Essentialknowledgestandards•  3.A.1:Achemicalchangemayberepresentedbyamolecular,

ionic,ornetionicequation•  3.B.3:Inoxidation-reduction(redox)reactions,thereisanet

transferofelectrons.Thespeciesthatloseselectronsisoxidized,andthespeciesthatgainselectronsisreduced

•  3.C.3:Electrochemistryshowstheinterconversionbetweenchemicalandelectricalenergyingalvanicandelectrolyticcells

•  5.E.4:ExternalsourcesofenergycanbeusedtodrivechangeincaseswheretheGibbsfreeenergychangeispositive

•  6.A.1:Inmanyclassesofreactions,itisimportanttoconsiderboththeforwardandreversereactions.

FLT•  Iwillbeableto:– Identifyredoxreactionsandjustifytheidentificationintermsofelectrontransfer– Designand/orinterprettheresultsofanexperimentinvolvingaredoxtitration– Makequalitativeorquantitativepredictionsaboutgalvanicorelectrolyticreactionsbasedonhalf-cellreactionsandpotentialsand/orFaraday’slaws– Analyzedataregardinggalvanicorelectrolyticcellstoidentifypropertiesoftheunderlyingredoxreactions

bycompletingElectrochemistryNotes

Electrochemistry

Recall

8

OxidationStates

Oxidation-Reduction•  Weneedtobeabletoassignoxidationstatestodeterminehowelectronsarebeingtransferredinareaction

•  Oxidationstates(oxidationnumbers)=indicatesthenumberofchargestheatomwouldhaveinamolecule/compoundifelectronsweretransferredcompletely

•  Rules:#1:ElementsintheirfreestatehaveanoxidationstateofZEROEx/N2,Na

Oxidation-Reduction•  Rules:

#2:MonoatomicionshaveoxidationstatesequaltotheirchargeEx/Li+hasanoxidationstateof+1,O2-hasanoxidationstateof-2#3:Incompounds,fluorinealwayshasanoxidationstateof-1Ex/NaF

Oxidation-Reduction•  Rules:

#4:Theoxidationnumberofoxygeninmostcompoundsis-2Ex/MgOButinhydrogenperoxideandperoxide,itis-1Ex/O2

2-

#5:Theoxidationnumberofhydrogenis+1exceptwhenbondedtometalsinbinarycompoundsEx/H2Ovs.LiH

Oxidation-Reduction•  Rules:

#6:Thesumoftheoxidationstatesmustbezeroforanelectricallyneutralcompound.Forpolyatomicions,itmustequalthechargeoftheion.Ex/CaH2Ex/NH4

+

– Generallyspeaking,metallicelementshavepositiveoxidationnumbers,whereasnonmetallicelementscanhavepositiveornegativeoxidationnumbers

–  ThehighestoxidationnumberanelementinGroups1A=7Acanhaveisitsgroupnumber

–  Transitionmetalshaveseveralpossibleoxidationnumbers.– And,yes,it’spossibletohavenon-integeroxidationstates

Oxidation-Reduction•  Assignoxidationstatestothefollowing:a)  Li2O

b)  HNO3c)  Cr2O7

2-

RedoxReactions

Oxidation-Reduction•  Oxidation–involvesthelossofelectrons•  Reduction–involvesthegainofelectrons•  LEOthelionsaysGER

Oxidation-Reduction•  Whenlookingatreactions,wecanassignoxidationstatestoseewhichsubstanceisoxidized,andwhichisreduced

Ca(s)+2H2O(l)àCa(OH)2(aq)+H2(g)

Oxidized(loste-s)

Reduced(gainede-s)

BalancingRedoxReactions

Balancing1.  Assignoxidationstates–  Focusonelementsthatarechanging

2.  Identifythesubstancesbeingoxidizedandreduced

Ca(s)+H2O(l)àCa(OH)2(aq)+H2(g)

Oxidized(loste-s)

Reduced(gainede-s)

0

+1

+2

0

Balancing3.  Separateintotwohalf-reactions–  Wewillbalancetheseseparatelyandthen

recombinethem

Ca(s)+H2O(l)àCa(OH)2(aq)+H2(g)

Oxidized(loste-s)

Reduced(gainede-s)

0

+1

+2

0

Ca(s)àCa(OH)2(aq) H2O(l)àH2(g)

Balancing4.  Balanceallatoms(likewenormallywould)

exceptforHandO5.  BalanceHandO•  ForACIDICsolutions:– BalanceObyaddingH2Ototheoppositeside– BalancetheH’sbyaddingH+totheoppositeside

•  ForBASICsolutions:– BalanceObyaddingH2Ototheoppositeside– BalanceHbyaddingdoublethenecessaryamountofH2Ototheoppositeside,andthenanequivalentamountofOH-totheotherside

Balancing6.  Balancechargesbyaddingthenecessary

numberofelectronstothemore(+)side7.  Makethenumberofe-sequalforbothhalf-

reactionsbymultiplyingoneorbothbyasmallwholenumber

8.  Addthetworxnstogether.Cancelliketermsasneeded.

Balancing•  Ex/BalanceAl(s)+Cu2+(aq)àAl3+(aq)+Cu(s)

Oxidized(loste-s)

Reduced(gainede-s)0

+2

+3

0Al(s)+Cu2+(aq)àAl3+(aq)+Cu(s)

Balancing•  Ex/BalanceAl(s)+Cu2+(aq)àAl3+(aq)+Cu(s)

OxidationHalf-RxnAl(s)àAl3+(aq)

Al(s)àAl3+(aq)+3e-2Al(s)à2Al3+(aq)+6e-

ReductionHalf-RxnCu2+(aq)àCu(s)

2e-+Cu2+(aq)àCu(s)6e-+3Cu2+(aq)à3Cu(s)

2Al(s)+3Cu2+(aq)à2Al3+(aq)+3Cu(s)

Balancing•  Let’strythisone…

MnO4-(aq)+Fe2+(aq)àFe3+(aq)+Mn2+(aq)

Oxidation:Fe2+àFe3+Reduction:MnO4-àMn2+

Balancing•  Trythisone:

Cl2(g)+H2O2(aq)àCl-(aq)+O2(g)

Let’sTryBasicSolutionRxns•  I-(aq)+MnO4

-(aq)àI2(aq)+MnO2(s)

OxidationHalf-RxnI-(aq)àI2(aq)2I-(aq)àI2(aq)

2I-(aq)àI2(aq)+2e-6I-(aq)à3I2(aq)+6e-

ReductionHalf-RxnMnO4

-(aq)àMnO2(s)

MnO4-(aq)àMnO2(s)+2H2O(l)

MnO4

-(aq)+4H2O(l)àMnO2(s)+2H2O(l)+4OH-

(aq)

MnO4-(aq)+4H2O(l)+3e-àMnO2(s)+2H2O(l)+4OH-

(aq)

2MnO4-(aq)+8H2O(l)+6e-à2MnO2(s)+4H2O(l)+8OH-

(aq)

4H2O(l)+2MnO4

-(aq)+6I-(aq)à2MnO2(s)+8OH-

(aq)+3I2(aq)

Balancing•  Trythisone:

Hg2+(aq)+H2(g)àHg(l)+H2O(l)

CW•  BalancingWS– #3ShouldbeSO3

2-(aq)+MnO4

-(aq)àSO4

2-(aq)+Mn2+(aq)

•  GR