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Chapter 18

“Electrochemistry”

RedoxReac)on

•  Oneormoreelementschangeoxida)onnumber– allsingledisplacement,andcombus)on,– somesynthesisanddecomposi)on

•  Alwayshavebothoxida)onandreduc)on– splitreac)onintooxida)onhalf‐reac)onandareduc)onhalf‐reac)on

•  Oxidizingagentisreactantmoleculethatcausesoxida)on(undergoesreduc)on)

•  Reducingagentisreactantmoleculethatcausesreduc)on(undergoesoxida)on)

Oxida)on&Reduc)on

•  Oxida/onistheprocessthatoccurswhen– oxida)onnumberofanelementincreases

– elementloseselectrons– half‐reac)onhaselectronsasproducts

•  Reduc/onistheprocessthatoccurswhen– oxida)onnumberofanelementdecreases

– elementgainselectrons– half‐reac)onshaveelectronsasreactants

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Recall:RulesforAssigningOxida)onStates

1.  Freeelementshaveanoxida)onstate=02.  Monatomicionshaveanoxida)onstateequaltotheir

charge3.  (a)thesumoftheoxida)onstatesofalltheatomsina

compoundis0 (b)thesumoftheoxida)onstatesofalltheatomsina

polyatomicionequalsthechargeontheion4.(a)GroupImetalshaveanoxida)onstateof+1inalltheir

compounds4.(b)GroupIImetalshaveanoxida)onstateof+2inalltheir

compounds

Recall:RulesforAssigningOxida)onStates

5.  Intheircompounds,nonmetalshaveoxida)onstatesaccordingtothetablebelow

Nonmetal Oxidation State Example F -1 CF4

H +1 CH4

O -2 CO2

Group 7A -1 CCl4

Group 6A -2 CS2

Group 5A -3 NH3

Oxida)on/Reduc)on

•  Oxida)onandreduc)onmustoccursimultaneously–  ifanatomloseselectronsanotheratommusttakethem

•  Thereactantthatreducesanelementinanotherreactantiscalledthereducingagent–  thereducingagentcontainstheelementthatisoxidized

•  Thereactantthatoxidizesanelementinanotherreactantiscalledtheoxidizingagent–  theoxidizingagentcontainstheelementthatisreduced

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Redox:Examples

H2S(g)+NO3–(aq)→ S(s)+NO(g)(inacid)

Redox:Examples

MnO2(s)+HBr(aq)→ MnBr2(aq)+Br2(l)(inacid)

ElectricalCurrent

•  Whenwetalkaboutthecurrentofaliquidinastream,wearediscussingtheamountofwaterthatpassesbyinagivenperiodof)me

•  Whenwediscusselectriccurrent,wearediscussingtheamountofelectricchargethatpassesapointinagivenperiodof)me– whetheraselectronsflowingthroughawireorionsflowingthroughasolu)on

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RedoxReac)ons&Current

•  Redoxreac)onsinvolvethetransferofelectronsfromonesubstancetoanother

•  Therefore,redoxreac)onshavethepoten)altogenerateanelectriccurrent

•  Inordertousethatcurrent,weneedtoseparatetheplacewhereoxida)onisoccurringfromtheplacethatreduc)onisoccurring

ElectricCurrentFlowingDirectlyBetweenAtoms

ElectricCurrentFlowingIndirectlyBetweenAtoms

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ElectrochemicalCells•  Electrochemistryisthestudyofredoxreac)onsthatproduceorrequireanelectriccurrent

•  Theconversionbetweenchemicalenergyandelectricalenergyiscarriedoutinanelectrochemicalcell

•  Spontaneousredoxreac)onstakeplaceinavoltaiccell– akagalvaniccells

•  Nonspontaneousredoxreac)onscanbemadetooccurinanelectroly/ccellbytheaddi)onofelectricalenergy

ElectrochemicalCells

•  Oxida)onandreduc)onreac)onskeptseparate– half‐cells

•  Electronflowthroughawirealongwithionflowthroughasolu)oncons)tutesanelectriccircuit

•  Requiresaconduc)vesolid(metalorgraphite)electrodetoallowthetransferofelectrons–  throughexternalcircuit

