PRE-AP-CHEMISTRY-CHAPTER-23

PRE-APCHEMISTRYCHAPTER23ELECTROCHEMISTRYTheinterconversionsofchemicalandelectricalenergyarecalledelectrochemicalprocesses,allofwhichinvolvethetransferofelectrons.Thisprocessoccursinadevicecalledanelectrochemicalcell.RemembertheactivityseriesofmetalsinChapter8?Weusedittodeterminereactivityinsinglereplacementreactions.ThemostactivemetalsarethegroupI&IImetals+Al,thenthetransitionmetals,Hisbyitself,andtheleastactivearethejewelrymetals(Cu,Hg,Ag,Au).Themoreactivemetals(higherontheactivityseries)canreplacethelessactivemetals(lowerontheactivityseries)inachemicalreaction.ACTIVITYSERIESOFMETALSElementLithiumPotassiumBariumDecreasingCalciumActivitySodiumMagnesiumAluminumZincIronCadmiumNickelTinLeadHydrogen(anonmetal)CopperMercurySilverGoldPlatinum*MetalsfromLitoNawillreplaceHfromwaterandacids;metalsfromMgtoPbwillreplaceHfromacidsonly.GroupI,II,andAlTransitionMetalsHydrogenJewelryMetalsThesesinglereplacementreactionsareonetypeofredoxreaction.Electronsarelostbythemoreactivemetalsandgainedbythelessactivemetals.Thehigherametalisontheactivityseries,theeasieritisoxidized.AgwillnotreactwithNaClbecauseAgcannotspontaneouslytransferelectronstoNa.AgislessactivethanNa.Electrochemicalcellsthatconvertchemicalenergyintoelectricalenergyarecalledvoltaicorgalvaniccells.Theenergyisproducedbyspontaneousredoxreactions.Voltaiccellscanbeseparatedintotwohalf-cells,oneinwhichoxidationoccursandoneinwhichreductionoccurs.Ahalfcellconsistsofametalrodorstripimmersedinasolutionofitsions.Thetwohalf-cellsareconnectedbyaporouspartitionorasaltbridge.Asaltbridgeisatubecontainingaconductingsolution.Ionspassthroughthesaltbridgetokeepthechargesbalanced.Electronspassthroughanexternalwire.Themetalrodsinvoltaiccellsarecalledelectrodes.Oxidationoccursattheanodeandreductionoccursatthecathode.(AnOxandaRedCat)Speciesundergoingreductionreceiveelectronsfromthecathode.Speciesundergoingoxidationdonateelectronstotheanode.Thedirectionofelectronflowisthereforefromtheanodetothecathode.(FATCAT)GalvanicCellanimationInavoltaiccell,theanodeisassignedanegativechargeandthecathodeisassignedapositivecharge.Linenotationisashorthandmethodforrepresentingelectrochemicalcells.Thegeneralformis:anode|anodeion||cathodeion|cathodeThedoubleverticallinerepresentsthesaltbridge.ForavoltaiccellinvolvingCuandZnasshownonthetopofpg680,thelinenotationwouldberepresentedas:Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s)Whenonlyions(ratherthansolids)areinvolvedintheredoxprocess,inertelectrodessuchasgraphiteorplatinumareused.Ag++Fe2+Fe3++Ag(s)Galvaniccell:Ag++Fe2+Fe3++Ag(s)Linenotation:Pt|Fe2+,Fe3+||Ag+|AgFe2+isoxidized,butwecan’tmakeasolidpieceofFe2+.Ag+isreduced.Oxidationhalf-reaction:Fe2+Fe3++e-Reductionhalf-reaction:Ag++e-AgAdrycellisavoltaiccellinwhichtheelectrolyteisapaste.Theflashlightbatteryisacommonexample.Theinexpensivedrycellbatteriesconsistofazinccontainerwhichactsastheanode,agraphiterodwhichactsasthecathodeandanelectrolytepasteofMnO2,ZnCl2,NH4ClandH2O.ThepHofthissolutionisacidicandtendstocorrodethezinccase.Zincisoxidizedandmanganeseisreduced.AnalkalinebatteryhastheNH4ClreplacedwithKOHorNaOHandisbasicinsteadofacidic.Thismakesthebatterylastlonger.Agroupofcellsconnectedtogethermakeupabattery.Aleadstoragebatteryiscommonlyusedinanautomobile.Inthistypeofbatterytheanodeislead.Thecathodeisleaddioxide.Theelectrolyte(batteryacid)issulfuricacid.ModernBatteryChemistry(FYIonly)Modernbatteriesuseavarietyofchemicalstopowertheirreactions.Typicalbatterychemistriesinclude:Zinc-carbonbattery-Alsoknownasastandardcarbonbattery,zinc-carbonchemistryisusedinallinexpensiveAA,CandDdry-cellbatteries.Theelectrodesarezincandcarbon,withanacidicpastebetweenthemthatservesastheelectrolyte.Alkalinebattery-AlkalinechemistryisusedincommonDuracellandEnergizerbatteries,theelectrodesarezincandmanganesedioxide,withanalkalineelectrolyte.Lithium-iodidebattery-Lithium-iodidechemistryisusedinpacemakersandhearingaidesbecauseoftheirlonglife.Lead-acidbattery-Lead-acidchemistryisusedinautomobiles,theelectrodesaremadeofleadandlead-oxidewithastrongacidicelectrolyte(rechargeable).Nickel-cadmiumbattery-Theelectrodesarenickel-hydroxideandcadmium,withpotassium-hydroxideastheelectrolyte(rechargeable).Nickel-metalhydridebattery-Thisbatteryisrapidlyreplacingnickel-cadmiumbecauseitdoesnotsufferfromthememoryeffectthatnickel-cadmiumsdo(rechargeable).Lithium-ionbattery-Withaverygoodpower-to-weightratio,thisisoftenfoundinhigh-endlaptopcomputersandcellphones(rechargeable).Zinc-airbattery-Thisbatteryislightweightandrechargeable.Zinc-mercuryoxidebattery-Thisisoftenusedinhearing-aids.Silver-zincbattery-Thisisusedinaeronauticalapplicationsbecausethepower-to-weightratioisgood.Afuelcellisanelectrochemicaldevicethat,inmostcases,combineshydrogenandoxygentoproduceelectricity,withwaterandheatasitsby-product.Aslongasfuelissupplied,thefuelcellwillcontinuetogeneratepower.Whataresomeusesforfuelcells?H2poweredcarsspacecraftmilitarysubmarinesTheelectromotiveforce(emf)orcellpotential(Eo)ofagalvaniccellisacombinationofthepotentialsofthetwohalf-reactions.Thestandardreferencethatal