Reduction Definition in Chemistry

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Oxidation and reduction occur in batteries. Erik Dreyer / Getty Images

Reduction Definition

A half-reaction in which a chemical species decreases its oxidation number, usually by gaining electrons. The other half of the reaction involves oxidation, in which electrons are lost. Together, reduction and oxidation form redox reactions (reduction-oxidation = redox). Reduction may be considered the opposite process of oxidation.

In some reactions, oxidation and reduction may be viewed in terms of oxygen transfer.

Here, oxidation is gain of oxygen, while reduction is loss of oxygen.

An old, less-common definition of oxidation and reduction examines the reaction in terms of protons or hydrogen. Here, oxidation is ​loss of hydrogen, while reduction is the gain of hydrogen.

The most accurate reduction definition involves electrons and oxidation number.

Examples of Reduction

The H+ ions, with an oxidation number of +1, are reduced to H2, with an oxidation number of 0, in the reaction:

Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)

Another simple example is the reaction between copper oxide and magnesium to yield copper and magnesium oxide:

CuO + Mg → Cu + MgO

Rusting of iron is a process that involves oxidation and reduction. Oxygen is reduced, while iron is oxidized. While it's easy to identify which species are oxidized and reduced using the "oxygen" definition of oxidation and reduction, it's harder to visualize electrons.

One way to do this is to rewrite the reaction as an ionic equation. Copper(II) oxide and magnesium oxide are ionic compounds, while the metals are not:

Cu2+ + Mg → Cu + Mg2+

The copper ion undergoes reduction by gaining electrons to form copper. The magnesium undergoes oxidation by losing electrons to form the 2+ cation.

Or, you can view it as magnesium reducing the copper(II) ions by donating electrons. Magnesium acts as a reducing agent. In the meantime, the copper(II) ions remove electrons from magnesium to form magnesium ions. The copper(II ions are the oxidizing agent.

Another example is the reaction that extracts iron from iron ore:

Fe2O3 + 3CO → 2Fe + 3 CO2

The iron oxide undergoes reduction (loses oxygen) to form iron while the carbon monoxide is oxidized (gains oxygen) to form carbon dioxide. In this context, iron(III) oxide is the oxidizing agent which gives oxygen to another molecule. Carbon monoxide is the reducing agent, which removes oxygen a chemical species.

OIL RIG and LEO GER To Remember Oxidation and Reduction

There are two acronyms that may help you keep oxidation and reduction straight.

OIL RIG - This stands for Oxidation Is Loss and Reduction Is Gain. The species that is oxidized loses electrons, which are gained by the species that is reduced.

LEO GER - "Leo the lion says grr." - This stands for Loss of Electrons = Oxidation while Gain of Electrons = Reduction

Another way to remember which part of the reaction is oxidized and which is reduced is to simply recall reduction mean reduction in charge.