Before a discussion of the cell reaction, it is necessary to introduce the notation used to represent electrochemical cells as cell diagrams.
Phase boundaries are represented by a vertical line, | (i.e. such a line is placed between species in different phases, such as a solid electrode and aqueous ions).
Different species in the same phase (as would be encountered in a redox electrode, where the electrolyte contains ions of an element in two different oxidation states) are separated by a comma.
By convention, a cell diagram is written with the electrodes on the outside. In electrode systems which use an inert metal electrode, this is placed on the outside with the more reduced member of the redox couple next to it.
A double vertical line, || , indicates a salt bridge or equivalent way of linking the two compartments.
The conventions are best illustrated with an example:
The Cu2+/Cu redox couple (corresponding to the reaction Cu2+(aq) + 2e– → Cu (s) ) is represented by
Cu (s) | Cu2+(aq)
The Fe3+/Fe2+ redox couple (corresponding to the reaction Fe3+(aq) + e– → Fe2+(aq) ) is represented:
Pt | Fe2+(aq) , Fe3+(aq)
When combined into an electrochemical cell with the copper system as the left hand electrode, the overall representation is:
Cu (s) | Cu2+(aq) || Fe3+(aq) , Fe2+(aq) | Pt
The cell reaction is the overall reaction which takes place in the cell, written on the assumption that the right hand electrode is the cathode, i.e. assuming that the spontaneous reaction is the one in which reduction is occurring in the right-hand compartment.
(It is simple, from a knowledge of the standard potentials of the two electrodes, to predict whether the right hand electrode is in fact the cathode. If it is, the reaction is spontaneous as written. If the left-hand electrode turns out to be the cathode, then the reverse of the cell reaction is spontaneous.)
To write the cell reaction corresponding to a cell diagram, the right-hand half reaction is written as a reduction, and the left-hand half-reaction, written as an oxidation, is added to it. (This is exactly the same as subtracting the left-hand equation written as a reduction, which is the formally correct procedure.)
Thus the cell reaction for the Cu2+/Cu and Fe3+/Fe2+ cell described above is as follows:
2Fe3+(aq) + Cu (s) → 2Fe2+(aq) + Cu2+(aq)
Note that without a knowledge of the standard electrode potentials of the two electrodes, it is not possible to tell whether the reaction as written is spontaneous or not.