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Chapter Outline





17.1
17.2
17.3
17.4
17.5




17.6
17.7
17.8
17.9
Redox Chemistry Revisited
Electrochemical Cells
Standard Potentials
Chemical Energy and Electrical Work
A Reference Point: The Standard Hydrogen
Electrode
The Effect of Concentration on Ecell
Relating Battery Capacity to Quantities of Reactants
Electrolytic Cells and Rechargeable Batteries
Fuel Cells
19 - 1
Electrochemical (Galvanic or Voltaic) Cells
The difference in electrical potential
between the anode and cathode is called:
e-
e-
• cell voltage or potential (Volts)
•electromotive force (E)
Cell Diagram
Zn (s) + Cu2+ (aq)
Cu (s) + Zn2+ (aq)
[Cu2+] = 1 M & [Zn2+] = 1 M
< Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s) >
Anode (+)
Cathode (-)
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The circuit is completed via
the “Salt Bridge”
e-
e-
Common salt bridge = Na2SO4
SO42- Na+
gaining (+)
charge
losing (+)
charge
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Chapter Outline





17.1
17.2
17.3
17.4
17.5




17.6
17.7
17.8
17.9
Redox Chemistry Revisited
Electrochemical Cells
Standard Potentials
Chemical Energy and Electrical Work
A Reference Point: The Standard Hydrogen
Electrode
The Effect of Concentration on Ecell
Relating Battery Capacity to Quantities of Reactants
Electrolytic Cells and Rechargeable Batteries
Fuel Cells
19 - 5
The cell voltage is the difference in potential
between the cathode and the anode:
0
0 = E0
Ecell
cathode - Eanode
cathode: Cu2+(aq) + 2e-  Cu(s) Ecathode
anode:
Zn2+(aq) + 2e-  Zn(s) Eanode
Ecathode and Eanode are called Standard Reduction
Potentials; measured and tabulated (Table A6.1)
Measured under Standard Conditions =
1 atm, 1.0 M, 298 K
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0
0 = E0
Ecell
cathode - Eanode
e-
e-
Cu2+(aq) + 2e-  Cu(s)
Zn2+(aq) + 2e-  Zn(s)
Sign conventions:
E > 0 spontaneous
E = 0 equilibrium
E < 0 nonspontaneous
Standard Reduction Potentials at 298 K
F2(g) + 2 e-  2 F-(aq)
+2.87 V
2 H3O+(aq) + 2 e-  H2(g) + 2 H2O(l)
Li(s) + e-  Li+(aq)
0.00 V
-3.045 V
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•
reactions are written as a
reduction: E0red
•
the more positive E0 is, the
greater the tendency for the
substance to be reduced
•
strong oxidizing agents at
the top
•
strong reducing agents at
the bottom
•
the half-cell reactions are
reversible
•
the sign of E0 changes
when the reaction is
reversed = “oxidizing
potential”
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The Zinc-Air Battery
Cell potentials when the number of electrons
transferred is different for each half reaction Changing the stoichiometric coefficients of a half-cell reaction
does not change the value of E0
Eo = -1.25 V*
an
( Zn(s) + 2 OH-(aq)  ZnO(s) + H2O(l) + 2e-
Cathode: O2(g) + 2H2O(l) + 4e-  4OH-(aq)
Net:
(
Anode:
2
Eocath = 0.401 V
2 Zn(s) + O2(g)  2 ZnO(s)
0
0 = E0
Ecell
cathode - Eanode
= 0.401 - (-1.25) = 1.65 V
*Eoanode is obtained by reversing the reaction and looking up Eored ZnO(s) + H2O(l) + 2 e-  Zn(s) + 2 OH-(aq)
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Chapter Outline





17.1
17.2
17.3
17.4
17.5




17.6
17.7
17.8
17.9
Redox Chemistry Revisited
Electrochemical Cells
Standard Potentials
Chemical Energy and Electrical Work
A Reference Point: The Standard Hydrogen
Electrode
The Effect of Concentration on Ecell
Relating Battery Capacity to Quantities of Reactants
Electrolytic Cells and Rechargeable Batteries
Fuel Cells
19 - 13
Chemical Energy and Electrical Work
 Gcell = welec = -C Ecell
• welec = work done by the cell
• C = charge (coulombs)
• Volts = J/C
 G = -nFEcell
• Faraday constant (F) is 9.65 × 104 C/(mol e-)
• n = number of moles of electrons
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“Button” Batteries
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Chapter Outline





17.1
17.2
17.3
17.4
17.5




17.6
17.7
17.8
17.9
Redox Chemistry Revisited
Electrochemical Cells
Standard Potentials
Chemical Energy and Electrical Work
A Reference Point: The Standard Hydrogen
Electrode
The Effect of Concentration on Ecell
Relating Battery Capacity to Quantities of Reactants
Electrolytic Cells and Rechargeable Batteries
Fuel Cells
19 - 17
A Reference Point: The Standard
Hydrogen Electrode
2 H3O+(aq) + 2 e-  H2(g) + 2 H2O(l)
0.00 V
|| H+ (1.00 M) | H2(g, 1.00 atm) | Pt
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Chapter Outline





17.1
17.2
17.3
17.4
17.5




17.6
17.7
17.8
17.9
Redox Chemistry Revisited
Electrochemical Cells
Standard Potentials
Chemical Energy and Electrical Work
A Reference Point: The Standard Hydrogen
Electrode
The Effect of Concentration on Ecell
Relating Battery Capacity to Quantities of Reactants
Electrolytic Cells and Rechargeable Batteries
Fuel Cells
19 - 21
The Effect of Concentration on Ecell
The Nernst Equation
for aA + bB = cC + dD, from Thermodynamics we know -
ΔG  ΔG o  RT ln Q and ΔG  nFE
 nFE  nFE o  RT ln Q
E  Eo 
E  Eo 
E  Eo 
RT
ln Q
nF
2.303RT
log Q
nF
Converting to base
10 log
0.0592
log Q at 25o C
n
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The Lead-Acid Battery
The Lead-Acid Battery
Cathode:
Anode:
0
0 = E0
Ecell
cathode - Eanode
= 1.685 - (-0.356) = 2.041 V
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The Lead-Acid Battery
Both cells kept at 4.5 M H2SO4
Ecell = 2.041V -
0.0592
log
2
1
2
2
[4.5 M] [4.5 M]
= 2.041 V -.(-0773) V = 2.1 V
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Eo and K
aA + bB = cC + dD
E  Eo 
at equilibrium -
0.0592
log Q at 25o C
n
0  Eo 
Eo 
0.0592
log Q
n
K
0.0592
log K
n
nEo
K  10
0.0592
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