31251484Chapter6Electrochemistry.docx

31251484Chapter6Electrochemistry.docx

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31251484Chapter6Electrochemistry

Electrolytes

Electrolytesarecompoundswhichwhenmoltenordissolvedinwaterconductelectriccurrentandaredecomposedintheprocess.

Non-electrolyte

Anon-electrolyteisaliquidwhichdoesnotallowthepassageofelectricity.

MoltenSolution

▪Thisiscomposedoflead（II）ions, Pb2+ ,andbromideions,Br-.ItschemicalformulaisthereforePbBr2.

▪AsuitableapparatuswhichcouldbeusedtocarryoutthiselectrolysisisshowninFigureabove.

▪Thebulbhelpstoshowwhenelectricityisflowinginthecircuit,anduntilthelead（II）bromideiscompletelymolten,thebulbdoesnotlightup.Thisconfirmsthatelectrolyteshavetobemoltenfortheionstostarttomovetotheelectrodesandtherebyconductelectricity.

AttheCathode

AttheAnode

Observation

▪Whenelectricityisflowing,asilverydepositofleadmetalformsonthecathode.Infact,asitismolten,itismorelikelytodripoffinamoltenblob.

Observation

▪Whenelectricityisflowing,brownfumesofbrominegasareseenattheanode.

Halfequation 

Pb2+ +2e--->Pb

Halfequation

2Br- --->Br2 +e

Explanation

▪Thelead（II）ions,astheyarepositive,movetothenegativecathode,whereeachiongainstwoelectronstoformaleadatom.

▪Anyreactionatacathodeinvolvedisagaininelectrons.Thisiscalledreductionormoreexactly,cathodicreduction.

Explanation

▪Thebromideions,astheyarenegative,movetothepositiveanode,whereeachlosesanelectrontoformabromineatom.

▪Thentwoofthesenewlyformedatomscombinetoformbrominegas.

▪Anyreactionatananodeinvolvesalossofelectrons.

▪Insummary,thelead（II）bromideissplitintoitscomponentelements :

PbBr2 --->Pb+Br2

ElectrolysisofAqueousSulphuricAcid

▪Assulphuricacidisaqueous,itiscomposednotonlyofhydrogenions（H+ ）andsulphateions（SO42-）,butalsoofhydroxideions（OH-）fromthewater.

H2SO4 +H2O-->2H+ +SO42- +H+ +OH-

▪TheapparatususedtocarryoutthiselectrolysisandcollectthegasesgivenoffisshowninFigure9.8.

▪Whenwehavemorethanonetypeofionmovingtoanelectrode,selectivedischarge（orpreferentialdischarge）takesplace.

▪Thismeansthattheionwhichcanloseorgainelectronswiththegreatesteaseisdischarged,andtheotherions,whicharehardertodischarge,remaininsolution.

▪Withtheelectrolyteaqueoussulphuricacid,migrationofionstotheelectrodesalsooccurs.

AttheCathode

AttheAnode

▪Herewehaveonlyoneion,thehydrogen,H+（aq）,andeachiongainsanelectrontobecomeahydrogenatom.

▪Twoofthesenewlyformedatomsthencombinetoformahydrogengasmolecule.

▪Herewehaveachoiceofeithersulphate,SO42-（aq）,orhydroxideOH- （aq）ions.

▪Hydroxideiseasiertodischarge,sooxygengasisgivenoffattheanode.

Equation:

2H+ +2e--->H2

Equation:

OH- +4e--->O2 +H2O

Notes

▪Withelectrolysisofaqueoussolutionsofdiluteacidsoralkalis,thevolumeofhydrogengivenoffatthecathodeisroughlytwicethatoftheoxygengasattheanode.

▪Accordingly,theelementsofwaterarelostandastheelectrolysiscontinues,theconcentrationoftheacidoralkaliincreases.

▪Essentially,theelectrolysisofaqueoussulphuricacidistheelectrolysisofwater,withhydrogenandoxygengasbeinggivenoffinaratioof2 :

1.

ExtractionofMetal

▪Theextractionofmetalsfromtheirores,inparticularaluminiumandsodium,isimportantindustrialusesofelectrolysis.

▪Thediagrambelowshowsthemethodsofextractionfordifferentmetals.

▪Wecanseethatthosemetalswhicharelessreactivethancarboninreactivityseriesareextractedfromtheirorebydisplacementreactionusingcarbon.Thiswillbediscussedindetailinchapter3,form5,OxidationandReduction.

▪Copperandmercurycanbeextractedfromtheirorebyburningdirectlyinair.

▪Silver（Ag）andgold（Au）neednoextractionbecausetheyexistaselementinnature.

▪Thosemetalswhicharemorereactivethancarbonareextractedbyelectrolysis.

ExtractionofAluminium

▪Aluminiumisthemostabundantmetalfoundintheearth'scrust.Itmakesupabout8%byweightoftheEarth’ssolidsurface.

▪Itisalsoaveryusefulmetalduetoitslowdensityandabilitytoresistcorrosion.

▪Themainsourceofaluminiumisbauxiteore（AluminiumOxide）.

▪Inindustry,aluminiumisextractedbyelectrolysisfrombauxiteore.

AddingCryolite

▪Inelectrolysis,moltenaluminiumoxidemustbeusedtoextractaluminium.Aluminiumoxidedecomposetoformaluminiumandoxideionswhenmelted.

