Overview: This section reviews
reduction-oxidation (redox) reactions, a common type of chemical reaction.
Redox reactions are reactions in which one species is reduced and another is oxidized. Therefore the oxidation
state of the species involved must change. These reactions are important for a number of applications, including
energy storage devices (batteries), photographic processing, and energy production and utilization in living systems
Reduction: A process in which an atom gains an electron and therefore decreases (or reduces its oxidation
number). Basically the positive character of the species is reduced.
Oxidation: A process in which an atom loses an electron and therefore increases its oxidation number. In
other words, the positive character of the species is increased.
Historically, the term "oxidation" was used because the redox reactions that were first systematically investigated
took place in oxygen, with oxygen being reduced and the other species being oxidized, hence the term oxidation reaction. However, it was later
realized that this case (oxidation reactions involving oxygen) was just one possible scenario. For example consider the redox reaction shown below.
In this process the Fe2+ ion is oxidized, but there is no oxygen involved in this reaction. The Ce4+ ion, which is reduced acts as the oxidizing agent. So oxidation reactions need not involve oxygen. This redox reaction is actually the sum of two separate half-reactions (a reduction half-reaction and an oxidation half-reaction).
Oxidation State: The condition of a species with a specified oxidation number. An element with a given
oxidation number exists in the corresponding oxidation state.