Redox

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:

        H₂ → 2H⁺ + 2e^-

+ 2e^- + F₂ → 2F^-

  ---------------------

   H₂ + F₂ → 2H⁺ + 2F^-

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