Electrochemistry and the Nernst Equation
LEARNING OBJECTIVES
The objectives of this experiment are to . . .
• construct galvanic cells and develop an electrochemical series based on potential differences
between
half-cells.
• understand the Nernst Equation.
BACKGROUND
Any chemical reaction involving the transfer of electrons from one substance to another is an
oxidation-reduction (redox) reaction. The substance losing electrons is oxidized and the
substance gaining
electrons is reduced. Let us consider the following redox reaction:
Zn(s)+Pb2+(aq)
Zn2+(aq)+Pb(s)
This redox reaction can be divided into an oxidation and a reduction half-reaction:
Zn (s)
Zn2+ (aq) + 2eoxidation half-reaction and
Pb2+ (aq) + 2 e-
Pb (s)
reduction half-reaction
Agalvaniccell(Figure1)isadeviceusedtoseparatearedoxreactionintoitstwo
componenthalf-reactionsinsuchawaythattheelectronsaretransferredthroughan
externalcircuitratherthanbydirectcontactoftheoxidizingagentandthereducingagent.
Thistransferofelectronsthroughanexternalcircuitiselectricity.Eachsideofthegalvanic
cellisknownasahalf-cell.Fortheredoxreactionabove,eachhalf-cellconsistsofan
electrode(themetalofthehalf-reaction)andasolutioncontainingthecorresponding
cationofthehalf-reaction.Theelectrodesofthehalf-cellsareconnectedbyawirealong
whichtheelectronsflow.Inthecell,oxidationtakesplaceatthezincelectrode,liberating
electronstotheexternalcircuit.Reductiontakesplaceattheleadelectrode,consuming
electronscomingfromtheexternalcircuit.
theelectrodeatwhichoxidationoccursiscalledanode.
theelectrodeatwhichreductionoccursiscalledthecathode.
Sinceoxidationreleaseselectronstotheelectrode,itisdesignatedthenegativeelectrodein
thegalvaniccell.Reductionremovestheelectronsfromthecathode;itisthepositive
electrode.Aszincatomsareoxidized,theexcesspositivecharge(Zn2+ions)accumulatesin
solutionaroundthezincanode.Likewise,excess3negativecharge(NO3-)accumulates
aroundtheleadcathodeasPb2+ionsareremovedfromsolutionofPb(NO3)2byreduction
toleadmetal.Theseexcesschargescreateanelectricfieldthatcausestheionstomigrate:
positiveions(cations)migratetowardthecathodeandnegativeions(anions)migrate
towardtheanode.Inordertomakethisflowofionsbetweenthetwohalf-cellspossible,
thecellsareconnectedbyaporousbarrier(orsaltbridge)throughwhichtheionsflow.The
barrierpreventsfreemixingofthetwosolutionsbutpermitslimitedmovementofionsso
thattheelectricalneutralityismaintainedineachhalf-cell.
Differentmetals,suchaszincandlead,havedifferenttendenciestooxidize;similarlytheir
ionshave
differenttendenciestoundergoreduction.Thecellpotentialofagalvaniccellisduetothe
differencein
tendenciesofthetwometalstooxidize(loseelectrons)ortheirionstoreduce(gain
electrons).Commonly,areductionpotential,whichisatendencytogainelectrons,isused
torepresenttherelativetendencyforagivenmetaliontoundergoreduction.
Thevoltagemeasuredinthecellistheresultofthetwohalf-reactions,andthemagnitude
ofthepotentialdependsontheconcentrationsoftheions,thetemperature,andpressure
ofgases.Whenallthe
concentrationsinthezinc/leadsystemare1molarandthetemperatureis25°C,thecell
voltageis0.63volts.Itwouldbeamonumentaltasktoassemblealistofallpossiblecells
andreporttheirvoltage.Insteadweusethepotentialofthehalf-reactions.Wecannot
measureanyhalf-cellpotentialdirectly,sowepickonehalfreaction,callitthestandard,
constructacell,measurethecellvoltageandreportthepotentialrelativetothestandard.
Thestandardthathasbeenchosenbyconventionis:
2 H+ (aq) + 2 eH2 (g) E ° = 0.00 V
Here the notation E ° is called the standard electrode potential and is the assigned potential of the
standard hydrogenelectrodewhentheconcentrationofH+is1Mandthepressureofthe
hydrogengasisoneatmosphere.Themeasuredcellvoltageusingthestandardhydrogen
electrodeisthereforethepotentialoftheotherhalfreaction.
Tablesofstandardhalf-reactionpotentialshavebeencomputed.Thereactionsby
conventionarewrittenasreductionsandhencethetablesarecalledtablesofstandard
reductionpotentials.AbriefexamplefollowsbelowinanexcerptfromaStandard
ReductionPotentialstable.
