Redox Reactions

Definition

A redox reaction (reduction-oxidation reaction) is a chemical reaction in which electrons are transferred and the oxidation state changes. An example is the oxidation of elemental magnesium with oxygen:

• 2 Mg (elemental magnesium) + O₂ (oxygen) 2 MgO (magnesium oxide).

In this process, magnesium is called a reducing agent. It gives off two electrons.

• Mg (magnesium elemental) Mg²⁺ (magnesium, cation) + 2 e^– (electron)

Oxygen is an oxidizing agent that accepts two electrons.

• O (oxygen) + 2 e^– (electron) O^2- (oxide)

Magnesium, the reducing agent, becomes and oxygen, the oxidizing agent, becomes .

Mnemonic

In English, there is the following mnemonic: OIL RIG.

• OIL: Oxidation Is Loss of eletrons.
• RIG: Reduction Is Gain of electrons

You can also remember that oxygen is the main oxidant and metals like to give up electrons. Many reactions can be derived from this.

Other examples

A redox reaction also takes place when iron rusts:

• 4 Fe (iron elemental) + 3 O₂ (oxygen) 2 Fe₂O₃ (iron oxide)

So redox reactions often lead to the formation of salts (ionic compounds) with metals. Combustion is oxidation, for example, the combustion of methane (natural gas):

• CH₄ (methane) + 2 O₂ (oxygen) CO₂ (carbon dioxide) + 2 H₂O (water)

Formation of zinc oxide:

• 2 Zn (zinc) + O₂ (oxygen) 2 ZnO (zinc oxide)

The participation of oxygen is not a condition for a redox reaction. Here, copper is reduced and iron is oxidized:

• CuSO₄ (copper sulfate) + Fe (elemental iron) FeSO₄ (iron sulfate) + Cu (copper elemental)

Reaction of calcium with chlorine gas:

• Ca (calcium elemental) + Cl₂ (chlorine gas) CaCl₂ (calcium chloride)

Oxidation number

Each atom or ion can be assigned an oxidation number (= oxidation state). The corresponding value can be 0, negative or positive.

• During oxidation, the oxidation number increases.
• In the reduction, the oxidation number decreases.

When 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⁺). The oxidation number of oxygen decreases from 0 to -2 in this reaction (Oxid: O^-2). For the detailed rules for assigning the oxidation number, please refer to the technical literature.

Electrolysis

Redox reactions also occur at both electrodes in electrolysis (see there).