Kinetic Theory of Gases (SL IB Physics)

• Ideal gases are described in terms of the kinetic theory
  • This is a modelled system used to approximate the behaviour of real gases

• A gas consists of atoms or molecules moving around randomly at high speeds
• The kinetic theory of gases models the thermodynamic behaviour of gases by linking:
  • The microscopic properties of particles e.g. mass and speed
  • The macroscopic properties of particles e.g. pressure and volume

• Kinetic theory and ideal gases are based on a set of the following assumptions:

1. 1. A gas consists of many identical molecules in a container. They all have the same mass
   2. The volume of the molecules is negligible compared to the volume of the container. This means they can be considered point particles
   3. The molecules are in continuous random motion at high speeds
   4. The molecules obey Newton's laws of motion
   5. The molecules collide elastically with each other and the walls of the container
   6. There are no intermolecular forces between the molecules except during collisions
   7. The time of a collision between molecules is negligible compared to the time between collisions
   8. External forces (e.g. gravity) are ignored
   9. The number of molecules of gas in a container is very large. Therefore the average behaviour (eg. speed) is usually considered
   10. Each particle exerts a force on the wall of the container with which it collides. This means the average of the forces produced by all gas particles results in a uniform gas pressure 

• Real gases will not always obey the ideal gas equation
• This is because the assumptions of the kinetic theory of gases may not always be valid
  • For example, the assumption that there are no intermolecular forces between gas molecules is not particularly realistic

• However, under certain conditions, they can be considered ideal gases
• An ideal gas is a good approximation of a real gas when:
  • The gas pressure is low
  • The gas density is low
  • Temperature is sufficiently higher than the boiling point of the substance

• At very high pressures, densities and low temperatures, real gases do not obey the kinetic theory model
• This is because at high pressures and densities:
  • Molecules are closer together
  • Therefore, there will be attractive forces between the molecules
  • The volume of the molecules cannot be considered negligible due to the high density

• At low temperatures:
  • A gas can change into a liquid, so the substance will no longer behave like a gas

• No gas is completely ideal, but air at normal room temperature and pressure, based on experiments, behaves very similar to an ideal gas

Real vs. ideal gases