1 cell potential, e electrons are “driven” from anode to cathode by an electromotive force or...
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11
CELL POTENTIAL, ECELL POTENTIAL, E
• Electrons are “driven” from anode to cathode by Electrons are “driven” from anode to cathode by an electromotive force or an electromotive force or emfemf..
• For Zn/Cu cell, this is indicated by a voltage of For Zn/Cu cell, this is indicated by a voltage of 1.10 V at 25 ˚C and when [Zn1.10 V at 25 ˚C and when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.
Zn and ZnZn and Zn2+2+,,anodeanode
Cu and CuCu and Cu2+2+,,cathodecathode
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons1.10 V1.10 V
1.0 M1.0 M 1.0 M1.0 M
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CELL POTENTIAL, CELL POTENTIAL, EE
• For Zn/Cu cell, For Zn/Cu cell, potentialpotential is is +1.10 V+1.10 V at 25 ˚C at 25 ˚C and when [Znand when [Zn2+2+] and [Cu] and [Cu2+2+] = 1.0 M.] = 1.0 M.
• This is the This is the STANDARD CELL STANDARD CELL POTENTIAL, EPOTENTIAL, Eoo
• ——a quantitative measure of the tendency of a quantitative measure of the tendency of reactants to proceed to products when all reactants to proceed to products when all are in their standard states at 25 ˚C. are in their standard states at 25 ˚C.
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
33
Calculating Cell VoltageCalculating Cell Voltage
• Balanced half-reactions can be added Balanced half-reactions can be added together to get overall, balanced together to get overall, balanced equation. equation.
Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)
Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e-CuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s)----------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s)(aq) + Cu(s)
If we know EIf we know Eoo for each half-reaction, we for each half-reaction, we could get Ecould get Eoo for net reaction. for net reaction.
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CELL POTENTIALS, ECELL POTENTIALS, Eoo
Can’t measure 1/2 reaction Eo directly. Therefore, measure it relative to a
STANDARD HYDROGEN CELL, SHE.STANDARD HYDROGEN CELL, SHE.
2 H2 H++(aq, 1 M) + 2e- <----> H(aq, 1 M) + 2e- <----> H22(g, 1 atm)(g, 1 atm)2 H2 H++(aq, 1 M) + 2e- <----> H(aq, 1 M) + 2e- <----> H22(g, 1 atm)(g, 1 atm)
EEoo = 0.0 V = 0.0 V
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Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V
Zn/Zn2+ half-cell hooked to a SHE.Eo for the cell = +0.76 V
Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Negative Negative electrodeelectrode
Supplier Supplier of of
electronselectrons
Acceptor Acceptor of of
electronselectrons
Positive Positive electrodeelectrode
2 H2 H++ + 2e- --> H + 2e- --> H22
ReductionReductionCathodeCathode
Zn --> ZnZn --> Zn2+2+ + 2e- + 2e- OxidationOxidation
AnodeAnode
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Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Volts
ZnH2
Salt Bridge
Zn2+ H+
Zn Zn2+ + 2e- OXIDATION ANODE
2 H+ + 2e- H2REDUCTIONCATHODE
- +
Overall reaction is reduction of HOverall reaction is reduction of H++ by Zn metal. by Zn metal.
Zn(s) + 2 HZn(s) + 2 H++ (aq) --> Zn (aq) --> Zn2+2+ + H + H22(g)(g) E Eoo = +0.76 V = +0.76 V
Therefore, Therefore, EEoo for for Zn ---> ZnZn ---> Zn2+2+ (aq) + 2e- (aq) + 2e- is is +0.76 V+0.76 V
Zn is a Zn is a (better) (poorer)(better) (poorer) reducing agent than H reducing agent than H22..
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Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell
EEoo = +0.34 V = +0.34 V
Acceptor Acceptor of of
electronselectrons
Supplier Supplier of of
electronselectrons
CuCu2+2+ + 2e- --> Cu + 2e- --> CuReductionReductionCathodeCathode
HH22 --> 2 H --> 2 H++ + 2e- + 2e-
OxidationOxidationAnodeAnode
PositivePositive NegativeNegative
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Cu/CuCu/Cu2+2+ and H and H22/H/H++ Cell Cell
Overall reaction is reduction of CuOverall reaction is reduction of Cu2+2+ by H by H22 gas. gas.
