Explain how increasing the temperature of a reaction increases the rate of reaction. In your answer, refer to the Maxwell-Boltzmann distribution and the activation energy.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student notices that a reaction between two solutions proceeds much faster when carried out at 50 °C than at 20 °C. The activation energy of the reaction is relatively high.
Model answer (5 marks)
The Maxwell‑Boltzmann distribution shows the spread of kinetic energies of the molecules in a sample.
At a higher temperature the molecules move faster and possess more kinetic energy.
Consequently the Maxwell‑Boltzmann curve shifts to the right; its peak moves to a higher energy.
A larger proportion of the molecules now have energies equal to or greater than the activation energy.
Thus there are more effective collisions per unit time, so the rate of reaction increases.
At a higher temperature the molecules move faster and possess more kinetic energy.
Consequently the Maxwell‑Boltzmann curve shifts to the right; its peak moves to a higher energy.
A larger proportion of the molecules now have energies equal to or greater than the activation energy.
Thus there are more effective collisions per unit time, so the rate of reaction increases.
Examiner tips
- Use the phrase "Maxwell‑Boltzmann distribution" and mention the shift to the right. Include "activation energy" and "effective collisions". Show the causal chain from temperature to energy distribution to rate.
Common mistakes
- Saying the distribution becomes narrower instead of shifting. Forgetting to mention the activation energy. Using vague terms like "more collisions" without linking to energy threshold.
Mark scheme (5 marks)
- The Maxwell-Boltzmann distribution shows the spread of energies (or speeds) of particles in a sample
- At higher temperature, particles move faster / have more (kinetic) energy
- The Maxwell-Boltzmann distribution curve shifts to the right / the peak moves to higher energy at higher temperature
- A greater proportion / more particles have energy equal to or greater than the activation energy
- Therefore there are more successful / effective collisions per unit time, so the rate of reaction increases
Key terms in this question
Maxwell-Boltzmann distribution · activation energy · rate of reaction
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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