Oxidation and reduction

Gain or loss of oxygen

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:

  • magnesium is oxidised to form magnesium oxide
  • copper(II) oxide is reduced to form copper

This is an example of a redox reaction because reduction and oxygen happen at the same time. Also in this reaction:

  • magnesium acts as a reducing agent because it can reduce copper(II) oxide
  • copper(II) oxide acts as an oxidising agent because it can oxidise magnesium
Question

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.

Question

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.

Loss or gain of electrons - Higher

In terms of electrons in a chemical reaction:

  • oxidation is the loss of electrons
  • reduction is the gain of electrons

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

curriculum-key-fact
It helps to remember OIL RIG - Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
Question

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+.

Question

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-.