Oxidation and reduction reactions

Oxidation and reduction, together known as redox reactions, are among the most important processes in chemistry. They involve the transfer of electrons between substances: oxidation is the loss of electrons by one species, and reduction is the gain of electrons by another. The two always occur together, because electrons released by one substance must be taken up by another. Many familiar phenomena — rusting, combustion, the working of batteries and biological respiration — are redox processes.

To keep track of electrons, chemists assign each atom an oxidation number (or oxidation state), a value indicating how many electrons it has gained or lost relative to its neutral form. During a reaction, an increase in an atom's oxidation number signals oxidation, while a decrease signals reduction. A substance that causes another to be oxidised is called an oxidising agent and is itself reduced; a reducing agent brings about reduction and is itself oxidised. Common categories of redox reaction include combination, decomposition, displacement and combustion reactions.

Balancing redox equations is a standard skill, since both the number of atoms and the total charge must be conserved. One common approach separates the overall change into half-reactions — one describing the oxidation and one the reduction — which are balanced for mass and charge and then recombined so the electrons lost equal the electrons gained. Understanding these reactions underpins broad areas of chemistry, from metallurgy and corrosion to electrochemistry and the energy reactions of living cells.

Frequently asked questions

What is a redox reaction?

It is a reaction in which electrons are transferred between substances, with one being oxidised (losing electrons) and another reduced (gaining electrons).

What does an oxidation number tell you?

It indicates how many electrons an atom has effectively gained or lost; a rise during a reaction means oxidation and a fall means reduction.

What is an oxidising agent?

It is a substance that causes another to be oxidised by accepting its electrons, and in doing so the oxidising agent is itself reduced.