Consider the reaction below:
The two elements involved, iron and chlorine, each change oxidation state; iron from +2 to +3, chlorine from 0 to &minus;1. There are then effectively two half reactions occurring. These changes can be represented in formulas by inserting appropriate electrons into each half reaction:
Given two half reactions it is possible, with knowledge of appropriate electrode potentials, to arrive at the complete (original) reaction the same way. The decomposition of a reaction into half reactions is key to understanding a variety of chemical processes. For example, in the above reaction, it can be shown that this is a redox reaction in which Fe is oxidised, and Cl is reduced. Note the transfer of electrons from Fe to Cl. Decomposition is also a way to simplify the balancing of a chemical equation. A chemist can atom balance and charge balance one piece of an equation at a time.
For example:
*  becomes 
* is added to 
* and finally becomes 
It is also possible and sometimes necessary to consider a half reaction in either basic or acidic conditions, as there may be an acidic or basic electrolyte in the redox reaction. Due to this electrolyte it may be more difficult to satisfy the balance of both the atoms and charges. This is done by adding , and/or  to either side of the reaction until both atoms and charges are balanced.
Consider the half reaction below:
, and  can be used to balance the charges and atoms in basic conditions, as long as it is assumed that the reaction is in water.
Again consider the half reaction below:
, and  can be used to balance the charges and atoms in acidic conditions, as long as it is assumed that the reaction is in water.
Notice that both sides are both charge balanced and atom balanced.
Often there will be both  and  present in acidic and basic conditions but that the resulting reaction of the two ions will yield water,  (shown below):