Limitations of the Ideal Gas Model
Specification Reference S1.5.2
Quick Notes
- Real gases deviate from ideal behavior, especially at low temperatures and high pressures.
- Limitations of the ideal gas model:
- Real particles have volume which cannot be ignored at high pressure.
- Real gases have intermolecular forces which become significant at low temperatures.
- Ideal gas laws are approximations and work best under moderate conditions.
Full Notes
Recap of Ideal Gas Assumptions
Ideal gases are based on key assumptions (see S1.5.1):
- Particles have no volume.
- No intermolecular forces exist.
- All collisions are elastic.
- Particles move in random, straight-line motion.
- Temperature is directly related to average kinetic energy.
These assumptions make calculations simpler, but real gases do not always follow them exactly.
When Do Real Gases Deviate?
Real gases deviate most under high pressure and at low temperatures.
High Pressure
- Particles are forced closer together.
- Their volume is no longer negligible.
- Gas compresses less than predicted by the ideal model.
Low Temperature
- Particles move slower, so attractive forces (like Van der Waals forces) have more effect.
- Particles may stick together briefly or condense.
- Pressure is lower than expected because particles do not hit the walls as often or as hard.
Why These Deviations Matter
In real life gases like CO2, NH3, and H2O show noticeable deviation due to strong intermolecular forces.
However, hydrogen and helium behave more ideally because they are small and have weak forces.
Understanding the limitations helps us know when ideal gas equations are accurate. We can choose more complex models when needed.
Summary
- The ideal gas model simplifies gas behavior, but real gases deviate at low temperatures and high pressures.
- Deviations occur because particles have volume and interact with each other.
- Ideal gas assumptions work best under normal lab conditions, not extreme environments.