In the field of chemistry, the term reduction is used to describe a process involving a gain of electrons by an atom, ion, or molecule. In a chemical reaction where one reactant is reduced (i.e., gain of electrons), one or more of the other reactants undergo oxidation (i.e., loss of electrons).
An example of a chemical reaction involving oxidation and reduction is the reaction of hydrogen with chlorine (shown below).
The two reactants on the left side of the chemical equation are both elements with an oxidation number equal to zero. But when these elements react to form the product, , the hydrogen has a higher oxidation number () and the chlorine has a lower oxidation number (). In this reaction, the hydrogen has undergone oxidation while the chlorine has undergone reduction.
When writing down an electrochemical half reaction, it is generally quite clear when the half reaction is written as a reduction half reaction. If the electrons appear as reactants (rather than as products) in the half reaction, then the half reaction has been written as an reduction process (see example below).
In the example above, the iron(III) cation gains an electron to become iron(II); thus, the iron(III) cation is reduced.