Page 143 - Instant notes

Electrochemical cells 129

Fig. 1. Examples of (a) an electrolytic
cell incorporating a salt bridge; (b) a
galvanic cell incorporating a porous
frit.

An alternative is to drive the reaction in the reverse or non-spontaneous direction. This is
achieved by connecting the electrodes to an external voltage supply and applying a
voltage to reverse the driving force for the flow of electrons, forcing them to move in the
opposite direction (Fig. 1a). This is often the method by which reactive or relatively
unstable species are made. This is called an electrolytic cell and by definition the anode
in a galvanic cell is the cathode in an electrolytic cell and vice versa. Again, transfer of
ions from the salt bridge or glass frit maintains electrical neutrality in the half-cells
during reaction.

Other half-cell reactions
The metal-insoluble salt electrode consists of a metal M coated with a porous insoluble
−
salt MX in a solution of X . A good example is the silver/silver chloride electrode (Fig.
2a) for which the half-cell reaction is , where the
reduction of solid silver chloride produces solid silver and releases chloride ion into
solution.

Fig. 2. (a) The silver/silver chloride
3+
half-cell; (b) the ferric (Fe )/ferrous
2+
(Fe ) half-cell; (c) the
fluorine/fluoride ion half-cell.