CuCu2+2+ (aq) + H (aq) + H22(g) ---> Cu(s) + 2 H(g) ---> Cu(s) + 2 H++(aq)(aq)
Measured Measured EEoo = +0.34 V = +0.34 V
Therefore, Therefore, EEoo for Cu for Cu2+2+ + 2e- ---> Cu + 2e- ---> Cu is is
Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
Volts
CuH2
Salt Bridge
Cu2+ H+
Cu2+ + 2e- Cu REDUCTION CATHODE
H2 2 H+ + 2e-OXIDATION ANODE
-+
+0.34 V+0.34 V
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Zn/Cu Electrochemical CellZn/Cu Electrochemical Cell
Zn(s) ---> ZnZn(s) ---> Zn2+2+(aq) + 2e-(aq) + 2e- EEoo = +0.76 V = +0.76 VCuCu2+2+(aq) + 2e- ---> Cu(s)(aq) + 2e- ---> Cu(s) EEoo = +0.34 V = +0.34 V------------------------------------------------------------------------------------------------------------------------------CuCu2+2+(aq) + Zn(s) ---> Zn(aq) + Zn(s) ---> Zn2+2+(aq) + Cu(s) (aq) + Cu(s)
EEoo (calc’d) = +1.10 V (calc’d) = +1.10 V
Cathode, Cathode, positive, positive, sink for sink for electronselectrons
Anode, Anode, negative, negative, source of source of electronselectrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons ++
1111
Uses of EUses of Eoo Values ValuesUses of EUses of Eoo Values Values
• Organize half-Organize half-reactions by reactions by relative ability to relative ability to act as oxidizing act as oxidizing agentsagents
• Table 20.1Table 20.1
• Use this to predict Use this to predict cell potentials and cell potentials and direction of redox direction of redox reactions.reactions.
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
Zn
Zn2+ ions
Cu
Cu2+ ions
wire
saltbridge
electrons
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TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS
TABLE OF STANDARD TABLE OF STANDARD REDUCTION POTENTIALSREDUCTION POTENTIALS
2
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H 0.00
Zn 2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
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Potential Ladder for Reduction Half-ReactionsPotential Ladder for Reduction Half-ReactionsFigure 20.11Figure 20.11
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Using Standard Potentials, EUsing Standard Potentials, Eoo
Table 20.1Table 20.1
• Which is the best oxidizing agent:
O2, H2O2, or Cl2? _________________
• Which is the best reducing agent:
Hg, Al, or Sn? ____________________
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Standard Redox Potentials, EStandard Redox Potentials, Eoo
Cu2+ + 2e- Cu +0.34
+2 H + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
Northwest-southeast rule:Northwest-southeast rule: product-favored product-favored reactions occur between reducing agent at reactions occur between reducing agent at
southeast corner (anode) and oxidizing agent southeast corner (anode) and oxidizing agent at northwest corner (cathode). at northwest corner (cathode).
Any substance on the right will reduce any Any substance on the right will reduce any substance higher than it on the left.substance higher than it on the left.
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Standard Redox Potentials, EStandard Redox Potentials, Eoo
Any substance on the right Any substance on the right will reduce any substance will reduce any substance higher than it on the left.higher than it on the left.
• Zn can reduce HZn can reduce H++ and and CuCu2+2+..
• HH22 can reduce Cu can reduce Cu2+2+ but but
not Znnot Zn2+2+
• Cu cannot reduce HCu cannot reduce H++ or or ZnZn2+2+..
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
Eo (V)
Cu2+ + 2e- Cu +0.34
2 H+ + 2e- H2 0.00
Zn2+ + 2e- Zn -0.76
oxidizingability of ion
reducing abilityof element
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Using Standard Potentials, EUsing Standard Potentials, Eoo
Table 20.1Table 20.1
• In which direction do the following reactions In which direction do the following reactions
go?go?
• Cu(s) + 2 AgCu(s) + 2 Ag++(aq) ---> Cu(aq) ---> Cu2+2+(aq) + 2 Ag(s)(aq) + 2 Ag(s)
• 2 Fe2 Fe2+2+(aq) +(aq) + Sn2+(aq) ---> 2 Fe3+(aq) + Sn(s)
• What is Eonet for the overall reaction?
