Lecture 39 Electrochemistry III

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Lecture 39 - Electrochemistry III
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Review

• Shorthand cell notation
• Reduction potentials
• Standard cell potential
• DGo -nFEcello

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Most cells are not at standard conditions!

• Qualitatively - use Le Chateliers principle
• e.g. 2 Al 3 Mn2 ? 2 Al3 3 Mn
• Eocell 0.48 V
• Under standard conditions, Al3 1.0 M
  Increase Al3(aq) to 2.0 M
• equilm shifts LEFT
• Ecell lt Eocell

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Most cells are not at standard conditions!

• Qualitatively - use Le Chateliers principle
• e.g. 2 Al 3 Mn2 ? 2 Al3 3 Mn
• Eocell 0.48 V
• Under standard conditions, Mn2 1.0 M
  Increase Mn2(aq) to 2.0 M
• equilm shifts RIGHT
• Ecell gt Eocell

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Most cells are not at standard conditions!

• Quantitatively
• DG DGo RT ln (Q)
• -nFE -nFEo RT ln (Q)

Nernst equation
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Most cells are not at standard conditions!
(at 25oC)
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for example,

• 2 Al(s) 3 Mn2(aq) 2 Al3(aq) 3 Mn(s)
• suppose Mn2(aq) 0.25 M
• and Al3(aq) 2.50 M

(n6)
Find E At 25oC
2.502
Q

400
0.253
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E Eo -
0.48 -
0.45 V
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Temperature also affects Ecell
Eo 0.46 V
suppose Cu2 0.001 M, Ag 1.0 M
Find E at 25oC and at 100oC
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Temperature also affects Ecell
0.001

0.001
1.02
0.55 V
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Temperature also affects Ecell
8.314(373)
E100C 0.46 -
ln(0.001)
2(96500)
0.57 V
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Useful Galvanic Cells

• A. Primary batteries
• (irreversible reactions, disposable)
• B. Secondary batteries
• (reversible reactions, rechargeable)

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A. Primary Batteries

• The dry cell
• (Leclanché cell, carbon battery)

C rod
MnO2, NH4Cl, ZnCl2, H2O
Zn can
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The Dry Cell

• Zn(s) Zn2(aq) 2 e-
• and
• 2 MnO2(s) 2 H(aq) 2 e- Mn2O3(s) H2O(l)
• Eocell 1.55 V

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The Dry Cell

• Two things kill the battery
• 1. The anode is destroyed
• Zn (s) 2 H(aq) Zn2(aq) H2(g)
• (no interaction with cathode, thus no usable
  current)

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The Dry Cell

• Two things kill the battery
• 2. The Nernst equation

as the reaction proceeds, Q increases, Ecell
decreases
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Alkaline Cells

• Zn(s) 2 OH- ZnO(s) H2O 2 e-
• 2 MnO2(s) H2O(l) 2 e- Mn2O3(s)
• 2 OH-(aq)
• Eo 1.5 V
• Zn does not corrode as fast at high pH

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B. Secondary (Rechargeable) Batteries
Ni-Cd Batteries
Ni(s) NiO(OH)(s) rod
KOH(s), Cd(OH)2(aq), Ni(OH)2(aq)
Cd can
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B. Secondary (Rechargeable) Batteries
Ni-Cd Batteries

• Cd 2 OH-(aq) Cd(OH)2(s) 2 e-
• and
• 2 NiO(OH)(s) 2 H2O(l) 2 e-
• 2 Ni(OH)2(s) 2 OH-(aq)
• Ecell 1.2 - 1.4 V

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B. Secondary (Rechargeable) Batteries
Ni-Cd Batteries

• big advantage
• products stick to electrodes
• recharging is possible !!
• BUT - Cd is toxic

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B. Secondary (Rechargeable) Batteries
NiMH (Nickel Metal Hydride) Batteries

• MH(s) OH- ? M(s) H2O e-
• NiO(OH)s H2O ? Ni(OH)2(s) OH-
• M Ni (V, Ti, Zr, Cr, Co, Fe)
• (No Cd!)

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Resistance, R

• R E / i

voltage
current
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Resistance, R

• R E / i

voltage
current
Power, P
P E i i2R