Oxidation & Reduction in Terms of OxygenOxford AQA IGCSE Chemistry: Revision notes
Section 1
How are oxidation and reduction defined in terms of oxygen?
The original, oldest definitions of oxidation and reduction are based on oxygen:
- Oxidation is the gain of oxygen by a substance
- Reduction is the loss of oxygen from a substance
These definitions are useful for describing reactions where oxygen is clearly added to or removed from a substance, such as combustion or metal extraction.
Section 2
How does this apply to metal extraction?
Many metal oxides are reduced to obtain the pure metal, because the metal oxide loses oxygen. For example, when iron oxide is heated with carbon in a blast furnace, the iron oxide loses oxygen and is reduced to iron. At the same time, the carbon gains oxygen and is oxidised to carbon dioxide.
Iron(III) oxide + carbon → iron + carbon dioxide. The iron oxide is reduced (loses oxygen); the carbon is oxidised (gains oxygen).
Section 3
How does this apply to combustion?
When a fuel burns in oxygen (combustion), the fuel gains oxygen, so it is oxidised. For example, when carbon burns in oxygen it is oxidised to carbon dioxide, and when a hydrocarbon fuel burns completely it is oxidised to carbon dioxide and water.
Section 4
How does the oxygen definition relate to the electron definition?
The oxygen-based definitions and the electron-based definitions (oxidation = loss of electrons, reduction = gain of electrons) describe the same underlying chemistry from two different angles, and both are accepted in exam answers as long as the correct definition is used for the correct context. A substance that gains oxygen is usually also losing electrons overall, and a substance that loses oxygen is usually also gaining electrons.
If a question specifically asks you to define oxidation or reduction 'in terms of oxygen', you must use the words 'gain' or 'loss' of oxygen — an electron-based answer will not gain the mark.
Must Know
- Oxidation = gain of oxygen; Reduction = loss of oxygen
- In the blast furnace, iron oxide is reduced (loses oxygen) to form iron
- In the blast furnace, carbon is oxidised (gains oxygen) to form carbon dioxide
- Combustion of a fuel is an oxidation reaction, as the fuel gains oxygen
- Always match the definition (oxygen or electrons) to what the question specifically asks for
That's the notes covered.
Carry on to the next subtopic.