How do you use the redox reaction to generate energy? The answer is the development of Electrochemical Cells of two types:
Electrochemical cells that use an oxidation-reduction reaction to generate an electric current are known as galvanic or voltaic cells. Because the potential of these cells to do work by driving an electric current through a wire is measured in units of volts, we will refer to the cells that generate this potential from now on as voltaic cells.
In other electrochemical cells an externally supplied electric current is used to drive a chemical reaction which would not occur spontaneously. Such cells are called electrolytic cells.
In an electrochemical cell, an electric potential is created between two dissimilar metals. This potential is a measure of the energy per unit charge which is available from the oxidation/reduction reactions to drive the reaction. It is customary to visualize the cell reaction in terms of two half-reactions, an oxidation half-reaction and a reduction half-reaction.
Reduced species à oxidized species + ne- Oxidation at anode
(George says remember this because both words start with a vowel)
Oxidized species + ne- à reduced species Reduction at cathode
The electromotive force (EMF) is the maximum potential difference between two electrodes of a voltaic cell. This quantity is related to the tendency for an element, a compound or an ion to acquire (i.e. gain) or release (loss) electrons. For example, the maximum potential between Zn and Cu of the cell above.
Zn (s) | Zn2+ (1 M) || Cu2+ (1 M) | Cu (s)
has been measured to be 1.100 V. A concentration of 1 M in an ideal solution is defined as the standard condition, and 1.100 V is thus the standard electromotive force, DE°, or standard cell potential for the Zn-Cu voltaic cell.
The standard cell potential, DE°, of a voltaic cell can be evaluated from the standard reduction potentials of the two half cells E°. The reduction potentials are measured against a standard hydrogen electrode (SHE):
Example: What is the potential for the cell: Zn | Zn2+(1.0 M) || Cu2+(1.0 M) | Cu
Solution
From a table of standard reduction potentials we have the following values
Cathode Cu2+ + 2e- à Cu . . . E° = 0.337
Anode Zn à Zn2+ + 2e- . . . E°= - 0.763
DE° = E°cathode - E°anode = 0.337 – (-0.763) = 1.100 V
Copper is the Cathode because it is the stronger oxidizing agent (electron withdrawing agent) as can be seen from its position relative to Zinc in a table of standard reduction potentials.
Problems: For the following electrochemical cells calculate the cell potential and write out the relevant half reactions for the cell. If no concentration given, assume standard conditions. Is the cell spontaneous as written?