Oxidation and reduction defined by oxygen

Chemical changes • Reactivity of metals

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Which substance is reduced when copper(II) oxide reacts with carbon to form copper?

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Copper(II) oxide is reduced because it loses oxygen to form copper metal.

Key concepts

What you'll likely be quizzed about

Definition of oxidation (oxygen)

Oxidation occurs when a substance gains one or more oxygen atoms. Addition of oxygen changes the chemical composition and often increases the oxidation state of the element involved. Oxidation in the oxygen sense applies only to reactions where oxygen atoms are added to reactants.

Definition of reduction (oxygen)

Reduction occurs when a substance loses one or more oxygen atoms. Removal of oxygen decreases the oxidation state of the element that loses oxygen. Reduction in the oxygen sense applies only to reactions where oxygen atoms are removed from reactants.

Identifying oxidised and reduced substances

The substance that gains oxygen is oxidised. The substance that loses oxygen is reduced. Example: When magnesium reacts with oxygen to form magnesium oxide, magnesium gains oxygen and is oxidised; oxygen is reduced only if oxygen accepts electrons or bonds, but in the oxygen-definition method oxygen itself is not described as oxidised or reduced in that reaction. In a reaction where copper(II) oxide reacts with carbon to give copper and carbon dioxide, copper(II) oxide loses oxygen and is reduced; carbon gains oxygen and is oxidised.

Oxidising and reducing agents (oxygen view)

An oxidising agent causes another substance to gain oxygen and is itself reduced by losing oxygen. A reducing agent causes another substance to lose oxygen and is itself oxidised by gaining oxygen. Example: Carbon acts as a reducing agent when it removes oxygen from metal oxides during reduction; oxygen gas often acts as an oxidising agent when it adds oxygen to other elements.

Examples with metals and non-metals

Burning of magnesium: Mg + O2 → MgO. Magnesium gains oxygen and is oxidised. Rusting of iron: 4Fe + 3O2 → 2Fe2O3. Iron gains oxygen and is oxidised. Reduction of copper(II) oxide by hydrogen: CuO + H2 → Cu + H2O. Copper(II) oxide loses oxygen and is reduced; hydrogen gains oxygen and is oxidised.

Limitations of the oxygen definition

The oxygen definition only applies clearly to reactions that involve oxygen transfer. Some redox reactions do not involve oxygen atoms but involve electron transfer; those reactions require electron-based definitions of oxidation and reduction. Statements about oxidised or reduced substances must specify that the oxygen definition is in use when oxygen atoms move between substances.

Key notes

Important points to keep in mind

Oxidation = gain of oxygen atoms; reduction = loss of oxygen atoms.

The substance that gains oxygen is oxidised; the substance that loses oxygen is reduced.

Oxidising agent causes gain of oxygen in another substance and loses oxygen itself.

Reducing agent causes loss of oxygen in another substance and gains oxygen itself.

Oxygen-definition applies only to reactions with oxygen atom transfer.

For reactions without oxygen transfer, use electron-based redox definitions instead.

Check the oxygen count for each substance before and after the reaction to identify changes.

Word equations and symbol equations both show oxygen gain or loss for identification.

Metal oxides often reduce to metals when oxygen is removed by a reducing agent.

Combustion reactions typically show oxidation of the fuel by oxygen.