SomeStandardReductionPotentialsat25°C
Thegreaterthetendencyoftheiontogainelectronsandundergoreduction,theless
Half-reaction
Potential(volts)
negative(orthemorepositive)thereductionpotentialoftheion.Inthezinc/leadcell,the
Cu 2+ + 2 eleadhasagreatertendencytoundergoreductionthanthezinc.
Cu (s)
+0.34
2 H+ + 2 ePb2+ + 2 e-
2+
Zn + 2 e-
2+
Mg + 2 e-
Li+ + e-
H (g)
0.00
Pb (s)
- 0.13
Zn (s)
- 0.76
Mg (s)
- 2.37
Li (s)
- 3.05
2
Inthezinc/leadcellthemeasuredpotentialof0.63voltscanbededucedfromthesumof
thepotentialsofthetwohalf-reactions.
Zn (s) Zn+2 (aq) + 2 e-
Pb (aq) + 2 e +2
-
Zn (s) + Pb+2 (aq) Pb (s)
Zn+2 (aq) + Pb (s)
0.76 V
- 0.13 V
0.63V
Note: The sign of the standard reduction potential for the zinc half reaction is reversed to give
the
potential of the oxidation half reaction.
In Part I of this experiment, other metal/ion half-cell combinations will be tried. From the data, a
table will be developed, listing various elements and ions in order of their tendency to gain or
lose electrons.
TheNernstEquation
Theoreticalpredictionsoftendencytogainelectronsareusedtopredictthevoltage
differencebetweentwoelectrodes.Thevoltagedifferencebetweenelectrodes,thecell
voltage,isalsocalledtheelectromotivecellforce,oremf(orE).Understandard
conditions(25°C,1Msolutionconcentration,1atmgaspressure),thesetheoretically
predictedvoltagesareknownasstandardemfs(orE°cell).
Inreality,standardconditionsareoftendifficult,ifnotimpossible,toobtain.TheNernst
Equationallowscellvoltagestobepredictedwhentheconditionsarenotstandard.Walter
Nernstdevelopedthefollowingequationinthelate1800'swhilestudyingthe
thermodynamicsofelectrolytesolutions:
⎛ 2.303RT ⎞
E cell = E 0cell − ⎜
(1)
⎟ log Q
⎝ nF ⎠
Inequation(1),Risthegasconstant(8.314Jmole-1K-1),Tisthetemperature(Kelvin),Fis
Faraday's
constant(96,485coulombs/mole),nisthenumberorelectronstransferredinthebalanced
oxidation/reductionreaction,andQisthereactionquotient,or([products]/[reactants]).If
thereactionsarecarriedoutatroomtemperature(25°C),theNernstequationbecomes
⎛ 0.0591 ⎞
E cell = E 0cell − ⎜
(2)
⎟ log Q
⎝ n ⎠
Note in equations (1) and (2) that if the reaction quotient is equal to 1, then Ecell = E°cell.
In Part II of this experiment, voltages will be measured at various solution concentrations for the
copper/zinc galvanic cell and compared with voltages calculated using the Nernst Equation.
SAFETY PRECAUTIONS
Safety goggles must be worn in the lab at all times. Any skin contacted by chemicals should be
washed
immediately.
EXPERIMENTALPROCEDURE
PartI:Galvaniccellsandtheelectrochemicalseries
1.Intoacentralcupofthe12-wellcontainer,carefullypourapproximately5mLof0.1M
KNO3.
2.Intothefourwellsup(#1),right(#2),bottom(#3)andleft(#4)ofthecentralwell,pour
about5mLofthemetallicsaltsolutionslistedbelow:
#10.1MCu(NO3)2
#20.1MZnSO4
#30.1MFe2+/Fe3+
#40.1MSnCl2
3.Withcleantweezers(donotuseyourfingers)take,onebyone,fourstripsoffilterpaper
anddipone
endintothecentralcup(whereimmersionintheKNO3solutionwillholdoneend),the
otherendinto
adifferentoneofeachofthefourouterwells.Anytwostripsoffilterpaperfromanytwoof
thefour
outerwells,togetherwiththeKNO3solutioninthecentralwell,makeyoursaltbridge.