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Standard Redox Standard Redox Potentials, EPotentials, Eoo
E˚E˚netnet = “distance” from “top” half-reaction = “distance” from “top” half-reaction
(cathode)(cathode) to “bottom” half-reaction to “bottom” half-reaction (anode)(anode)
E˚E˚netnet = E˚ = E˚cathodecathode - E˚ - E˚anodeanode
EEoonetnet for Cu/Ag+ reaction = +0.46 V for Cu/Ag+ reaction = +0.46 V
2020
Volts
Cd Salt Bridge
Cd2+
Fe
Fe2+
Volts
Cd Salt Bridge
Cd2+
Fe
Fe2+
Cd --> CdCd --> Cd2+2+ + 2e- + 2e-oror
CdCd2+2+ + 2e- --> Cd + 2e- --> Cd
Fe --> FeFe --> Fe2+2+ + 2e- + 2e-oror
FeFe2+2+ + 2e- --> Fe + 2e- --> Fe
EEoo for a Voltaic Cell for a Voltaic Cell
All ingredients are present. Which way does All ingredients are present. Which way does reaction proceed?reaction proceed?
2121
From the table, you see From the table, you see
•• Fe is a better reducing Fe is a better reducing agent than Cdagent than Cd
•• CdCd2+2+ is a better is a better oxidizing agent than oxidizing agent than FeFe2+2+
Volts
Cd Salt Bridge
Cd2+
Fe
Fe2+
Volts
Cd Salt Bridge
Cd2+
Fe
Fe2+
EEoo for a Voltaic Cell for a Voltaic Cell
Overall reactionOverall reactionFe + CdFe + Cd2+2+ ---> Cd + Fe ---> Cd + Fe2+2+
EEoo = E˚ = E˚cathodecathode - E˚ - E˚anodeanode
= (-0.40 V) - (-0.44 V) = (-0.40 V) - (-0.44 V) = +0.04 V= +0.04 V
2222More About More About Calculating Cell VoltageCalculating Cell Voltage
Assume IAssume I-- ion can reduce water. ion can reduce water.
2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2
2 H2O + 2e- ---> H2 + 2 OH- Cathode2 I- ---> I2 + 2e- Anode-------------------------------------------------2 I- + 2 H2O --> I2 + 2 OH- + H2
Assuming reaction occurs as written, Assuming reaction occurs as written,
E˚E˚netnet = E˚ = E˚cathodecathode - E˚ - E˚anodeanode
= (-0.828 V) - (+0.535 V) = = (-0.828 V) - (+0.535 V) = -1.363 V-1.363 V
Minus E˚ means rxn. occurs in opposite Minus E˚ means rxn. occurs in opposite
directiondirection
2424
Michael FaradayMichael Faraday1791-18671791-1867
Originated the terms anode, Originated the terms anode, cathode, anion, cation, cathode, anion, cation, electrode.electrode.
Discoverer of Discoverer of
• electrolysiselectrolysis
• magnetic props. of mattermagnetic props. of matter
• electromagnetic inductionelectromagnetic induction
• benzene and other organic benzene and other organic chemicalschemicals
Was a popular lecturer.Was a popular lecturer.
2525
EEoo and ∆G and ∆Goo
EEoo is related to ∆G is related to ∆Goo, the free , the free energy change for the reaction.energy change for the reaction.
∆∆GGoo = - n F E = - n F Eoo where F = Faraday constant where F = Faraday constant
= 9.6485 x 10= 9.6485 x 1044 J/V•molJ/V•mol
and n is the number of moles of and n is the number of moles of electrons transferredelectrons transferred
Michael FaradayMichael Faraday1791-18671791-1867
2626
EEoo and ∆G and ∆Goo
∆∆GGoo = - n F E = - n F Eoo
For a For a product-favoredproduct-favored reaction reaction
Reactants ----> ProductsReactants ----> Products
∆∆GGo o < 0 and so E < 0 and so Eo o > 0 > 0
EEoo is positive is positive
For a For a reactant-favoredreactant-favored reaction reaction
Reactants <---- ProductsReactants <---- Products
∆∆GGo o > 0 and so E > 0 and so Eo o < 0 < 0
EEoo is negative is negative
2727E at Nonstandard E at Nonstandard ConditionsConditions
• The The NERNST EQUATIONNERNST EQUATION• E = potential under nonstandard conditionsE = potential under nonstandard conditions
• n = no. of electrons exchangedn = no. of electrons exchanged
• ln = “natural log”ln = “natural log”
• If [P] and [R] = 1 mol/L, then E = E˚If [P] and [R] = 1 mol/L, then E = E˚
• If [R] > [P], then E is ______________ than E˚If [R] > [P], then E is ______________ than E˚
• If [R] < [P], then E is ______________ than E˚If [R] < [P], then E is ______________ than E˚
E EÞ - 0.0257 V
n ln
[Products][Reactants]
E EÞ - 0.0257 V
n ln
[Products][Reactants]