It is convenient to separate an overall redox reaction into two half-reactions (see previous page) because the reduction and oxidation processes which make up the overall reaction of an electrochemical cell are separated in space.
Reduction occurs in one electrode compartment and oxidation in the other, and the process occurring in a given compartment is described by one or other of the half-reactions that make up the overall cell reaction.
As the cell reaction progresses, electrons are released by the oxidation process which is occurring in one of the compartments. The electrode in this compartment must be connected to the other electrode by some form of conducting wire (which may or may not be linked to a circuit to do electrical work) or the circuit will be incomplete and no current will flow. Thus the electrons released by oxidation in one compartment can travel through this wire to the other electrode, where they can bring about the reduction reaction happening in that compartment.
This concept can be simply illustrated by a cell consisting of two metal/metal ion electrode systems, each system being composed of a metal electrode dipped in an aqueous solution of a salt of that metal.
The half reaction for this type of electrode system indicates that in one compartment, containing electrode A, the metal ions receive electrons (from the electrode) being reduced to form neutral metal atoms which are then deposited on the electrode. Electrode A is replenished with electrons from the other electrode in the cell (B), to which it is connected by a conducting wire which will complete the circuit. The reaction occurring at B will be the reverse, oxidation process, where the metal atoms in the electrode lose electrons to the electrode to form metal ions which go into solution. The excess of electrons at electrode B gives rise to a potential that causes the electrons to move through the wire to electrode A, which has been rendered electron-deficient by giving up electrons to ions in solution.
Note that it is not possible to have a spontaneous cell reaction in either direction if the two electrode compartments are identical in every respect. (Clearly, in such a case there is no driving force for reaction, as neither electrode has the greater tendency to be oxidised or reduced.) It is not, however, necessary for the two compartments to contain different redox couples. A reaction may be observed if the two compartments are identical except for the concentration of the electrolyte – such a cell is called an electrolyte concentration cell. In this case a driving force for reaction exists until the two concentrations have equalised.