A student is investigating two reactions, Reaction X and Reaction Y, both producing the same product. Reaction X is exothermic with a high activation energy. Reaction Y is also exothermic but has a much lower activation energy. The student adds a catalyst to Reaction X. Explain how the catalyst increases the rate of Reaction X, and explain how the reaction profile of Reaction X would change after the catalyst is added.

OCR A-Level Chemistry B: Salters (H433) — 5.2 Energy · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Reaction X: exothermic, high activation energy, slow rate at room temperature. Reaction Y: exothermic, low activation energy, fast rate at room temperature.

Model answer (5 marks)

The catalyst provides a different reaction pathway
The catalyst lowers the activation energy
A greater proportion of particles now have energy at least equal to the (lower) activation energy
The catalyst is not used up during the reaction
On the reaction profile, the peak (activation energy barrier) would be lower, but the overall energy change (difference in energy between reactants and products) would remain the same

Examiner tips

  • Use the exact wording from the mark scheme; each point counts as one mark
  • Show the catalyst’s effect on the energy diagram – lower peak, same ΔH

Common mistakes

  • Confusing the catalyst with a reactant or product
  • Saying the catalyst changes the overall ΔH instead of just the activation energy

Mark scheme (5 marks)

  1. The catalyst provides a different reaction pathway
  2. The catalyst lowers the activation energy
  3. A greater proportion of particles now have energy at least equal to the (lower) activation energy
  4. The catalyst is not used up during the reaction
  5. On the reaction profile, the peak (activation energy barrier) would be lower, but the overall energy change (difference in energy between reactants and products) would remain the same

Key terms in this question

activation energy · catalyst · reaction profile

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