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AS · Practice questions · The Boltzmann distribution and activation energy

Spread of energies.

Six original Cambridge-style questions on the Boltzmann distribution: drawing and labelling it, its features and the effect of temperature.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Same area, lower peak, more beyond Ea.

01
Analysis
[3 marks]

Sketch the Boltzmann distribution for a gas. Label the axes and mark the activation energy.

  • Axes: number (or proportion) of molecules on y, energy on x. ✓
  • Curve from the origin, rising to a peak, then a long tail approaching but not touching the x-axis. ✓
  • A vertical line for Ea well to the right of the peak. ✓
02
Analysis
[2 marks]

On the same axes, add the curve for the same gas at a higher temperature and describe two differences.

  • The peak is lower and shifted to the right. ✓
  • The curve is broader, with more molecules at high energy; the area under both curves is the same. ✓
03
Explain
[3 marks]

Use the Boltzmann distribution to explain why increasing the temperature increases the rate of reaction.

  • At higher temperature the distribution shifts to higher energies. ✓
  • A greater proportion of molecules have energy equal to or greater than Ea. ✓
  • So there are more successful collisions per unit time (collision frequency also rises slightly). ✓
04
Explain
[2 marks]

Explain why the area under the curve does not change when the temperature is raised.

  • The area represents the total number of molecules. ✓
  • Heating does not change the number of molecules, only how their energies are spread. ✓
05
Recall
[1 marks]

Define activation energy.

  • The minimum energy that colliding particles must have for a reaction to occur. ✓
06
Explain
[2 marks]

A student says: “Heating a reaction lowers the activation energy.” Explain why this is wrong.

  • The activation energy is fixed for a given reaction pathway; temperature does not change it. ✓
  • Heating increases the number of molecules with energy above Ea; only a catalyst lowers Ea. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.