Oxidation and reduction
Francine Taylor-Campbell, Contributor

From left: Dunoon Technical High School teacher, Brenton McLean, and students, Nico Tyndale, Terry-Ann Goodridge, Donald Hall and Sanya Smith and principal Samuel Thompson. Dunoon produced one of the teams with the best overall project in the Access to Information Students' Challenge Competition. - Norman Grindley/Deputy Chief Photographer

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 take 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.

1. Oxidation was originally defined as the gain of oxygen by a substance.

Eg. 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.

QUES 1

In the reaction 2H2 SO 3 (aq) + O2 (g) = 2H2 SO2 (aq), give the formulae of the substance which is oxidised and the product of the oxidation.

2. Oxidation also occurs when hydrogen is lost by a substance.

E.g. When ammonia burns in pure oxygen

4NH3 (g) + 3O2 (g) = 2N2 (g) + 6H2O (g) ...... 3

Ammonia loses hydrogen and is oxidised to nitrogen.

3. When electronic structures become fully understood, it was realised that when a substance was oxidised it lost electrons and oxidation was defined as a process in which electrons were lost.

E.g. In reaction one, magnesium atoms lose electrons to form magnesium ions

2Mg (s) = 2Mg 2+ (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+ O 2- or 2MgO (s)

This concept is not so easily applied to covalent compounds e.g. in reactions two and three.

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

Thus in the reaction

2CO (g) + O2 (g) = 2CO2 (g) ...... 4

The valency of carbon in carbon monoxide is two and in carbon dioxide is four.

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.

Continue to review this topic until next week's lesson.

Francine Taylor-Campbell is an independent contributor.