4.Holdacoppermetalstripwithcleantweezersand,ontopofapieceofscratchpaper,
sandthestrip
toremoveanyoxidecoating.(DONOTSANDTHESTRIPONTHELABBENCH!!)Oneend
(2cm)ofthestripshouldbebentandimmersedintotheCu(NO3)2solutioninitshalfcell
(#1).The
rest(3cm)shouldextendouttotheedgeofthecellandshouldbebentovertherim.There
the
electricalleads(alligatorclips)fromtheinterfacewillbeattachedlater.Repeatthesame
procedure
withthezincmetalstripandplaceitincup#2.Insertaninertelectrode(nichromewire)
intothe#3
solution,whichcontainsFe2+andFe3+atequalconcentrations.Insertastripoftinfoilinto
cell#4.
ThepiecesofmetalsandtheFe2+/Fe3+solutionwillbetheelectrodesofthegalvaniccells.
Thetinwill
bethereferenceelectrode;thatis,wewillmeasureallcellvoltagesrelativetothereduction
oftin:
Sn2+(aq)+2e-
Sn(s)
E°cell=0.00V
5.StartingwiththeSn/Cucombination,measurethevoltageproducedfromthegalvanic
cells.Usingtheredandblackleadwires(probes)attachedtotheredandblackpostsatV,
securethetinfoiland
coppermetaltothedigitalvoltmeterwiththealligatorclips.Donotallowtheclipstocome
incontact
withthesolutions.PositiontheprobessothatyoureadapositivevoltagefortheSn/Cu
system.
Recordthevoltage(note:theinterfacedisplaysvoltagesinvoltswhenusingthe
voltageleads).
Leavetheprobesattachedonlylongenoughtogetavoltagereading,thendisconnectto
minimize
chemicalchangesbythecurrentflow.
Theredprobeisthepositiveterminal;theblackisthenegativeterminal.Themeasured
voltagewillhaveapositivesigniftheblackprobeisontheanodeandtheredprobeison
thecathode.Identifyandrecordwhichmetalservesastheanodeandwhichasthecathode.
6.MeasurethevoltagefromtheSn/ZnsystemandtheSn/Fesystem,alwayshavingthe
sameprobeon
theSnasyoudidfortheSn/Cucombination.
7.MeasurethevoltageoftheCu/Zncell,Cu/FeandFe/Zncells.Besuretokeepthe
polaritiescorrect
asyoudothis.
PartII:TheNernstEquation
Inthispartoftheexperiment,youwillexaminetheeffectofsolutionconcentrationonthe
cellvoltageforthereaction:
Cu2+(aq)+Zn(s) Cu(s)+Zn2+(aq) (1)
TheNernstEquationallowsyoutocalculateE°cellasafunctionofthereactantand
productconcentrations.
Fortheabovereactionat25°C,theNernstEquationbecomes:
Ecell=E°cell-(0.0591/2)log{[Zn2+]/[Cu2+]}
(2)
Remember,solidsandpureliquidsarenotincludedintheQexpression.Theoretically,E°cell
fortheabovereactionis1.10V.Thus,thevalueforEcellcanbecalculated,knowing[Zn2+]
and[Cu2+].
1.Setupfivezinc/coppercellsusingthefollowingZn2+andCu2+solutionconcentrations
(inmoleper
liter).Thecellsshouldbeassembledincups1through5,withZn/ZnSO4inthecentralcup.
Cell# [Cu2+](M)
[Zn2+](M)
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
2.Usingthetweezers,dipfivefilterpaperstripsin0.1MKNO3inasmallbeaker.Use
approximately5mLofKNO3.Insertthestripswithoneendinthecentercup,theotherend
intooneoftheoutercups
justbeforeyouaretomeasurethepotentialofthatcombination.
3.Youshouldnowhavethefivenumberedcupseachcontainingadifferentcoppersolution
withacopperstripattachedasinPartI.Thecentralcupcontains1Mzincsolutionwitha
stripofzincplacedinthesolution.Thefilterpaperstrips,moistenedwithKNO3actassalt
bridgesforeachcell.
4.Measurethevoltage(Ecell)fromeachoftheabovehalf-cellcombinations.Recordthis
voltageinthe
DataTableinyournotebook.
5.DetermineEcellforanunknownCu2+concentration.
DATAANALYSIS
PartI:Galvaniccellsandtheelectrochemicalseries
Makeatablethatindicatestherelativepositionofthereductionreactionsyouobserved
withrespect
tothetinhalf-reaction.Placethereactionforyourmostpositivevoltageontop,andyour
reactionfor
yourleastpositive(ormostnegative)voltageonthebottom.
PartII:TheNernstEquation
CalculateEcellat25°Cforeachoftheabovehalf-cellcombinations.
GraphEcell(observed)versuslog[Cu2+].
FromyourplotofEcell(observed)versuslog[Cu2+],determinetheunknowncopperion
concentrationinthesolution.