AQA A-Level Chemistry Paper 1, June 2025: Question 6

Correct Answers

At negative electrode:
H₂ + 2OH⁻ → 2H₂O + 2e⁻

At positive electrode:
O₂ + 2H₂O + 4e⁻ → 4OH⁻

Key Knowledge

• Negative Electrode (Anode): Hydrogen gas is oxidised. In alkaline conditions, H₂ reacts with OH⁻ to form water and release electrons.
• Positive Electrode (Cathode): Oxygen gas is reduced. O₂ reacts with water and gains electrons to form OH⁻ ions.
• State symbols are not required for these marks, but charges must balance perfectly.

Exam Technique

If you struggle to remember the alkaline half-equations, start with the simpler acidic ones and "neutralise" them:

1. Write acidic oxidation: H₂ → 2H⁺ + 2e⁻
2. Add 2OH⁻ to both sides: H₂ + 2OH⁻ → 2H⁺ + 2OH⁻ + 2e⁻
3. Combine H⁺ and OH⁻ to make water: H₂ + 2OH⁻ → 2H₂O + 2e⁻

Common Errors

• Writing the standard acidic half-equations involving H⁺ ions. The question explicitly states alkaline conditions.
• Forgetting to balance the charges (e.g., omitting the e⁻ or writing 2e without the negative charge).

Correct Answer

The OH⁻ ions are produced in one half-equation (at the positive electrode) and used/consumed in the other half-equation (at the negative electrode).

Key Knowledge

Look at the stoichiometry of the two half-equations:

• Positive electrode produces 4OH⁻ .
• Negative electrode consumes 2OH⁻ per H₂ molecule (which scales to 4OH⁻ for the overall reaction 2H₂ + O₂ → 2H₂O ).
• Therefore, there is no net consumption of the KOH electrolyte.

Correct Answer

• M1: Electrons are released/produced at the negative electrode (or hydrogen is oxidised).
• M2: These electrons pass through the motor/external circuit to the positive electrode (where oxygen is reduced).

Exam Technique

To secure both marks, you must describe a complete circuit pathway for the electrons:

1. State where the electrons come from (oxidation at the negative electrode).
2. State how they travel (through the external circuit/motor) and where they go (to the positive electrode).

Correct Answer

Carbon dioxide / CO₂ (in the air would react with the alkaline KOH electrolyte).

Key Knowledge

KOH is a strong base. Carbon dioxide is an acidic gas. They react readily to form potassium carbonate:

2KOH + CO₂ → K₂CO₃ + H₂O

This neutralises the hydroxide ions, reducing the concentration of the electrolyte and lowering the cell's efficiency.

Correct Answer

Liquid hydrogen occupies less volume than gaseous hydrogen (or minimises the chance of escape).

Common Errors

❌ Do not mention: "It is cheaper", "It is safer", or "It is easier to transport". The mark scheme explicitly ignores cost, safety, and transport unless linked directly to volume or pressure reduction.

Correct Answer

Graph C

Description of Graph C: A horizontal straight line showing a constant EMF value over time.

Key Knowledge

Unlike standard sealed batteries (which run out of reactants, causing concentration to drop and EMF to fall to zero), a fuel cell is a continuous flow system.

Reactants (H₂ and O₂) are continuously fed into the cell, so their concentrations at the electrodes remain constant. Consequently, the cell maintains a constant EMF during operation.

Correct Answer

They have the same overall reaction / same overall equation.

Key Knowledge

Whether the cell operates in acidic or alkaline conditions, the net chemical change is always the combustion of hydrogen:

2H₂(g) + O₂(g) → 2H₂O(l)

Because the starting reactants and final products are identical, the thermodynamic driving force (free energy change, ΔG) is the same, resulting in an identical standard EMF of +1.23 V.