Learning focus
Explain chemical changes using particles, collisions, equilibrium and electron transfer; interpret graphs and industrial conditions; and apply the ideas to unfamiliar reactions.
Oxidation by oxygen gain
A substance is oxidised when it gains oxygen. Magnesium is oxidised in 2Mg + O2 -> 2MgO. Carbon monoxide is oxidised to carbon dioxide when it gains oxygen.
Reduction by oxygen loss
A substance is reduced when it loses oxygen. Copper(II) oxide is reduced to copper in CuO + H2 -> Cu + H2O. The hydrogen gains oxygen and is oxidised.
Simultaneous process
Oxidation and reduction always occur together because oxygen transferred from one substance is gained by another. Such a reaction is redox. Name the species, not only the element, when stating what is oxidised or reduced.
Limitations
The oxygen definition cannot describe every redox reaction, such as electron transfer between iron(II) and chlorine. Electron and oxidation-number definitions provide a more general framework.
Practical or data skill
Analyse metal-oxide reactions with carbon, carbon monoxide or hydrogen and identify the oxidised and reduced substances.
Examination tip
If one substance gains oxygen, identify which other substance loses or supplies it.
Review questions and suggested answers
Question 1
Define oxidation using oxygen.
Suggested answer
Gain of oxygen.
Question 2
What is reduced in CuO + H2 -> Cu + H2O?
Suggested answer
Copper(II) oxide.
Question 3
Why is the reaction redox?
Suggested answer
Oxidation and reduction occur simultaneously.
