Redox

de:Redoxreaktion et:Redoksreaktsioon fr:Oxydo-réduction pl:Reakcja redoks nl:Redoxreactie In chemistry, a redox reaction (reduction–oxidation reaction) is a chemical reaction which consists of an oxidation reaction and a reduction reaction. That is, electrons are transferred from one specicies, which is oxidised/oxidized, to the other, which is reduced. In a redox reaction the oxidation numbers of the two species are changing.

Contents

1 Oxidising/Oxidizing/Reducing agents

2 Former meaning (Oxygen/Hydrogen)

3 Examples of redox reactions

4 Redox reactions in biology

5 See also

Oxidising/Oxidizing/Reducing agents

Substances or reactions having the capability to oxidize (reduce) are said to be oxidative (???) and known as oxidising agents/oxidizing agents (reducing agents), oxidants (reductants) or oxidisers/oxidizers (???).

Former meaning (Oxygen/Hydrogen)

Formerly, oxidation (reduction) simply meant the addition (removing) of oxygen or the removing (addition) of hydrogen (hence the name oxidation). However, when the terms are now used it is normally in the more general sense.

Examples of redox reactions

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

Here are some other examples:

iron(II) oxidises/oxidizes to iron(III):

Fe²⁺ → Fe³⁺ + e^-

hydrogen peroxide reduces to hydroxide:

H₂O₂ + 2 e^- → 2 OH^-

overall equation for the above:

2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O

iron(II) oxidises/oxidizes to and oxygen reduces to iron(III) oxide (commonly known as rusting or tarnishing):

4Fe + 3O₂ → 2 Fe₂O₃.

Another example is the burning of hydrocarbons to produce water, carbon dioxide, some partially oxidized forms, and heat energy. Complete oxidation of materials containing carbon produces carbon dioxide, which is linked to global warming because it absorbs certain wavelengths of infrared light.

In organic chemistry, stepwise oxidation of a hydrocarbon produces water and, successively, an alcohol, an aldehyde or a ketone, carboxylic acid, and then a peroxide.

In inorganic chemistry terms, incompletely oxidized carbon takes the form of carbonate, bicarbonate or carbon monoxide.

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, oxidises/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.