3.4 Deviation from Ideal Gas Law

Why real gases aren't ideal

The ideal gas law (PV = nRT) assumes gas particles have no volume and no intermolecular forces. Real gases deviate from this most under high pressure (particles squeezed close together, volume matters) and low temperature (particles move slowly, attractive forces matter more).

Two causes of deviation

At high pressure, particle volume becomes significant relative to container volume, making real gas volume *larger* than predicted. At low temperature, intermolecular attractions pull particles together, making real pressure *lower* than predicted. Both effects cause the ideal gas law to fail.

Which gases deviate most

Gases with larger molar mass and stronger intermolecular forces (polar molecules, hydrogen bonding) deviate more from ideal behavior than small, nonpolar gases like He or H₂. This is because larger, more polar molecules have greater volume and stronger attractions between particles.

Key terms

Ideal gas law assumptions

Gas particles have negligible volume and no intermolecular forces.

When do gases deviate from ideal behavior?

At high pressure (volume matters) and low temperature (intermolecular forces matter).

Effect of high pressure on real gases

Particle volume becomes significant, making real volume larger than ideal gas law predicts.

Effect of low temperature on real gases

Intermolecular attractions become significant, making real pressure lower than ideal gas law predicts.

Which gases behave most ideally?

Small, nonpolar gases with weak intermolecular forces, like He and H2.

Try an AP-style question

Under which conditions does a real gas behave most ideally?

1. Low temperature and high pressure
2. High temperature and high pressure
3. High temperature and low pressure
4. Low temperature and low pressure