Explain why the pressure of a fixed amount of gas increases when its temperature is raised at constant volume.

IB DP Chemistry Standard Level (2023 syllabus) — S1.1 Introduction to the particulate nature of matter · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Increasing the temperature raises the average kinetic energy of the gas particles. The faster particles collide with the walls more often and each collision changes the particle’s momentum by a larger amount. The greater frequency and larger momentum change of collisions means a larger average force on the walls, so the pressure rises while the volume and amount of gas stay constant.

Examiner tips

  • Use the word ‘increase’ to show cause and effect; link temperature to kinetic energy, then to collision frequency and force, then to pressure.
  • Show the chain of reasoning in a logical order; avoid unnecessary words.
  • Mention that volume is constant so the only variable changing is kinetic energy of particles.

Common mistakes

  • Failing to connect temperature with kinetic energy; writing only ‘temperature rises’ without explanation.
  • Confusing pressure with volume or number of particles; forgetting to state that volume is constant.
  • Using vague terms like ‘more energy’ without specifying kinetic energy or momentum change.

Mark scheme (4 marks)

  1. Increasing temperature increases the average kinetic energy (or speed) of the gas particles.
  2. Faster-moving particles collide with the walls of the container more frequently.
  3. Each collision imparts greater force to the walls because the particles are moving faster (greater momentum change per collision).
  4. Greater frequency and force of collisions with the walls results in higher pressure (since volume is constant, the number of particles and their confinement remain unchanged).

Key terms in this question

pressure · temperature

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