Explain why the ideal gas equation becomes less accurate at low temperatures when applied to a sample of sulfur dioxide gas.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
At low temperatures the average kinetic energy of SO₂ molecules is lower, so the molecules move more slowly. The attractive van der Waals forces between SO₂ molecules become significant and are no longer negligible. Because the molecules do not have enough kinetic energy to overcome these attractive forces, they spend more time close together and are drawn towards one another. Consequently the measured pressure is lower than that predicted by the ideal gas equation (or the actual volume is lower than the ideal value), because the ideal gas equation assumes no intermolecular forces.
Examiner tips
- Use the word ‘attractive’ to describe the forces, not just ‘forces’.
- Show the link between lower kinetic energy and increased importance of attractions.
- Mention the effect on pressure or volume explicitly.
Common mistakes
- Saying the gas behaves like a liquid instead of explaining reduced kinetic energy.
- Forgetting to mention that the ideal gas equation assumes no intermolecular forces.
Mark scheme (4 marks)
- At low temperatures, the average kinetic energy of SO₂ molecules is lower / molecules move more slowly.
- Intermolecular (attractive) forces between SO₂ molecules become significant / are not negligible.
- Molecules do not have sufficient kinetic energy to fully overcome these attractive forces, so they spend more time close together / are drawn towards one another.
- The measured pressure is lower than that predicted by the ideal gas equation / the actual volume is lower than the ideal value, because the ideal gas equation assumes no intermolecular forces.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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