Consider the reaction below:
(1) Cl2 + 2Fe2+ -> 2Cl- + 2Fe3+
The two elements involved, iron and chlorine, each change oxidation state - iron from 2+ to 3+, chlorine from 0 to 1-. There are then effectively two half-reactions occurring. These changes can be represented in formulas by inserting appropriate electrons in to each half-reaction:
(3) Fe2+ -> Fe3+ + e- (4) Cl2 + 2e- -> 2Cl-In the same way given two half-reactions it is possible, with knowledge of appropriate electrode potentials, to arrive at the full (original) reaction.