Explain why the pressure of an ideal gas increases when its temperature is raised at constant volume.

IB DP Chemistry Standard Level (2023 syllabus) — S1.5 Ideal gases · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Raising the temperature increases the average kinetic energy of the gas particles, so they move faster. Faster particles collide with the walls more often and each collision transfers more momentum, giving a larger force on the walls. With the volume fixed, the increased force per unit area raises the pressure.

Examiner tips

Use the word ‘average kinetic energy’ and ‘momentum’ to show understanding of the kinetic‑theory basis.
Explain both the increased collision frequency and the larger impulse per collision.
Show the link between force per area and pressure.
Mention that volume is constant so the only variable changing is the force on the walls.

Common mistakes

Failing to mention that temperature raises kinetic energy rather than just speed.
Confusing pressure with force or volume, or ignoring the constant volume condition.
Using vague terms like ‘more energy’ without linking to kinetic theory.

Mark scheme (4 marks)

Raising temperature increases the average kinetic energy (or speed) of the gas particles.
Faster-moving particles collide with the walls of the container more frequently.
Each collision imparts greater force / impulse to the wall because the particles have greater momentum.
Because the volume is constant, the greater force and frequency of wall collisions results in higher pressure.

Key terms in this question

ideal gas · pressure · constant volume