•  Ionexchangebetweenthetwohalvesofthesystem– electrolyte

Electrodes

•  Anode– electrodewhereoxida)onoccurs

– anionsa^ractedtoit– connectedtoposi)veendofba^eryinelectroly)ccell

–  losesweightinelectroly)ccell

•  Cathode– electrodewherereduc)onoccurs

– ca)onsa^ractedtoit– connectedtonega)veendofba^eryinelectroly)ccell

– gainsweightinelectroly)ccell

•  electrodewherepla)ngtakesplaceinelectropla)ng

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VoltaicCell

CurrentandVoltage

•  Thenumberofelectronsthatflowthroughthesystempersecondisthecurrent(unit=Ampere)– 1Aofcurrent=1Coulombofchargeflowingbyeachsecond– 1A=6.242x1018electrons/second– Electrodesurfaceareadictatesthenumberofelectronsthatcanflow

•  Thedifferenceinpoten)alenergybetweenthereactantsandproductsisthepoten/aldifference(unit=Volt)– 1Vofforce=1Jofenergy/Coulombofcharge–  thevoltageneededtodriveelectronsthroughtheexternalcircuit

– amountofforcepushingtheelectronsthroughthewireiscalledtheelectromo/veforce,emf

CellPoten)al•  Thedifferenceinpoten)alenergybetweentheanode/thecathodeinavoltaiccelliscalledthecellpoten/al

•  Thecellpoten)aldependsontherela)veeasewithwhichtheoxidizingagentisreducedatthecathodeandthereducingagentisoxidizedattheanode

•  Thecellpoten)alunderstandardcondi)onsiscalledthestandardemf,E°cell– 25°C,1atmforgases,1Mconcentra)onofsolu)on

– sumofthecellpoten)alsforthehalf‐reac)ons

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CellNota)on

•  Shorthanddescrip)onofVoltaiccell•  Electrode|electrolyte||electrolyte|electrode•  Oxida)onhalf‐cellonleg,reduc)onhalf‐cellontheright

•  Single|=phasebarrier– ifmul)pleelectrolytesinsamephase,acommaisusedratherthan|

– ogenuseaninertelectrode•  Doubleline||=saltbridge

Fe(s) | Fe2+(aq) || MnO4-(aq), Mn2+(aq), H+(aq) | Pt(s)

StandardReduc)onPoten)al•  Ahalf‐reac)onwithastrongtendencytooccurhasalarge+half‐cellpoten)al

•  Whentwohalf‐cellsareconnected,theelectronswillflowsothatthehalf‐reac)onwiththestrongertendencywilloccur

•  Wecannotmeasuretheabsolutetendencyofahalf‐reac)on,wecanonlymeasureitrela)vetoanotherhalf‐reac)on

•  Weselectasastandardhalf‐reac)onthereduc)onofH+toH2understandardcondi)ons,whichweassignapoten)aldifference=0v– standardhydrogenelectrode,SHE

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Half‐CellPoten)als

•  SHEreduc)onpoten)alisdefinedtobeexactly0v•  Half‐reac)onswithastrongertendencytowardreduc)onthantheSHEhavea+valueforE°red

•  Half‐reac)onswithastrongertendencytowardoxida)onthantheSHEhavea‐valueforE°red

•  E°cell=E°oxida)on+E°reduc)on– E°oxida)on=‐E°reduc)on– whenaddingE°valuesforthehalf‐cells,donotmul/plythehalf‐cellE°values,evenifyouneedtomul)plythehalf‐reac)onstobalancetheequa)on

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Example

•  CalculateE°cellforthereac)onat25°CAl(s)+NO3

−(aq)+4H+

(aq)→Al3+(aq)+NO(g)+2H2O(l)

Example•  SketchandLabeltheVoltaicCell

Fe(s)Fe2+(aq) Pb2+(aq)Pb(s),WritetheHalf‐Reac)onsandOverallReac)on,andDeterminetheCellPoten)alunderStandardCondi)ons.

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Example•  ox:Fe(s)→Fe2+(aq)+2e−E°=+0.45V

•  red:Pb2+(aq)+2e−→Pb(s)E°=−0.13V•  tot:Pb2+(aq)+Fe(s)→Fe2+(aq)+Pb(s)E°=+0.32V

Predic)ngWhetheraMetalWillDissolveinanAcid

•  Acidsdissolveinmetalsifthereduc)onofthemetalioniseasierthanthereduc)onofH+

(aq)

• Metalswhoseionreduc)onreac)onliesbelowH+reduc)ononthetablewilldissolveinacid

E°cell,ΔG°andK

•  Foraspontaneousreac)on– onetheproceedsintheforwarddirec)onwiththechemicalsintheirstandardstates

– ΔG°<1(nega)ve)– E°>1(posi)ve)– K>1

•  ΔG°=−RTlnK=−nFE°cell– nisthenumberofelectrons

– F=Faraday’sConstant=96,485C/mole−

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Example

•  CalculateΔG°forthereac)onI2(s)+2Br−(aq)→Br2(l)+2I−(aq)

