ElectrochemistryOxidation-Reduction

Dr. Ron RusaySpring 2004

© Copyright 2004 R.J. Rusay© Copyright 2004 R.J. Rusay

Oxidation-Reduction

• Oxidation is the loss of electrons.• Reduction is the gain of electrons.• The reactions occur together. One does not

occur without the other.• The terms are used relative to the change in

the oxidation state of the reactant(s).

Oxidation State(Oxidation Number)

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Cu 2+ Cu (s)

H2 (g) 2 H +

0

0

+2 e-

- 2 e-

Use oxidation numbers to determine what is oxidized and what is reduced.

Number of electrons gained must equal the number of electrons lost.

- 2 e-

+2 e-

Voltaic CellsVoltaic Cells

Voltages of Some Common Voltaic Cells

Voltaic Cell Voltage (V)

Common alkaline battery 1.5

Lead-acid car battery (6 cells = 12 V) 2.0

Calculator battery (mercury) 1.3

Electric eel (~ 5000 cells in 6-ft eel = 750 V) 0.15

Nerve of giant squid (across cell membrane) 0.070

Automotive Application: the Discharge and Recharging of a Lead-Acid Battery

Lithium Solid-State Battery

Nickel-Cadmium(Nicad) Battery

Flow Batteries (Fuel Cells)

The Hydrogen Oxygen Fuel Cell

Anode (oxidation):H2 (g) + CO3

2-(l) H2O(g) + CO2 (g) + 2 e-

Cathode (reduction): 1/2 O2 (g) + CO2 (g) + 2 e- CO3

2-(l)

Overall (cell) reaction:H2 (g) + 1/2 O2 (g) H2O(g) Ecell = 1.2 V

Other Fuel Cells:2 NH3 (g) + 3/2 O2 (g) N2 (g) + 3 H2O(l)

N2H4 (g) + O2 (g) N2 (g) + 2 H2O (l)

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O(l)

Chemistry of Batteries

Comparison of Voltaic and Electrolytic Cells

Electrode

Cell Type G Ecell Name Process Sign

Voltaic < 0 > 0 Anode Oxidation -

Voltaic < 0 > 0 Cathode Reduction +

Electrolytic > 0 < 0 Anode Oxidation +

Electrolytic > 0 < 0 Cathode Reduction -

Electrochemical Reactions

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Electrolysis: Chrome Plating

Aluminum ProductionUsing an Electrolytic Cell

Do you recycle? It takes only 5% of the energy to produce a can from recycled aluminum vs. aluminum ore.