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.

Example of a redox reaction

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

Redox reactions in biology

Much biological energy is stored and released by means of redox reactions. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of water into molecular oxygen. The reverse reaction, respiration, oxidizes sugars to produce carbon dioxide and water. As intermediate steps, the reduced carbon compounds are used to reduce [[NAD+]], which then contributes to the creation of a proton gradient, which drives the synthesis of ATP and is maintained by the reduction of oxygen.

See also

• Calvin cycle
• electrochemistry
• Citric acid cycle