Deviations from Ideal Behavior
Quick Notes
- The ideal gas law assumes gas particles have no volume and experience no intermolecular forces.
- Real gases deviate from this behavior, especially at low temperatures and high pressures.
- At low temperatures: particles move slower, and intermolecular attractions become more significant.
- At high pressures: gas particles are closer together, so their own volume affects total volume.
- These deviations explain why real gases may not obey PV = nRT perfectly under all conditions.
Full Notes
The ideal gas law (PV = nRT) provides a useful model for gas behavior (see Ideal Gas Law), but it is based on assumptions that do not always hold true for real gases. Under certain conditions, gases exhibit non-ideal behavior due to intermolecular forces and particle volume.
Why Gases Deviate from Ideal Behavior
The two main causes of deviation are:
- Interparticle (intermolecular) attractions
- Finite volume of gas particles
These effects are especially noticeable under non-ideal conditions of very low temperature and very high pressure.
Effect of Intermolecular Forces
At low temperatures, gas particles move more slowly. This gives intermolecular forces (such as London dispersion forces or dipole-dipole interactions) more opportunity to pull particles together. The particles don't have enough kinetic energy to overcome the strength of attraction that exists between them.
Result:
Gas particles are attracted to one another, so they collide with container walls less frequently and with less force.
This causes the observed pressure to be lower than predicted by the ideal gas law.
Effect of Particle Volume
The ideal gas law assumes that gas particles have no volume, but real particles occupy space.
At high pressures, particles are pushed closer together, and their actual size becomes significant compared to the volume of the container.
Result:
The space available for particle movement is less than expected.
This causes the observed volume to be greater than predicted by the ideal gas law.
Summary of Deviations
- Ideal gases behave according to PV = nRT under most moderate conditions.
- Real gases follow this model less accurately when particles are close together or moving slowly.
- Gases behave more ideally at:
- High temperature (faster movement reduces attractions)
- Low pressure (particles are farther apart)
- Some real gases (like He or H2) behave nearly ideally over a wide range. Others (like NH3 or CO2) deviate more strongly due to stronger intermolecular forces or larger molecular size.
If you see a question about gases condensing or approaching liquefaction, think about intermolecular attractions. If pressure is very high, think about particle volume. These are clues that the gas might not follow ideal behavior.
Summary
- The ideal gas law works well under many conditions, but real gases deviate due to:
- Intermolecular attractions, which reduce pressure at low temperatures.
- Particle volume, which reduces available space at high pressures.
- Understanding when and why these deviations occur helps explain why gases condense and why PV = nRT is sometimes only an approximation.