Redox

O Level / IGCSE Chemistry · Chemical reactions. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A redox reaction is one in which oxidation and reduction happen together. One substance cannot be oxidised unless another is reduced at the same time.

Oxidation is gain of oxygen, loss of electrons, or an increase in oxidation number. Reduction is loss of oxygen, gain of electrons, or a decrease in oxidation number.

The oxidation number shows how many electrons an atom has lost or gained control of in a substance. It is written as a Roman numeral in names, such as iron(III) oxide. An oxidising agent oxidises another substance and is itself reduced. A reducing agent reduces another substance and is itself oxidised.

Rules to remember

Common mistake

Students mix up the agent with what happens to it. The oxidising agent is the substance that gets reduced, and the reducing agent is the substance that gets oxidised.

Worked example

In the reaction 2FeCl2 + Cl2 → 2FeCl3, which substance is oxidised and which is the oxidising agent?

  1. Iron: +2 in FeCl2 (since each Cl is −1) and +3 in FeCl3. The number goes up, so iron is oxidised.
  2. Chlorine in Cl2: 0 (uncombined element) and −1 in FeCl3. The number goes down, so chlorine is reduced.
  3. The substance that is reduced is the oxidising agent.

Answer: Iron(II) chloride (the Fe2+ ions) is oxidised. Chlorine, Cl2, is the oxidising agent.

Practice questions

Try each one, then open the answer.

1. What is the oxidation state of manganese in KMnO4?

  1. A
    +3
  2. B
    +5
  3. C
    +7
  4. D
    −1
Show answer

Answer: C. K is +1, each O is −2 (total −8). For a neutral compound Mn must be +7.

2. In the reaction 2Mg + O2 → 2MgO, magnesium is

  1. A
    reduced, because it gains oxygen and gains electrons
  2. B
    neither oxidised nor reduced, because it only combines
  3. C
    oxidised, because it gains electrons from the oxygen
  4. D
    oxidised, because it gains oxygen and loses electrons
Show answer

Answer: D. Oxidation is gain of oxygen or loss of electrons: Mg → Mg2+ + 2e−. Oxygen is reduced.

3. Mg + Zn2+ → Mg2+ + Zn. Which statement about this reaction is correct?

  1. A
    magnesium is the reducing agent and is oxidised
  2. B
    zinc ions are oxidised
  3. C
    magnesium is the oxidising agent and is reduced
  4. D
    zinc ions are the reducing agent
Show answer

Answer: A. Magnesium loses two electrons (oxidised) and gives them to Zn2+, which is reduced to zinc. The substance that is oxidised is the reducing agent.

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