Oxidation and reduction
Francine Taylor-Campbell, Contributor

We will start this topic by first focusing on the basics that need to be known on oxidation and reduction. Use this lesson to review the important facts, so that you can answer the questions.

IMPORTANT POINTS

• A reaction in which oxidation and reduction takes place is called a REDOX reaction.
• Oxidation and reduction can be defined in terms of loss and gain of electrons, oxygen and hydrogen and a change in oxidation number.
• Oxidation was originally defined as the gain of oxygen by a substance.
  For example, when magnesium burns in oxygen
• 2Mg (s) + O2 (g) = 2MgO … 1
  magnesium is oxidised as it gains oxygen to form magnesium oxide or when carbon monoxide burns 2CO (g) + O2 (g) = 2CO2 (g) … 2
  it is oxidised to carbon dioxide.
• Oxidation also occurs, when hydrogen is lost by a substance. For example, when ammonia burns in pure oxygen
  4NH3 (g) + 3O2 (g) = 2N2 (g) + 6H2O (g) … 3
  Ammonia loses hydrogen and is oxidised to nitrogen.
• When electronic structures became fully understood, it was realised that when a substance was oxidised it lost electrons and hence oxidation was defined as the process by which electrons were lost. For example, in reaction 1, magnesium atoms lose electrons to form magnesium ions
  2Mg (s) = 2Mg2+(s) + 4e … A
  These electrons are gained by oxygen molecules
  O2 (g) + 4e = 2O2- (s) … B
  On adding A and B we get 2Mg (s) + O2 (g) = 2Mg2+O2- or 2MgO (s)

This concept is not so easily applied to covalent compounds, for example in reactions 2 and 3.

• The concept of oxidation numbers overcomes this problem. Oxidation number is numerically the same as valency but has a + or a - sign.

Thus, in the reaction

2CO (g) + O2 (g) = 2CO2 (g) … 4

The valency of carbon in carbon monoxide is 2 and in carbon dioxide is 4.

The oxidation numbers are +2 and +4, that is, the oxidation number of carbon increases from 2 to 4. Thus oxidation is a process involving an increase in oxidation number.

In the reaction 4NH3 (g) + 3O2 (g) = 2N2 (g) + 3H2O (g) the oxidation number of nitrogen in ammonia is -3 and in nitrogen is zero. It increases from -3 to 0.

Attempt the following question.

In the reaction 2H2SO3 (aq) + O2 (g) = 2H2SO4 (aq), give the formulae of the substance which is oxidised and the product of the oxidation.

A student examines a slide through a microscope while the other writes. - contributed

Francine Taylor-Campbell is an independent contributor.