Explain why the pressure exerted by a real gas is lower than that predicted by the ideal gas equation at high pressures.

IB DP Chemistry Higher 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)

The ideal gas model assumes no intermolecular forces between particles.
At high pressures the molecules are forced closer together, so intermolecular (attractive) forces become significant.
These attractive forces pull the molecules toward the bulk of the gas, reducing the velocity/momentum with which they strike the container walls.
Therefore the measured pressure is less than the value calculated using the ideal gas equation (PV = nRT).

Examiner tips

Use the word ‘explain’ to describe the cause and effect; mention both the lack of forces in the ideal model and the attraction at high pressure.
Show the logical sequence: ideal assumption → high pressure → attraction → reduced momentum → lower pressure.
Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

Confusing repulsive with attractive forces; the pressure drop is due to attractive forces.
Failing to link the attraction to reduced momentum on the walls.
Writing a general statement about real gases without referencing high pressure or the ideal equation.

Mark scheme (4 marks)

The ideal gas model assumes no intermolecular forces between particles.
At high pressures, molecules are forced closer together, so intermolecular (attractive) forces become significant.
Attractive forces pull molecules toward the bulk of the gas, reducing the velocity/momentum with which they strike the container walls.
Therefore the measured pressure is less than the value calculated using the ideal gas equation (PV = nRT).

Key terms in this question

ideal gas · real gas · pressure