Chapter Outline - WOU Homepage · 17.3 Standard Potentials ... The Standard Hydrogen Electrode ... 19 - 1 Chapter Outline Electrochemical (Galvanic or Voltaic) Cells

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17.1 Redox Chemistry Revisited

17.2 Electrochemical Cells

17.3 Standard Potentials

17.4 Chemical Energy and Electrical Work

17.5 A Reference Point: The Standard Hydrogen

Electrode

17.6 The Effect of Concentration on Ecell

17.7 Relating Battery Capacity to Quantities of Reactants

17.8 Electrolytic Cells and Rechargeable Batteries

17.9 Fuel Cells

19 - 1

Chapter Outline

Electrochemical (Galvanic or Voltaic) Cells

The difference in electrical potential

between the anode and cathode is called:

• 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 (-)

e- e-

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The circuit is completed via

the “Salt Bridge”

Common salt bridge = Na2SO4

losing (+)

charge

Na+

gaining (+)

charge

SO42-

e- e-

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Electrode




17.9 Fuel Cells

19 - 5

Chapter Outline

The cell voltage is the difference in potential

between the cathode and the anode:

E0 = Ecathode - Eanode cell 0 0

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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Cu2+(aq) + 2e- Cu(s)

Zn2+(aq) + 2e- Zn(s)


Sign conventions:

E > 0 spontaneous

E = 0 equilibrium

E < 0 nonspontaneous

e- e-

Standard Reduction Potentials at 298 K

F2(g) + 2 e- 2 F-(aq) +2.87 V

Li(s) + e- Li+(aq) -3.045 V

2 H3O+(aq) + 2 e- H2(g) + 2 H2O(l) 0.00 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 -

Anode: Zn(s) + 2 OH-(aq) ZnO(s) + H2O(l) + 2e-

Cathode: O2(g) + 2H2O(l) + 4e- 4OH-(aq)

Changing the stoichiometric coefficients of a half-cell reaction

does not change the value of E0

ZnO(s) + H2O(l) + 2 e- Zn(s) + 2 OH-(aq)

*Eoanode is obtained by reversing the reaction and looking up Eo

red -

Eoan = -1.25 V*


Eocath = 0.401 V

Net: 2 Zn(s) + O2(g) 2 ZnO(s)

= 0.401 - (-1.25) = 1.65 V

(

(

2

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Electrode




17.9 Fuel Cells

19 - 13

Chapter Outline

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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Electrode




17.9 Fuel Cells

19 - 17

Chapter Outline

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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Electrode




17.9 Fuel Cells

19 - 21

Chapter Outline

The Effect of Concentration on Ecell

The Nernst Equation

nFEΔG and Qln RTΔGΔG o

for aA + bB = cC + dD, from Thermodynamics we know -

Qln RTnFEnFE o

Qln nF

RTEE o

C25at Q log n

0.0592EE oo

Converting to base

10 log Q log

nF

2.303RTEE o

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The Lead-Acid Battery


Anode:

Cathode:


= 1.685 - (-0.356) = 2.041 V

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Both cells kept at 4.5 M H2SO4

Ecell = 2.041V - 0.0592

2 log

1

[4.5 M]2

[4.5 M]2

= 2.041 V -.(-0773) V = 2.1 V

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Eo and K

Q log n

0.0592E0 o

K log n

0.0592Eo

aA + bB = cC + dD

C25at Q log n

0.0592EE oo

at equilibrium -

0.0592nEo

10K

K

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Chapter Outline - WOU Homepage · 17.3 Standard Potentials ... The Standard Hydrogen Electrode ... 19 - 1 Chapter Outline Electrochemical (Galvanic or Voltaic) Cells