SL and HL: The kinetic theory of gases models a gas as many particles in constant random motion. An ideal gas is a simplified model whose particles obey specific assumptions, allowing relationships such as to describe its behaviour.
The Reasoning and Mechanism
In IB Physics B.3 Gas laws, kinetic theory connects microscopic particle motion to macroscopic quantities such as pressure and temperature.
Gas particles collide with the container walls and change momentum. By Newton's second law, this rate of change of momentum produces a force on the wall. Since pressure is force per unit area, , repeated collisions create gas pressure.
The kelvin temperature measures the particles' average translational kinetic energy:
Therefore, increasing temperature increases the particles' average speed and kinetic energy. At fixed volume, their collisions with the walls become more frequent and involve larger momentum changes, so pressure increases.
| Ideal-gas assumption | Meaning |
|---|---|
| Particles are point-like | Their individual volumes are negligible compared with the gas volume. |
| Motion is random | Particles move continuously in all directions and obey Newton's laws. |
| No intermolecular forces act | Particles do not attract or repel one another except during collisions. |
| Collisions are perfectly elastic | Total kinetic energy is conserved in particle-particle and particle-wall collisions. |
| Collision time is negligible | Collisions occur over a much shorter time than the interval between them. |
| Many particles are present | Their collective behaviour can be described statistically. |
Real gases approximate ideal behaviour most closely at low pressure, where particles are widely separated, and high temperature, where intermolecular attractions are relatively insignificant.
A common misconception is that ideal-gas particles have no volume at all. The model assumes their volume is negligible compared with the container volume, not that real molecules are literally dimensionless.
Exam Technique
For an IB Explain question, link collisions to momentum change, force, and pressure. If asked when the model is valid, state high temperature and low pressure, then explain why intermolecular forces and particle volume become less significant.
