Fluorine, chlorine, bromine and iodine are all halogens in Group 7. A student claims that chlorine is a more powerful oxidising agent than iodine, and uses a displacement reaction to support this claim. Explain what is meant by the term 'oxidising agent' and explain, in terms of electron transfer, why chlorine is able to displace iodine from a solution of potassium iodide.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.2 Group 7 (halogens) · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

An oxidising agent is a substance that gains electrons and is itself reduced.
Chlorine is a stronger oxidising agent than iodine because it has a higher oxidising power (reactivity decreases down Group 7).
In the displacement reaction, each Cl₂ molecule accepts one electron from an I⁻ ion:
Cl₂ + 2 I⁻ → 2 Cl⁻ + I₂
Thus chlorine atoms gain electrons to form Cl⁻, while iodide ions lose electrons and are oxidised to I₂.

Examiner tips

  • Define oxidising agent as electron‑acceptor and reduced; mention higher oxidising power of Cl₂; show electron transfer stoichiometry; keep answer concise and use correct symbols.

Common mistakes

  • Confusing oxidising agent with reducing agent; writing the reaction without electron transfer; using wrong oxidation states or omitting the formation of I₂.

Mark scheme (4 marks)

  1. An oxidising agent gains electrons (and is itself reduced)
  2. Chlorine is more reactive than iodine (higher oxidising power / higher reactivity down Group 7 decreases)
  3. Each chlorine atom gains one electron (from the iodide ions) to form chloride ions (Cl⁻)
  4. Iodide ions lose electrons and are oxidised to form iodine molecules (I₂)

Key terms in this question

oxidising agent · displacement · halogens

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