NonstandardCondi)ons‐theNernstEqua)on

•  ΔG=ΔG°+RTlnQ•  E=E°‐(0.0592/n)logQat25°C•  whenQ=K,E=0•  usetocalculateEwhenconcentra)onsnot1M

E°atNonstandardCondi)ons

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Example

•  CalculateEcell at25°Cforthereac)on3Cu(s)+2MnO4

−(aq)+8H+

(aq)→2MnO2(s)+Cu2+(aq)+4H2O(l)

Concentra)onCells

•  Itispossibletogetaspontaneousreac)onwhentheoxida)onandreduc)onreac)onsarethesame,aslongastheelectrolyteconcentra)onsaredifferent

•  Thedifferenceinenergyisduetotheentropicdifferenceinthesolu)ons–  themoreconcentratedsolu)onhaslowerentropythantheless

concentrated

•  Electronswillflowfromtheelectrodeinthelessconcentratedsolu)ontotheelectrodeinthemoreconcentratedsolu)on

LeClanche’AcidicDryCell•  electrolyteinpasteform

– ZnCl2+NH4Cl•  orMgBr2

•  anode=Zn(orMg)Zn(s)→Zn2+(aq)+2e‐

•  cathode=graphiterod•  MnO2isreduced

2MnO2(s)+2NH4+(aq)+2H2O(l)+2e‐

→2NH4OH(aq)+2Mn(O)OH(s)

•  cellvoltage=1.5v•  expensive,nonrechargeable,heavy,easilycorroded

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AlkalineDryCell

•  samebasiccellasacidicdrycell,exceptelectrolyteisalkalineKOHpaste

•  anode=Zn(orMg)

Zn(s)→Zn2+(aq)+2e‐

•  cathode=brassrod•  MnO2isreduced

2MnO2(s)+2NH4+(aq)+2H2O(l)+2e‐

→2NH4OH(aq)+2Mn(O)OH(s)

•  cellvoltage=1.54v

•  longershelflifethanacidicdrycellsandrechargeable,li^lecorrosionofzinc

LeadStorageBa^ery

•  6cellsinseries

•  electrolyte=30%H2SO4

•  anode=Pb

Pb(s)+SO42‐(aq)→PbSO4(s)+2e‐

•  cathode=PbcoatedwithPbO2

•  PbO2isreduced

PbO2(s)+4H+(aq)+SO42‐(aq)+2e‐

→PbSO4(s)+2H2O(l)

•  cellvoltage=2.09v•  rechargeable,heavy

FuelCells

•  Likeba^eriesinwhichreactantsareconstantlybeingadded– soitneverrunsdown!

•  AnodeandCathodebothPtcoatedmetal

•  ElectrolyteisOH–solu)on•  AnodeReac)on:

2H2+4OH–

→4H2O(l)+4e‐•  CathodeReac)on:

O2+4H2O+4e‐→4OH–

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ElectrochemicalCells

•  Inallelectrochemicalcells,oxida)onoccursattheanode,reduc)onoccursatthecathode

•  Involtaiccells,– anodeisthesourceofelectronsandhasa(−)charge– cathodedrawselectronsandhasa(+)charge

•  Inelectroly)ccells– electronsaredrawnofftheanode,soitmusthaveaplacetoreleasetheelectrons,the+terminaloftheba^ery

– electronsareforcedtowardtheanode,soitmusthaveasourceofelectrons,the−terminaloftheba^ery

Electrolysis

•  Electrolysisistheprocessofusingelectricitytobreakacompoundapart

•  Electrolysisisdoneinanelectroly)ccell•  Electroly)ccellscanbeusedtoseparateelementsfromtheircompounds– generateH2fromwaterforfuelcells– recovermetalsfromtheirores

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ElectrolysisofWater

ElectrolysisofPureCompounds

• Mustbeinmolten(liquid)state•  Electrodesnormallygraphite

•  Ca)onsarereducedatthecathodetometalelement

•  Anionsoxidizedatanodetononmetalelement

ElectrolysisofNaCl(l)

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Corrosion

•  Corrosionisthespontaneousoxida)onofametalbychemicalsintheenvironment

•  Sincemanymaterialsweuseareac)vemetals,corrosioncanbeaverybigproblem

Rus)ng

•  Rustishydratediron(III)oxide• Moisturemustbepresent

– waterisareactant– requiredforflowbetweencathodeandanode

•  Electrolytespromoterus)ng– enhancescurrentflow

•  Acidspromoterus)ng– lowerpH=lowerE°red