AQA A-Level Chemistry AS Paper 1, June 2022: Question 18

✅ Correct Answer: D

Statement D is correct: 2I⁻ + Cl₂ → I₂ + 2Cl⁻

Chlorine is higher in Group 7 than iodine, meaning chlorine is a stronger oxidising agent than iodine. Consequently, iodide ions (I⁻) are strong enough reducing agents to reduce chlorine (Cl₂) to chloride ions (Cl⁻) while being oxidised to iodine (I₂).

💡 Key Knowledge

• Oxidising power of halogens: Decreases down Group 7:
  Cl₂ > Br₂ > I₂ (strongest to weakest oxidiser).
• Reducing power of halide ions: Increases down Group 7:
  I⁻ > Br⁻ > Cl⁻ > F⁻ (strongest to weakest reducer).
• Reaction of halides with conc. H₂SO₄:

  Halide	Redox Occurs?	Reduction Product(s)
  Cl⁻	No (only acid-base)	None (HCl gas only)
  Br⁻	Yes	SO₂ (sulfur oxidation state: +6 → +4)
  I⁻	Yes (powerful)	SO₂ (+4), S (0), and H₂S (-2)

🧠 Evaluation of Options (Process of Elimination)

• Option A is INCORRECT: Cl⁻ is not a strong enough reducing agent to reduce concentrated H₂SO₄. Only an acid-base reaction occurs:
  NaCl + H₂SO₄ → NaHSO₄ + HCl . No SO₂ is formed.
• Option B is INCORRECT: Br⁻ is only strong enough to reduce sulfur from +6 to +4 (forming SO₂ gas). Only iodide ions (I⁻) are strong enough reducers to produce elemental sulfur (S) or hydrogen sulfide (H₂S).
• Option C is INCORRECT: For Br⁻ to reduce I₂, Br⁻ would have to be oxidized to Br₂ while reducing I₂ to I⁻. But iodine is a weaker oxidising agent than bromine, so this reaction is not feasible.
• Option D is CORRECT: Chlorine oxidises iodide because Cl₂ is a stronger oxidising agent than I₂. Looking at it from the halide perspective: I⁻ is a stronger reducing agent than Cl⁻, so I⁻ reduces Cl₂ to Cl⁻.

❌ Common Misconceptions & Examiner Traps

• Confusing halogens with halides: Students frequently mix up the trend in halide reducing ability (increases down) with halogen oxidising ability (decreases down). Remember: larger ions lose electrons more easily (better reducers).
• Mixing up Br⁻ and I⁻ reduction products: A classic AQA trap is asserting that bromide reduces H₂SO₄ all the way to S or H₂S. Bromide stops strictly at SO₂.
• Forgetting oxidation states: Track oxidation numbers to avoid confusion:
  • In Option D: Cl goes from 0 in Cl₂ to -1 in Cl⁻ (reduction). I goes from -1 in I⁻ to 0 in I₂ (oxidation). Hence, iodide ions indeed reduce chlorine!