Oxidation and Reduction

Oxidation and reduction describe chemical changes

Iron is widely used to make products, but corrosion can shorten their useful lives. When iron reacts with oxygen from the air, the iron is oxidised. Oxidation and reduction describe changes that occur in many reactions, not just corrosion.

There are two ways to recognise these changes: by following oxygen or by following electrons. In either case, identify the starting substance or particle and compare it with what it becomes.

Following oxygen through a reaction

In terms of oxygen, oxidation is the gain of oxygen and reduction is the loss of oxygen. A substance does not need to gain or lose all its oxygen: the important point is whether oxygen is added or removed during the reaction.

Consider the reaction between zinc oxide and carbon:

$$\mathrm{ZnO + C \rightarrow Zn + CO}$$

Zinc oxide becomes zinc. Oxygen has been removed from the zinc oxide, so zinc oxide is reduced.

Carbon becomes carbon monoxide. Oxygen has been added to the carbon, so carbon is oxidised.

The oxygen removed from zinc oxide ends up combined with carbon. Oxidation and reduction therefore occur together in this reaction: it is a redox reaction. To identify each change, follow the substances across the equation rather than simply noticing that oxygen is present somewhere.

Following electrons instead

Some redox reactions do not involve oxygen at all. A more general explanation follows the transfer of electrons:

• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.

Electrons have a negative charge. When a neutral metal atom loses electrons, it becomes a positive ion. When a positive metal ion gains enough electrons, it becomes a neutral atom.

These are not two unrelated meanings of oxidation and reduction. The electron definition gives a wider way of describing redox reactions, including those where there is no oxygen to track.

Identifying the species in an electron-transfer reaction

Magnesium displaces copper from copper(II) sulfate solution:

$$\mathrm{Mg + CuSO_4 \rightarrow MgSO_4 + Cu}$$

The particles that change are magnesium atoms and copper(II) ions. Sulfate ions remain unchanged, so the reaction can be represented more simply by an ionic equation:

$$\mathrm{Mg + Cu^{2+} \rightarrow Mg^{2+} + Cu}$$

Each magnesium atom loses two electrons and becomes a magnesium ion. Magnesium atoms are oxidised. Each copper(II) ion gains two electrons and becomes a copper atom. Copper(II) ions are reduced.

The electrons lost by magnesium are gained by copper(II) ions. This explains why oxidation and reduction occur together: the transferred electrons leave one species and enter another.

Reading half-equations

A half-equation shows just one side of the electron transfer. For magnesium, the oxidation half-equation is:

$$\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$$

The electrons appear on the right because magnesium releases them. The total charge is balanced: the neutral atom on the left has charge zero, and the positive ion plus two negative electrons on the right also has total charge zero.

For copper(II) ions, the reduction half-equation is:

$$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$$

The electrons appear on the left because the copper(II) ion receives them. Its positive charge is cancelled by the two electrons, producing a neutral copper atom.

When deciding which species is oxidised or reduced, look for the actual loss or gain of electrons. Oxygen does not have to be involved.

Two ways to identify redox

Oxidation and reduction occur together in a redox reaction.

Track the starting species

$\mathrm{ZnO + C \rightarrow Zn + CO}$

• Zinc oxide is reduced: loses oxygen.
• Carbon is oxidised: gains oxygen.

Read electron transfer

$\mathrm{Mg \rightarrow Mg^{2+} + 2e^-}$ — magnesium atoms are oxidised.

$\mathrm{Cu^{2+} + 2e^- \rightarrow Cu}$ — copper(II) ions are reduced.

The number of electrons lost equals the number gained.