A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.

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.

See also: Electrode potential