In terms of oxygen in a chemical reaction:
For example, magnesium reacts with copper(II) oxide:
magnesium + copper(II) oxide → magnesium oxide + copper
In this reaction:
This is an example of a redox reaction because reduction and oxygen happen at the same time. Also in this reaction:
Aluminium reacts with zinc oxide:
aluminium + zinc oxide → aluminium oxide + zinc
Explain which substance is oxidised and which substance acts as an oxidising agent.
Aluminium is oxidised because it gains oxygen to form aluminium oxide. Zinc oxide acts as an oxidising agent because it can oxidise aluminium.
Carbon monoxide reacts with iron(III) oxide:
carbon monoxide + iron(III) oxide → carbon dioxide + iron
Explain which substance is reduced and which substance acts as a reducing agent.
Iron(III) oxide is reduced because it loses oxygen to form iron. Carbon monoxide acts as a reducing agent because it can reduce iron(III) oxide.
In terms of electrons in a chemical reaction:
These definitions are useful in reactions that do not involve oxygen. For example, magnesium reacts with chlorine to form magnesium chloride:
Mg + Cl2 → MgCl2
Magnesium chloride contains Mg2+ ions and Cl- ions. The balanced equation can be split into two half equations:
Mg → Mg2+ + 2e- oxidation, because magnesium atoms lose electrons
Cl2 + 2e- → 2Cl- reduction, because chlorine gains electrons
Zinc reacts with copper(II) sulfate solution:
Zn + CuSO4 → ZnSO4 + Cu
Explain, in terms of electrons, which substance is oxidised.
Zinc is oxidised because zinc atoms lose electrons to form zinc ions, Zn2+.
Chlorine reacts with sodium bromide solution:
Cl2 + 2NaBr → 2Cl- + Br2
Explain, in terms of electrons, which substance is reduced.
Chlorine is reduced because it gains electrons to form chloride ions, Cl-.