AQA A-Level Chemistry Paper 3, June 2022: Question 11

✅ Correct Answer

C — the surface area of the platinum electrode

Platinum acts solely as an inert catalyst and conductive solid surface for electron transfer. Changing its surface area affects only the rate of electron exchange, not the equilibrium position or potential difference.

💡 Key Knowledge

The SHE is based on the reversible redox equilibrium:

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

• Concentration: Changing [H⁺] shifts equilibrium (Le Chatelier’s principle), altering E.
• Pressure: Changing p(H₂) shifts equilibrium, altering E.
• Temperature: Altering T affects the equilibrium constant (ΔH is non-zero), altering E.
• Catalyst surface area: Increases the speed at which equilibrium is reached, but does not alter the position of equilibrium.

🧠 Exam Technique & Examiner Tips

• Watch the negative stem: The question asks which has no effect. Underline words like not or no immediately.
• Connect voltage to equilibrium: Electrode potential (E) directly reflects the position of equilibrium. Any condition that shifts the equilibrium shifts the voltage:
  • Higher [H⁺] → shifts right → E becomes more positive.
  • Higher p(H₂) → shifts left → E becomes more negative.
• Remember the role of Pt: Platinum is heterogeneous and chemically inert. It does not appear in the equilibrium expression.

❌ Common Misconceptions

• Kinetics vs Thermodynamics Confusion: Students often think that a larger platinum surface area provides "more sites for reaction" and therefore generates a greater potential difference. Potential is an intensive thermodynamic property (volts), not extensive (current/rate).
• Misunderstanding standard definition: Thinking that the SHE is always 0.00 V under any temperature or pressure. It is 0.00 V by definition only at standard conditions (298 K, 100 kPa, 1.00 mol dm⁻³ H⁺).