyouofferanexplanationforwhytheyaredifferent?
Note: E° for Fe31/Fe is –0.036 V.
Part V. More Exercises Using the Nernst Equation
WiththevaluesyoucalculatedinPartII,usethespreadsheettomakeaplotofEcell(calc) versuslog([Zn21]/[Cu21]).Graphalinearregressionplotforthedata.Includeacopyof thisgraphwithyourreport.Whatistheslopeofthisline?Whatdoestheslopeofthe linerepresent(Equation11)?Whatshouldtheslopeofthelinebeequalto?Whatis yourvalueforthey-intercept?Whatisthesignificanceoftheyintercept;thatis,what doesthey-interceptrepresent?Whatshouldthey-interceptbeequalto?
Pre-Lab Questions
Make sure you can answer these before you enter the lab!
1.Giventhefollowinghalf-cellreactionsoccurringinagalvaniccell:
Zn0→Zn2112e–
Cu2112e–→Cu0
a.Whichhalfreactionistheoxidationhalfreaction?
b.Whichhalfreactionisthereductionhalfreaction?
c.Writetheoverallreactionwhichresultsfromthetwohalfreactionsabove.
2.What is the function of the salt bridge in a galvaniccell?
3.Usingyouranswerfrom1candtheNernstequation,calculatetheEcellat25°Cif [Zn21] 5 0.10 M and [Cu21] 5 1.0 M. The E°cell for the correct overall reaction in 1c is 1.10V.
10
Data Summary
Part I. Galvanic Cells and the Electrochemical Series
Cell Combination / Cell VoltagePb/Zn
Pb/Cu
Pb/Ni
Pb/Pb
Cell Combination / Cell Voltage Prediction / Cell Voltage Experimental
Cu/Zn
How well did your prediction match the experimental value?
If not, what is a possible explanation?
Electrochemical Series Table
Reaction / Cell Voltage Compared toPb21/Pb0 Half-cell
Part II. The Nernst Equation
[Cu21] / [Zn21] / [Zn21]/[Cu21] / log{[Zn21]/ [Cu21]} / Ecell(meas) / Ecell(calc)1.01 / 1.0
0.10 / 1.0
0.010 / 1.0
0.0010 / 1.0
0.00010 / 1.0