From observations to a molecular model
The gas laws are empirical: they describe relationships found from experiments. Kinetic theory instead starts from a theoretical model of many moving particles and uses it to explain macroscopic quantities such as pressure and volume.
The kinetic-theory model assumes that a gas contains a very large number of particles undergoing continuous random motion.
- The gas particles are identical, so each has the same mass.
- Particles behave as hard spheres and collisions are perfectly elastic.
- The volume of the particles themselves is negligible compared with the volume of the container.
- The duration of a collision is negligible compared with the time between collisions.
- No intermolecular forces act except during collisions.
- External forces such as gravity are ignored.
- Between collisions, particles move in straight lines.
- The overall molecular motion is random and Newton's laws apply.
- The number of particles is sufficiently large for average behaviour to be considered.
Pressure arises because collisions with the container walls change the particles' momentum. The combined force from a very large number of such collisions produces a measurable pressure.
Exam Tip: The assumptions of kinetic theory are commonly tested directly, so be ready to recall the full set, including negligible particle volume, perfectly elastic collisions and no intermolecular forces.