Explain why a real gas such as ammonia deviates more significantly from ideal gas behaviour at high pressure and low temperature than at low pressure and high temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An ideal gas assumes no intermolecular forces; real gases such as ammonia have attractive forces.
At low temperature the kinetic energy of the molecules is low, so the attractive forces become significant compared with the kinetic energy, producing a larger deviation.
An ideal gas also assumes the volume of the particles is negligible; real molecules occupy a finite volume.
At high pressure the molecules are forced close together, so the volume occupied by the molecules is a larger fraction of the total volume, again increasing the deviation.
At low temperature the kinetic energy of the molecules is low, so the attractive forces become significant compared with the kinetic energy, producing a larger deviation.
An ideal gas also assumes the volume of the particles is negligible; real molecules occupy a finite volume.
At high pressure the molecules are forced close together, so the volume occupied by the molecules is a larger fraction of the total volume, again increasing the deviation.
Examiner tips
- Use the word "attract" to show knowledge of intermolecular forces. Show the two conditions (high pressure, low temperature) and link each to a specific cause. Keep the answer concise – 4 marks, so one sentence per point.
- Mention both the attractive forces and finite volume to cover all scheme points.
Common mistakes
- Confusing high temperature with low temperature. Forgetting to mention the finite volume of molecules. Using vague terms like "more gas-like" instead of specific causes.
Mark scheme (4 marks)
- An ideal gas assumes no intermolecular forces between particles; real gas molecules (e.g. ammonia) do experience intermolecular forces / attractions.
- At low temperature, molecules move more slowly / have lower kinetic energy, so intermolecular attractions have a greater effect relative to kinetic energy, causing greater deviation.
- An ideal gas assumes the volume of gas particles themselves is negligible compared to the container volume; real gas molecules occupy a finite / non-negligible volume.
- At high pressure, molecules are forced closer together, so the actual volume occupied by the molecules becomes a significant fraction of the total volume, causing greater deviation from ideal behaviour.
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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