AQA A-Level Chemistry Paper 3, 2024: Question 16

Catalyst & Rate: What changes at constant temperature?

What this question tests 1 mark • AO1

Collision theory language: successful collisions vs collision frequency.
Effect of a catalyst at constant temperature: catalyst provides an alternative pathway with lower activation energy.
Interpreting statements about average particle energy (only changes with temperature, not catalyst).

Examiner focus: this is a precision wording question. You score the mark by linking the catalyst to a decrease in activation energy, which increases the proportion of particles with E ≥ Eₐ (i.e., successful collisions).

Question 16 Multiple choice

Part (16): Why does the rate increase when a catalyst is added at constant temperature?

You are choosing the one statement that correctly explains the rate increase without changing temperature.

✅ Correct answer (from mark scheme)

C — The proportion of successful collisions increases because there is a decrease in activation energy.

Mark breakdown (1 mark): Awarded for selecting C exactly.

💡 Key knowledge (what you must know)

A catalyst provides an alternative reaction pathway with a lower activation energy (Eₐ).
At constant temperature, the average kinetic energy of particles stays the same.
Lower Eₐ means a greater fraction of collisions have energy ≥ Eₐ → more successful collisions → faster rate.

Link to Maxwell–Boltzmann idea: the distribution doesn’t shift (T constant), but the Eₐ threshold is lower, so the area beyond Eₐ increases.

🧠 Exam technique (how to spot the right option fast)

Underline the constraint: “at constant temperature”. This immediately eliminates any option claiming average energy increases.
Next, check the mechanism: catalysts change Eₐ, not how often particles collide (collision frequency is mainly affected by concentration/pressure and temperature).
Pick the option that says: successful collisions ↑ because Eₐ ↓ .

What distinguished top responses: choosing the option that uses the correct collision theory term “proportion of successful collisions” and the correct cause “decrease in activation energy”.

❌ Common errors (why the other options are wrong)

A (wrong): says collision frequency increases due to Eₐ decreasing. Lower Eₐ does not make particles collide more often; it makes a higher fraction of collisions successful.
B (wrong): says collision frequency increases because average energy increases. Average energy only increases if temperature increases.
D (wrong): says proportion of successful collisions increases because average energy increases. Again, average energy does not increase when adding a catalyst at constant temperature.

Examiner insight: many students confuse “rate increases” with “collisions happen more often”. In catalyst questions, the key phrase is almost always lower activation energy → more successful collisions.

Quick recall 1-mark wording

High-scoring phrasing you can reuse

✅ Model statement

“A catalyst increases the rate by providing an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful.”

❌ Avoid saying

“Catalyst increases collision frequency.”
“Catalyst increases the energy of particles.”
“Catalyst increases the number of collisions” (unless you also correctly explain it’s via Eₐ and success rate—better to avoid).