Explain why the rate of a gas-phase reaction increases when the pressure is increased at constant temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Increasing pressure reduces the volume of the reaction vessel, so the concentration of the gas‑phase reactants rises.
Higher concentration means more reactant molecules occupy the same space, leading to a greater number of collisions per unit time.
Because the temperature is constant, the fraction of molecules that possess energy equal to or greater than the activation energy does not change.
Thus, with more collisions occurring and the same fraction of them having sufficient energy, a larger proportion of collisions result in reaction, so the overall rate increases.
Higher concentration means more reactant molecules occupy the same space, leading to a greater number of collisions per unit time.
Because the temperature is constant, the fraction of molecules that possess energy equal to or greater than the activation energy does not change.
Thus, with more collisions occurring and the same fraction of them having sufficient energy, a larger proportion of collisions result in reaction, so the overall rate increases.
Examiner tips
- Mention concentration change first, then collision frequency, then unchanged energy fraction, finally rate increase.
- Use terms: concentration, collisions, activation energy, rate of reaction.
- Keep answer concise – 4 points, one sentence each.
Common mistakes
- Confusing pressure with temperature effects; stating temperature changes.
- Using vague phrases like ‘more molecules’ without linking to concentration and collisions.
- Over‑explaining activation energy fraction or omitting the final rate‑increase statement.
Mark scheme (4 marks)
- Increasing pressure decreases the volume, so the concentration (number of moles per unit volume) of the reactant molecules increases.
- The increased concentration leads to more frequent collisions between reactant molecules per unit time.
- Since temperature is unchanged, the fraction of molecules with energy greater than or equal to the activation energy remains the same.
- Therefore, more collisions per unit time have sufficient energy (and correct orientation) to result in reaction, so the overall rate increases.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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