Redox processes: electron transfer and changes in oxidation number

AS Level Chemistry · Electrochemistry. 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 electrons are transferred. Oxidation is loss of electrons and reduction is gain of electrons. The two always happen together.

The oxidation number of an atom is the charge it would have if the compound were made of ions. It lets you follow electrons in any reaction. If the oxidation number of an element goes up, the element is oxidised. If it goes down, the element is reduced.

The oxidising agent takes electrons from another species and is itself reduced. The reducing agent gives electrons and is itself oxidised. In disproportionation, the same element is both oxidised and reduced in one reaction.

Rules to remember

Common mistake

Students mix up the agent and the change: the oxidising agent is the species that is reduced, not the one that is oxidised.

Worked example

Chlorine reacts with hot aqueous sodium hydroxide: 3Cl2 + 6NaOH → 5NaCl + NaClO3 + 3H2O. What happens to the chlorine, and what type of reaction is this?

  1. Chlorine in Cl2 has oxidation number 0.
  2. In NaCl it is −1, so 5 chlorine atoms are reduced (total decrease = 5 × 1 = 5).
  3. In NaClO3: (+1) + Cl + 3(−2) = 0, so Cl = +5. One chlorine atom is oxidised (total increase = 5).
  4. Increase equals decrease, so the equation balances, and the same element is both oxidised and reduced.

Answer: Chlorine goes from 0 to −1 and to +5: this is disproportionation.

Practice questions

Try each one, then open the answer.

1. Iodate(V) ions react with iodide ions in acid: IO3− + xI− + 6H+ → 3I2 + 3H2O. What is x?

  1. A
    1
  2. B
    3
  3. C
    6
  4. D
    5
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Answer: D. Iodine in IO3− goes from +5 to 0, gaining 5 electrons; each I− goes from −1 to 0, losing 1, so 5 I− are needed. Check: 1 + 5 = 6 iodine atoms = 3I2, and charge −1 − 5 + 6 = 0.

2. In the reaction 2Fe3+ + 2I− → 2Fe2+ + I2, which species is the oxidising agent?

  1. A
    I−
  2. B
    Fe2+
  3. C
    Fe3+
  4. D
    I2
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Answer: C. The oxidising agent gains electrons and is itself reduced. Fe3+ gains an electron to become Fe2+, so Fe3+ is the oxidising agent; iodide loses electrons and is the reducing agent.

3. What is the oxidation number of chromium in the dichromate(VI) ion, Cr2O72−?

  1. A
    +3
  2. B
    +6
  3. C
    +7
  4. D
    +12
Show answer

Answer: B. Oxygen is −2, so seven oxygens give −14. For the total to be −2, the two chromium atoms must add to +12, i.e. +6 each. The +12 option forgets there are two Cr atoms.

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