AQA A-Level Chemistry AS Paper 1, June 2022: Question 19

✅ Correct Answer

A: OF₂

Award 1 mark for selecting option A (AO1 recall & application).

📐 Step-by-Step Evaluation

• A (OF₂): Fluorine is always −1 in compounds.
  Oxidation state of O + 2(−1) = 0 → O = +2
• B (H₂O): Hydrogen is +1.
  2(+1) + Oxidation state of O = 0 → O = −2
• C (O₂): Uncombined element in its standard state.
  O = 0
• D (H₂O₂): Peroxide. Hydrogen is +1.
  2(+1) + 2(O) = 0 → 2(O) = −2 → O = −1

Comparing values: +2 > 0 > −1 > −2

💡 Key Knowledge: Rules for Oxygen

• Standard rule: Oxygen is almost always −2 in compounds (e.g. H₂O, CO₂).
• Peroxides: Oxygen is −1 (e.g. H₂O₂, Na₂O₂).
• Elemental form: Pure elements always have an oxidation state of 0 (e.g. O₂).
• Compounds with Fluorine: Fluorine is the most electronegative element, so F is always −1. This forces oxygen to have a positive oxidation state (+2 in OF₂).

❌ Common Misconceptions & Traps

• Automatic "Oxygen is −2": Assuming oxygen is always −2 without checking the bonded partner.
• Confusing "highest" with numerical magnitude: Choosing −2 (in H₂O) because 2 looks larger than 0 or 1, forgetting that oxidation state is an algebraic scale where positive numbers are greater than negative numbers (+2 > −2).
• Ignoring Fluorine's priority: Forgetting that fluorine outranks oxygen in electronegativity and always takes −1.