.The oxidation alone and the reduction alone are each called a half-reaction because two half-reactions always occur together to form a whole reaction. Oxidation originally implied a reaction with oxygen to form an oxide. Later, the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions.

In any given oxidation-reduction reaction, there are two half-reactions – oxidation half- reaction and reduction half-reaction. The sum of these two half-reactions is the oxidation- reduction reaction. The two elements involved, iron and chlorine, each change oxidation state; iron from +2 to +3, chlorine from 0 to -1.