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In chemistry, a redox reaction is a chemical reaction which consists of an oxidation reaction and a reduction reaction. That is one species gains electrons -- it is reduced -- at the cost of the other, which is oxidized. In a redox reaction the oxidation numbers of the two species are changing.

A good example is the reaction between hydrogen and fluorine:

H2 + F2 → 2HF

We can write this overall reaction as two half-reactions: an oxidation reaction:

H2 → 2H+ + 2e-

and a reduction reaction:

F2 + 2e- → 2F-

Elements always have an oxidation number of zero. In the first half reaction hydrogen is oxidized from an oxidation number of zero to an oxidation number of +1. In the second half reaction fluorine is reduced from an oxidation number of zero to an oxidation number of -1

When adding the reactions together the electrons cancel and the ions combine to form hydrogen fluoride:

2H+ +2F- → 2HF

As another example, consider the oxidation of iron(II) to iron(III):

Fe2+ --> Fe3+ + e

and the reduction of hydrogen peroxide:

H2O2 + 2 e --> 2 OH-

The two processes occur together in the following redox reaction:

2Fe2+ + H2O2 + 2H+ --> 2Fe3+ + 2H2O

See also: electrochemistry