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Galvaniccells

Pleaseturnonyourclicker.

2

Au(s)

Au3+

Au3+

Au3+

1.09

voltmeter

Galvaniccells

Halfreactions

Cathode,Reduction

Au3+(aq)+3e Au(s)

Anode,Oxidation

Cu(s)Cu2+(aq)+2e

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Standardreduction potentials(25oC)

REACTION Eo(V)

Au3+(aq)+3e Au(s) 1.42

Cu2+(aq)+2e Cu(s) 0.3402

ThehalfcellreactionwiththelargerEo isthecathode.

Theanodeiswrittenontheleft:

Cu|Cu2+Au3+|Au

Eo= Eo(cathode) Eo(anode)=1.42 0.3402=1.08V

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Clickerquestion

Agalvaniccelliscomposedofastandardhalf

cellwithareductionpotentialof 1.00Vanda

secondstandardhalfcellwithareduction

potentialof 0.100V.Whichhalfcellcontainsthecathode?

A)Eo =1.00V

B)Eo =0.100V

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Thevoltageinthediagramiscloseto1.08V,but

slightlydifferent. Why?

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Au(s)Ag3+

Au3+

Au3+

Au3+

1.09

voltmeter

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ConcentrationdependenceNernstequation: Q

nF

RTo ln EE

m

n

][reactants

[products]Q

Au3+(aq)+3e Au(s) cathode,reducon

Cu(s) Cu2+(aq)+2e anode,oxidation

2Au3+(aq)+3Cu(s) 2Au(s)+3Cu2+(aq)

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32

][Au

][Cu

Q23

32

][Au

][Culn

608.1

F

RTE

2{ }

7

3{ }

Experimentalconcerns

Connecttocom side

ofvoltmeter.

Anode

Doesthereaction

produceorconsume

electrons?

Connectto+V

sideof

voltmeter.

cathode

Thescaleonthevoltmetershouldbesettomaximum

sensitivityforanonscalereading.Itshouldbesetto

readadirectcurrentvoltagesource ( not )

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1.09

voltmeter

Au(s)

Au3+

Au3+

Au3+

Au3+

ExperimentalconcernsThescaleonthevoltmetershouldbesettomaximum

sensitivityforanonscalereading.Itshouldbesetto

readadirectcurrentvoltagesource ( not )

Connectionsneedtobetight.

Donotinsertthe

probeintotheend

ofthealligatorclip.

MakesketchesinyournotebookforALLcellsshowing

whichconnectionsarenecessarytoobtainapositivevoltage.

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Safety

Allofthemetalsaltsaretoxic.

TheyhaveanMSDSHealthratingof2 moderate

or 3severe(poison)

Somearealsostrongoxidants.

ThenitrateshaveanMSDSReactivityratingof

3severe(oxidizer)

Theywilldamageyourskin.

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PartA

Youwillmakeacopper/leadgalvaniccell.

Youwillneedtomakecoppersulfateand

leadnitratesolutions.

Youcalculatetheamountofeachthatyouneedfortheprelabhomework.Youshould

bringthosenumberswithyoutolab.

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Clickerquestion

Ifyouweremakingasolutionina50ml

volumetricflask,howmuchwaterwouldyou

usetoinitiallydissolvethesolid?

A) 10ml

B) 25ml

C) 50ml

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PartBunknowns

Youwillbegivenasheetwith3unknowns.AskyourTAtogiveyouthe3metalsforelectrodes

andthe3matchingsolutions.

Youwillmakeagalvaniccellwitheachoftheunknownscomparedtocopper.

Fromthevoltages,youwillbeabletoidentifytheunknowns.(Youllneedtodothisbeforeyouleavethelab.)

Theunknownelectrodeswillbefromalistof5possibilities(Ag,Sn,Zn,Al,orMg).

TheconcentrationsofthesolutionsaresuchthatthelogtermintheNernstequationwillnotcontributetothepotentialdifference.

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6wellplate

PartA PartB

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V

1

2

3

1

2

3

Connecttheelectrodestothevoltmeter

betweenthecopperandeachunknown

togetapositivereading.

PartC Concentrationcell

Cu2+(aq)+2e Cu(s) Cu(s)Cu2+(aq)+2e

cathode(reduction) anode(oxidation)

V

Cu2+ (0.05M) Cu2+(

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H2C2O4(aq) H+(aq)+HC2O4

(aq) Ka1=5.9x102

HC2O4 (aq) H+(aq)+C2O4

2(aq) Ka2=6.4x105

NearlyalltheH+ comesfromthefirststepsincetheequilibriumconstantis3ordersofmagnitudelarger.

[H+

] [HC2O4

]Ka2=6.4x10

5=[H+][C2O42]/[HC2O4

]

[C2O42]

Ksp=[Cu2+][C2O4

2]

Warning:TheKspyoucalculateisverydifferentfromthevalueinthetext.Youwillnotneedtofigureoutapercentdeviation.However,youwillbeaskedforsomepossiblereasonsforanydiscrepancyandyouwillbeaskedtosuggestexperimentstotestwhetherthosereasonscouldaccountforthediscrepancy.

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Experimentalconcerns

Makesketchesinyournotebookofeach

setup.Youwillbeabletodeterminewhichisthecathode

andwhichistheanodebytheconnectionthat

resultsinapositivevoltage.

Bepreparedtodothisinthelab youllneedto

knowhowtodoittodetermineyourunknowns.

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Excesschemicaldisposal

Thesolutionsareneutralexceptfortheones

containingoxalicacid.

Theneutralsolutionscanjustbeputinthebig

drumswithoutneedingtoaddbakingsoda.

Anysolutionscontainingoxalicacidneedtobe

neutralizedwithbakingsodaandthenputin

thedrums.

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Similarities

GalvanicCells

Oxidationoccursattheanode.

Reductionoccursatthe

cathode.

Electronstravelinthewires.

Ionstravelinsolution.

Thesolutionsareelectrically

neutral

Acompletecircuitis

necessary.

ElectrolyticCells

Oxidationoccursattheanode.

Reductionoccursatthe

cathode.

Electronstravelinthewires.

Ionstravelinsolution.

Thesolutionsareelectrically

neutral.

Acompletecircuitis

necessary.

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Differences

GalvanicCells

Twoconnectedhalfcellsproduce voltage.

Theidentityofthecathodeandanodeisdeterminedbythereductionpotentials.

Theelectricalneutralityofthesolutionsaremaintainedwiththesaltbridgesincethereactionstakeplaceinseparatebeakers.

Galvaniccellpotentialsaredependentontheconcentrationofthesolutions. (Nernsteqn.)

ElectrolyticCells

Abatteryorpowersupplyisrequired.

Theidentityofthecathodeandanodeisdeterminedbythebattery,NOT bythereductionpotentials.

Theelectricalneutralityofthesolutionismaintainedbysimultaneousreactionsatthecathodeandanodesincethereactionstakeplaceinthesamebeaker.

Theconcentrationofthesaltdoesnotchangetheamountplated,aslongasthereissufficientsaltpresent.

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Reminders

Prelabhwonexperiment3duenextweek

Experiment2postlabassignmentisduenext

week

Remembertobringyoursketchoftheelectrolytic

cellapparatustolabnextweek.

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