AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 7

✅ Correct Answers (Choose 2)

• Error 1: The activation energy arrow must start from the reactants line (up to the peak), not from the products level.
• Error 2: The reaction is exothermic (releases energy), so the products line (2H₂O) should be lower than the reactants line (2H₂ + O₂).

💡 Key Knowledge

• The prompt states: "The reaction releases energy". This explicitly defines it as exothermic.
• In an exothermic reaction: Energy_products < Energy_reactants.
• Activation energy ( E _a) is always measured from the reactants' energy level to the highest point of the curve.

🧠 Exam Technique

Read the question stem carefully! It told you energy was released. Students who missed this tried to describe it as an endothermic reaction profile rather than pointing out that the diagram wrongly showed an endothermic profile.

❌ Common Errors

• Saying the activation energy arrow is "pointing the wrong way" without stating it starts from the wrong horizontal baseline.
• Stating "the overall energy change arrow is upside down" — the mark scheme explicitly ignores the direction of this arrow.

✅ Correct Answers (Any 2)

• Takes less time to refuel (compared to long recharging times for batteries).
• Can travel further before refuelling / greater driving range.
• No toxic chemicals to dispose of at the end of the cell's lifespan.
• No loss of efficiency / does not lose capacity or range over time or in cold weather.

🧠 Exam Technique

• Comparative words are essential: say "faster refuelling" rather than just "it refuels".
• Converse arguments about rechargeable batteries are allowed (e.g. "batteries take hours to recharge" or "batteries contain toxic metals that are hard to recycle").

❌ Common Errors

• "Produces only water / no pollutants": While true for the fuel cell in operation, rechargeable electric cars also produce zero exhaust emissions, so this is not a comparative advantage over rechargeable cells.
• Vague statements like "cheaper" or "greener" without qualified scientific explanation.

✅ Correct Half-Equations (Give Any 1)

Acidic electrolyte:

• Negative electrode (oxidation): H₂ → 2H⁺ + 2e⁻
• Positive electrode (reduction): O₂ + 4H⁺ + 4e⁻ → 2H₂O

Alkaline electrolyte:

• Negative electrode: H₂ + 2OH⁻ → 2H₂O + 2e⁻
• Positive electrode: O₂ + 2H₂O + 4e⁻ → 4OH⁻

💡 Key Knowledge

The simplest and most reliable equation to memorise for AQA GCSE is the oxidation of hydrogen at the negative electrode:

H₂ → 2H⁺ + 2e⁻

Remember: Oxidation is loss of electrons (OIL), occurring at the negative terminal of a fuel cell.

❌ Common Errors

• Writing the overall equation ( 2H₂ + O₂ → 2H₂O ) instead of a half-equation involving electrons ( e⁻ ).
• Unbalanced charges or incorrect electron placement (e.g. writing + 2e⁻ on the reactant side for hydrogen oxidation).

✅ Correct Answers (Any 2)

• Hydrogen is not shown as diatomic molecules / not shown as H₂ .
• Particles are shown as solid spheres (atoms/molecules are mostly empty space).
• Does not show the (weak intermolecular) forces between particles.
• Does not show the movement or speed of the particles.
• Model is only two-dimensional (2D), whereas real gas exists in three dimensions (3D).

🧠 Exam Technique

Whenever asked for limitations of the particle model in chemistry, standard specification recall points apply: no forces shown, particles shown as inelastic solid spheres, and 2D limitation. For hydrogen specifically, pointing out that it is diatomic ( H₂ ) rather than individual single atoms is a top-level response.

❌ Common Errors

• Writing "particles are too far apart" (gas particles really are far apart).
• Vague points like "not drawn to scale" without specifying what is inaccurate.

✅ Correct Answers (Any 1)

• Store under higher pressure (compress the gas).
• Cool the gas / condense into a liquid / store at low temperature.
• Absorb / adsorb into a solid material (e.g. metal hydride matrix).

💡 Key Knowledge

Gases have massive amounts of empty space between particles. According to Boyle's and Charles's Gas Laws:

• Increasing pressure forces gas particles closer together ( Volume ∝ 1/Pressure ).
• Decreasing temperature reduces kinetic energy, causing contraction or condensation into a dense liquid.

❌ Common Errors & Traps

• Prefix failure: Forgetting that MJ is megajoules (10⁶) and doing 58 ÷ 290 = 0.2 mol , forgetting to convert to kJ first.
• Dividing instead of multiplying: Calculating 200 ÷ 24 instead of 200 × 24 for molar gas volume.
• Unit muddling: Trying to convert 24 dm³ into cm³ unnecessarily and losing track of powers of 10.

🧠 Exam Technique: Error Carried Forward (ecf)

• Always write out your conversions clearly! Even if you incorrectly converted 58 MJ, you can still gain marks 2, 3, and 4 via error carried forward if your method is clear.
• Check the reasonableness of your final value: 100 km requires a substantial volume of fuel gas, so an answer of 4.8 dm³ or 0.2 dm³ should instantly alert you to a unit slip.