AQA A-Level Chemistry AS Paper 1, June 2025: Question 20

✅ Correct Answer

D: SO₂ loses electrons and the oxidation state of sulfur increases.

• A reducing agent donates (loses) electrons to another species, thereby reducing that species.
• In losing electrons, the reducing agent itself undergoes oxidation.
• Oxidation always leads to an increase in oxidation state (more positive).

💡 Key Knowledge

• OIL RIG: O xidation I s L oss of electrons; R eduction I s G ain of electrons.
• Reducing Agent: Causes reduction in another species → must itself be oxidised → loses electrons → oxidation state increases.
• Example reaction: When SO₂ acts as a reducing agent in solution, it is oxidised to sulfate:
  SO₂ + 2H₂O → SO₄²⁻ + 4H⁺ + 2e⁻
  Here, the oxidation state of sulfur increases from +4 in SO₂ to +6 in SO₄²⁻.

🧠 Exam Technique: Two-Step Elimination

1. Step 1 (Electron Transfer): A reducing agent gives away electrons so that another chemical can gain them. Therefore, SO₂ must lose electrons. Eliminate options A and B immediately.
2. Step 2 (Oxidation State Change): Electrons are negatively charged. When a species loses negative charges, its oxidation state must become more positive (it increases). This eliminates C.
3. Conclusion: Only option D satisfies both conditions.

❌ Common Misconceptions & Traps

• Confusing the agent with the process: Thinking a reducing agent itself gets reduced (leading to selecting A). Remember: the agent causes the effect on something else!
• Sign confusion with electrons: Forgetting that losing negative charge makes the oxidation number go up, not down (leading to trap C).
• Overcomplicating the question: You do not need to recall a specific complex equation—applying pure core definitions solves this question in under 15 seconds.