Edexcel A-Level Chemistry Paper 3, June 2023: Question 5

✅ Correct Answers

• Y: Platinum / Pt (electrode)
• Z: Manganese(II) nitrate / Mn(NO₃)₂ AND Potassium manganate(VII) / KMnO₄ (solution)
• Acid: Sulfuric acid (H₂SO₄)

💡 Key Knowledge

• A platinum electrode is required because both the oxidized and reduced species in the half-cell are aqueous ions (solution phase), meaning they cannot act as an electrical contact themselves.
• The half-cell requires both Mn²⁺ ions and MnO₄⁻ ions alongside a source of H⁺ ions to function.

❌ Common Errors

• Using hydrochloric acid instead of sulfuric acid (Cl⁻ ions would be undesirably oxidized by manganate(VII)).
• Omitting either of the required ionic components for solution Z. Both manganese(II) and manganate(VII) must be present.

🧠 Exam Technique

• Names or chemical formulae are both accepted by examiners, but ensure formulas have correct ionic charges if used.
• Concentrations do not need to be specified unless standard conditions are explicitly demanded.

✅ Correct Answer & Mark Breakdown (5 Marks)

• Final State: Chromium(II) / Cr²⁺ (1 mark)
• Step 1 (Cr(VI) to Cr(III)): E-theta cell = (+1.33) - (-0.76) = +2.09 V (Positive, feasible) (1 mark)
• Step 2 (Cr(III) to Cr(II)): E-theta cell = (-0.41) - (-0.76) = +0.35 V (Positive, feasible) (1 mark)
• Step 3 (Cr(II) to Cr(0)): E-theta cell = (-0.91) - (-0.76) = -0.15 V (Negative, not feasible) (1 mark)
• Conclusion link: First two reductions occur because E-theta cell is positive, but the final reduction does not occur because E-theta cell is negative. (1 mark)

📐 Step-by-Step E-theta Calculation Strategy

1. Calculate E-theta cell = E-theta (reduction) - E-theta (oxidation) for each successive half-equation. Remember that zinc acts as the reducing agent and is itself oxidized (E-theta = -0.76 V).
2. Check if the calculated E-theta cell value is positive (indicating a feasible reaction).
3. Stop the sequence when the calculated E-theta cell value becomes negative.

❌ Common Errors

• Forgetting to test the final step (Cr²⁺ to Cr) and incorrectly assuming all transition metal ions are reduced completely to metal atoms.
• Incorrectly subtracting electrode potential values, leading to wrong signs for E-theta cell.

🧠 Exam Technique

• You must quote the specific E-theta cell values for each stage to gain full credit. Simply stating "the numbers are positive" will not score top marks.

✅ Correct Answer

NO₃⁻ + 2H⁺ + e⁻ ⇌ NO₂ + H₂O (or using = )

💡 Key Knowledge

In standard cell representations, the right-hand side of the double vertical line ( || ) represents the reduction process (cathode). You must balance the half-equation using electrons and hydrogen ions as indicated by the species present in the diagram.

❌ Common Errors

• Including state symbols when the question explicitly states: "State symbols are not required."
• Incorrect stoichiometry for electrons or protons.

🧠 Exam Technique

Multiples of the balanced equation are accepted as long as all stoichiometric coefficients are correctly proportioned.

✅ Correct Answers

• Direction: Electrons move from the negative electrode to the positive electrode through the external circuit. (1 mark)
• Justification: Hydrogen is oxidized (loses electrons) at the negative electrode, while oxygen is reduced (gains electrons) at the positive electrode. (1 mark)

💡 Key Knowledge

• Negative electrode (Anode): H₂ → 2H⁺ + 2e⁻
• Positive electrode (Cathode): ½O₂ + 2H⁺ + 2e⁻ → H₂O

❌ Common Errors

• Stating that electrons flow through the internal membrane (electrons cannot cross the proton-exchange membrane; only ions do).
• Mixing up oxidation and reduction locations.

🧠 Exam Technique

To secure both marks, link the physical direction of flow directly to the chemical terms oxidation and reduction.

✅ Correct Answers (Any one of)

• Produces harmless products / water only (compared to toxic exhaust pollutants like CO, CO₂, NOₓ, or unburned hydrocarbons).
• Less reliant on finite fossil fuels / renewable energy source.
• More efficient energy production.
• Can be smaller and lighter as a power unit.

❌ Common Errors

• Vague answers such as just saying "more efficient" without context (though "more efficient energy production" is credited, simple single-word descriptions lack rigor).
• Ignoring that hydrogen production itself can require fossil fuels unless specified as green hydrogen. Stick closely to the mark scheme points regarding the vehicle operation.