chapter 18faculty.scf.edu/gambinc/chm 2046/chm 2046 lecture notes...4/8/08 1 chapter 18...
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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