Al2O3 --->2Al3+ +3O2-

▪However,themeltingpointofaluminiumoxideisveryhigh（over2000°C）,soanotheraluminiumcompoundcalledcryolite（Na3AIF6）isaddedtolowerdownthemeltingpoint（about980oC）.

▪Thediagramaboveshowshowaluminiumisextractedfrommoltenaluminiumoxidebyelectrolysis.

▪Graphiteisusedastheanodeandcathode.

▪Duringelectrolysis,thealuminiumionsareattractedtowardsthegraphitecathode.

▪Theionsisdischargedandbecomemoltenaluminiummetal.

▪Thepartialequationofthisreactionisasfollow:

Al3+ +3e--->Al

▪Attheanode,oxygengaswhichalsohascommercialvalueiscollected.Thepartialequationofthisreactionisasfollow:

2O2- --->O2 +4e

▪Atthetemperatureof980°C,theoxygenburnsthecarbonanode.Thereforetheanodehastobereplacedperiodically.

▪Also,thiscelluseslargequantitiesofelectricity,andthereforeneedscheapsourcesofpower.

Extractionofsodiumchloride

▪Inindustry,sodiumisextractedfrommoltensodiumchloride.Moltensodiumchlorideisputintotheapparatusasshowinginthediagramabove.

▪Whensodiumchlorideismelted,thesodiumandchlorideionsdisassociatetobecomefreelymoveions,asshowninthechemicalequationbelow.

NaCl--->Na+ +Cl-

▪Inthiselectrolyticcell,graphitewasusedasanodewhileironisusedascathode.

▪Thenegativechlorideionsareattractedtotheanodeandthendischargedtoformchlorinegas.

2Cl- --->Cl2 +2e

▪Sincechlorinegasisalsosignificantinindustry,itiscollectedandstored.

▪Incathode,thesodiumionsaredischargedtoformsodiumatom.

Na+ +e--->Na

▪Duetohightemperature,thesodiummetalformedisinmoltenform.

▪Metalsodiumhavelowerdensity.Thereforeitmovesupwardandbeencollected.

PurificationOfCopper

▪Intherefiningorpurificationofcopper,theimpurecopperismadetheanodeandathin,purecopperplateisusedasacathode.

▪Theelectrolyteisusuallyacidifiedcopper（II）sulphatesolution.

▪Whenelectricityflows,thecopperdissolvesfromtheimpureanodeandgoesintosolutionascopperions.

▪Impuritiesinthecopperdonotdissolve,andinsteadfallofftheanodeasanodesludge.Atthecathode,thecopperionsaredepositedaspurecoppermetal.

▪Reactioninanode（impurecopper）

Inanode,thecopperatomsfromtheelectrodeareionisedtoformcopper（II）ions.

Cu--->Cu2+ +2e

▪Reactionincathode（purecopper）

Cu2+Cu--->Cu+2e

Electroplating

Electroplating:

CoatingwithaThinProtectiveLayerofMetal

▪Averycommonuseofelectrolysisistoformathinprotectivecoatingofametalonthesurfaceofanotherwhichislikelytocorrode.

▪Thediagramaboveillustratetheelectroplatingofakeywithcopper.

▪Inthisprocess,weneedtomakethecathodetheobjectforplating（thekey.

▪Theanodeisthenmadeofthemetalwewishtoplatewith（copper）,andtheelectrolyteneedstobeasolutionofasaltofthismetal（copper（II）sulphate）.

Anode

▪Inanode,thecopperatomsfromtheelectrodeareionisedtoformcopper（II）ions.

Cu--->Cu2+ +2e

Cathode

▪Incathode,thecopperionsaredischargedtoformcopperatomandthendepositonthesurfaceofthekey

Cu2+ --->Cu+2e

CellsandBatteries

▪Adevicewhichconvertschemicalenergyintoelectricalenergyiscalledacellorbattery.Batteryisacollectionofcells.

▪Acellconsistsofapairofdissimilarmetalsinanelectrolyte.

▪Figureaboveshowsanexampleofasimplevoltaiccellconsistofamagnesiumelectrodeandacopperelectrodeimmerseinmagnesiumsulphatesolution.

▪Whenchemicalreactionhappens,themorereactivemetal,magnesium,dissolvesinthemagnesiumsulphatesolutionandbecomemagnesiumions,therebyproducingelectrons,asshowninthehalfequationbelow:

Mg--->Mg2+ +2e

▪Aselectronsareproduced,themagnesiumactsasthenegativeelectrode.

▪Theseelectronsthentraveltothecopperelectrode.

▪Thehydrogenionsaroundthecopperelectrodereceivetheelectronsandaredischargedtoproducebubblesofhydrogengas:

2H+ +2e--->H2

▪Aselectronsaretakenin,thecopperisthepositiveelectrode.

▪Thisproductionandmovementofelectronsiselectricity,soelectricalenergyhasbeengeneratedandthegalvanometerisdeflected.*Overall,thechemicalreactioncanberepresentedbytheionicequation:

Mg+2H+ --->Mg2+ +H2

▪Involtaiccell,thenegativeelectrodeistheanodewhereasthepositiveelectrodeisthecathode,whichistheoppositeoftheelectrolyticcell.