Explain how the addition of a catalyst increases the rate of a chemical reaction.

IB DP Chemistry Standard Level (2023 syllabus) — R2.2 How fast? The rate of chemical change · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A catalyst offers an alternative reaction pathway that has a lower activation energy.
This lower activation energy means that a larger fraction of the colliding molecules possess the energy required to react.
Consequently, the reaction rate increases.
The catalyst is not consumed in the overall reaction, so it can repeatedly catalyse further reactions.

Examiner tips

  • Use the term "alternative reaction pathway" and "lower activation energy" – these are key phrases. Show the logical link: lower Ea → more molecules above threshold → faster rate. Mention that the catalyst is unchanged to satisfy the last point.
  • common_mistakes
  • :
  • Confusing a catalyst with a reactant or product. Failing to state that the catalyst is not consumed. Using vague terms like "makes it faster" without explaining the mechanism.

Mark scheme (4 marks)

  1. A catalyst provides an alternative reaction pathway (mechanism).
  2. This alternative pathway has a lower activation energy.
  3. A greater proportion/fraction of colliding particles now have energy greater than or equal to the (new, lower) activation energy.
  4. The catalyst is not consumed/used up in the overall reaction, so it can continue to catalyse further reactions.

Key terms in this question

catalyst

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