How does a reaction take place

Redox reactionsPharmaWikiA redox reaction is a chemical reaction in which electrons are transferred. Redox stands for reduction and oxidation. Redox reactions are of fundamental importance for life, for example for energy production, metabolism and photosynthesis. They are also used in the synthesis of active pharmaceutical ingredients and excipients.

synonymous: reduction, oxidation, reduction-oxidation reaction


A redox reaction (reduction-oxidation reaction) is a chemical reaction in which electrons are transferred and in which the oxidation number of the chemical elements changes.

An example is the oxidation of elemental magnesium with oxygen:

Magnesium is referred to as a reducing agent. It gives off two electrons.

Oxygen is an oxidizing agent that accepts two electrons.

  • O (oxygen) + 2 e- (Electron) → O2- (Oxide)

Magnesium, the reducing agent, will oxidized and oxygen, the oxidizing agent, becomes reduced.

Redox reaction with magnesium and oxygen. Illustration © PharmaWiki

Memory aid

In English there is the following memory aid: OIL RIG.

  • OIL: Oxidation Is Loss of Electronics
  • RIG: Reduction Is Gain of electrons

You can also remember that oxygen is the most important oxidizing agent and metals like to donate electrons. Many reactions can be derived from this.

Further examples

A redox reaction also takes place when iron rusts:

Redox reactions with metals often lead to the formation of salts (ionic compounds).

Burns are oxidations, for example the combustion of methane (natural gas):

  • CH4 (Methane) + 2 O2 (Oxygen) → CO2 (Carbon dioxide) + 2H2O (water)

Formation of zinc oxide:

The involvement of oxygen is not a requirement for a redox reaction. Here copper is reduced and iron is oxidized:

Reaction of calcium with chlorine gas:

Oxidation number

An oxidation number (= oxidation level) can be assigned to each atom or ion. The corresponding value can be 0, negative, or positive.

  • During the oxidation, the oxidation number increases.
  • The oxidation number drops during the reduction.

If magnesium (Mg) is oxidized to the magnesium ion, the oxidation number increases from 0 to +2. This corresponds to the charge of the ion (Mg2+). In this reaction, the oxidation number of oxygen drops from 0 to -2 (oxide: O-2). For the detailed rules for assigning the oxidation number, we refer to the specialist literature.


Redox reactions also take place on both electrodes during → electrolysis (see there).

  • Specialist literature
  • Chemistry textbooks

Conflicts of Interest: None / Independent. The author has no relationships with the manufacturers and is not involved in the sale of the products mentioned.

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This article was last changed on 3/3/2021.
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