AQA A-Level Chemistry AS Paper 1, June 2023: Question 18

✅ Correct Answer

Option B: VO²⁺ → V³⁺ + H₂O

Adding 2H⁺ and 1e⁻ to the left-hand side gives:

VO²⁺ + 2H⁺ + e⁻ → V³⁺ + H₂O

Check:

• Atoms: Left = 1 V, 1 O, 2 H; Right = 1 V, 1 O, 2 H (Balanced)
• Total Charge: Left = (+2) + 2(+1) + (-1) = +3; Right = +3 (Balanced)

💡 Key Knowledge

• Standard Redox Half-Equation Protocol:
  1. Balance elements other than O and H.
  2. Balance oxygen atoms using H₂O.
  3. Balance hydrogen atoms using H⁺.
  4. Balance overall charge using e⁻.
• Vanadium Chemistry: Vanadium exists in four common oxidation states in aqueous solution:
  VO₂⁺ (+5, yellow) → VO²⁺ (+4, blue) → V³⁺ (+3, green) → V²⁺ (+2, violet).

📐 Step-by-Step Analysis of All Options

Let's systematically balance each option with H⁺ and e⁻ on the left-hand side to find the required ratio of 2H⁺ + 1e⁻:

A: CH₃CHO → CH₃CH₂OH

CH₃CHO + 2H⁺ + 2e⁻ → CH₃CH₂OH

Requires 2H⁺ and 2e⁻ (Not 1e⁻).

B: VO²⁺ → V³⁺ + H₂O

VO²⁺ + 2H⁺ + e⁻ → V³⁺ + H₂O

Requires 2H⁺ and 1e⁻ (Matches the question exactly).

C: HNO₂ → NO + H₂O

Right side has 2 O's and 2 H's; Left has 1 H and 2 O's. Balancing requires 1 H⁺ and 1 e⁻:

HNO₂ + H⁺ + e⁻ → NO + H₂O

Requires 1H⁺ and 1e⁻ (Not 2H⁺).

D: O₂ → H₂O₂

O₂ + 2H⁺ + 2e⁻ → H₂O₂

Requires 2H⁺ and 2e⁻ (Not 1e⁻).

🧠 Exam Technique: The Charge Shortcut

In multiple choice questions under time pressure, use overall charge conservation:

• The question specifies adding 2H⁺ (+2) and 1e⁻ (-1) to the left.
• Net change added to the left-hand side = (+2) + (-1) = +1 .
• Therefore, before adding them, the LHS must have had a net charge 1 unit less than the RHS:
  Charge(LHS) + 1 = Charge(RHS)
• Checking option B: VO²⁺ has charge +2; RHS (V³⁺ + H₂O) has charge +3.
  +2 + 1 = +3 — Instant match!

❌ Common Student Errors

• Confusing VO²⁺ and VO₂⁺: Students frequently confuse the dioxovanadium(V) ion (VO₂⁺, where V is +5) with the oxovanadium(IV) ion (VO²⁺, where V is +4). Balancing VO₂⁺ reduction to V³⁺ requires 4H⁺ and 2e⁻.
• Balancing atoms but forgetting charge: Option A (ethanal to ethanol) needs 2 H atoms, tempting students to quickly pick A without checking how many electrons are needed for charge neutrality. Neutral reactant + 2H⁺ requires 2e⁻ to form a neutral product.
• Miscounting H atoms in HNO₂: In option C, students sometimes miss the H already present on the left in nitrous acid (HNO₂), leading them to incorrectly add 2H⁺ instead of 1H⁺.

Mark Scheme Allocation:
• B is the sole correct response [1 mark, AO1].
Note: No working is required to be shown for multiple choice items, but quick jotting of oxidation states or charge differences prevents slips.