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Oxidation is the loss of electrons, gain of oxygen or loss of hydrogen.

Reduction is the gain of electrons, loss of oxygen or gain or hydrogen.

These examples show how to explain oxidation and reduction. Often you can explain it in terms of change in oxygen content or hydrogen content but sometimes an explanation in terms of electrons is required.

Example 1

Magnesium is oxidised when it reacts with oxygen to form magnesium oxide. Explain why this is an oxidation reaction.

magnesium + oxygen → magnesium oxide

2Mg + O₂ → 2MgO

Explanation:

Magnesium gains oxygen and gain of oxygen is oxidation.

or

Magnesium also loses electrons and loss of electrons is oxidation. This is shown by the equation below (higher tier):

Mg → Mg²⁺ + 2e^-

Example 2

Copper(II) oxide can be reduced to form copper when it reacts with hydrogen. Explain why copper(II) oxide is described as being reduced.

copper(II) oxide + hydrogen → copper + water

CuO + H₂ → Cu + H₂O

Explanation:

Copper(II) oxide loses oxygen and loss of oxygen is reduction.

or

Copper(II) ions gain electrons and gain of electrons is reduction.

This is shown by the equation below (higher tier):

Cu²⁺ + 2e^-→ Cu

Example 3

Nitrogen reacts with hydrogen to form ammonia according to the equation below.

N₂(g) + 3H₂(g) → 2NH₃(g)

Explain why nitrogen is described as being reduced.

Explanation:

Nitrogen gains hydrogen and gain of hydrogen is reduction.

This one cannot be explained in terms of oxygen (as this is no oxygen in the reaction) and all the substances involved are covalent so there is no loss of gain of electron.

Usually, oxidation and reduction take place simultaneously in a reaction. We call this type of reaction a redox reaction.

Example 4

Magnesium reacts with copper(II) sulfate solution according to the equation:

Mg(s) + CuSO₄(aq) → MgSO₄(aq) → Cu(s)

Explain, in terms of electrons, why this reaction is a redox reaction.

The sulfate ion, SO₄^2-, is the spectator ion in this reaction so the ionic equation is:

Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s)

The half equations are (higher tier):

Mg → Mg²⁺ + 2e^-

Cu²⁺ + 2e^- → Cu

Explanation:

Magnesium loses electrons and loss of electrons is oxidation. Copper(II) ions gain electrons and gain of electrons is reduction. Redox is when oxidation and reduction occur simultaneously in the same reaction.