A student notices that a packet of crisps contains a small sachet labelled 'keep dry — contains silica gel, a desiccant catalyst for packaging reactions.' A chemistry teacher corrects this, explaining that silica gel is not a catalyst but that catalysts are used in many industrial processes to increase reaction rates. Explain how a solid catalyst increases the rate of a chemical reaction. In your answer, refer to activation energy and the Maxwell-Boltzmann distribution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Catalysts are widely used in industry to increase the rate of chemical reactions without being used up in the reaction. The Maxwell-Boltzmann distribution shows the spread of energies of particles in a reaction mixture at a given temperature.
Model answer (5 marks)
A solid catalyst provides an alternative reaction pathway that has a lower activation energy than the uncatalysed route.
Because the activation energy is lower, a larger proportion of the reacting molecules have kinetic energies equal to or greater than this new, lower threshold.
On the Maxwell‑Boltzmann distribution curve this means the area under the curve to the right of the lower activation energy is greater.
Consequently, more collisions have sufficient energy to overcome the barrier per unit time.
Thus the overall rate of the reaction increases while the catalyst itself is not consumed.
Because the activation energy is lower, a larger proportion of the reacting molecules have kinetic energies equal to or greater than this new, lower threshold.
On the Maxwell‑Boltzmann distribution curve this means the area under the curve to the right of the lower activation energy is greater.
Consequently, more collisions have sufficient energy to overcome the barrier per unit time.
Thus the overall rate of the reaction increases while the catalyst itself is not consumed.
Examiner tips
- Use the phrase "alternative reaction pathway" and "lower activation energy". Show the link to the Maxwell‑Boltzmann curve and the increased area. Explain that more successful collisions give a higher rate.
- Remember to mention that the catalyst is not consumed – this is a key point in the mark scheme.
Common mistakes
- Confusing silica gel as a catalyst; students may write it is a catalyst. Failing to mention the lower activation energy or the increased area under the Maxwell‑Boltzmann curve. Using vague wording like "more energy" without linking to the distribution or activation energy.
Mark scheme (5 marks)
- A catalyst provides an alternative reaction pathway
- The alternative pathway has a lower activation energy
- A greater proportion / more particles have energy greater than or equal to the (new, lower) activation energy
- Reference to the Maxwell-Boltzmann distribution: the area under the curve to the right of the (lower) activation energy is larger
- Therefore more successful collisions per unit time occur, increasing the rate of reaction
Key terms in this question
activation energy · Maxwell-Boltzmann distribution · catalyst
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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