Redox reactions form an important class of reactions in which oxidation and reduction occur simultaneously.
Oxidation is defined as the addition of oxygen or electronegative element to a substance or removal of hydrogen or electropositive element from a substance.
Reduction is defined as the removal of oxygen or electronegative element from a substance or addition of hydrogen or electropositive element to a substance.
Oxidation: Loss of electron(s) by any species.
Reduction: Gain of electron(s) by any species.
Oxidising agent: Acceptor of electron(s).
Reducing agent: Donor of electron(s).
Oxidation number denotes the oxidation state of an element in a compound ascertained according to a set of rules formulated on the basis that electron pair in a covalent bond belongs entirely to more electronegative element.
Redox reactions are classified into four categories: combination, decomposition, displacement and disproportionation reactions. In a disproportionation reaction an element in one oxidation state is simultaneously oxidised and reduced.
The redox reactions find wide applications in the study of electrode processes and cells.