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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