Gas Behaviour: Kinetic-Molecular Theory

The gas laws we’ve seen so far (including the Ideal Gas Equation) are empirical, that is, they have been derived from experimental observations. The mathematical forms of these laws closely describe the macroscopic behavior of most gases at pressures less than about 1 or 2 atm. Although the gas laws describe relationships that have been verified by many experiments, they do not tell us why gases follow these relationships.

The kinetic molecular theory (KMT) is a simple microscopic model that effectively explains the gas laws you are familiar with. This theory is based on the following five postulates:

  1. Gases are composed of particles that are in continuous motion, travelling in straight lines and changing direction only when they collide with other particles or with the walls of a container.
  2. The gas particles are negligibly small compared to the distances between them.
  3. The pressure exerted by a gas in a container results from collisions between the gas particles and the container walls.
  4. Gas particles exert no attractive or repulsive forces on each other or the container walls; therefore, their collisions are elastic (do not involve a loss of energy).
  5. The average kinetic energy of the gas particles is proportional to the kelvin temperature of the gas.

(Note: The term “particle” is used to refer to the individual chemical species that compose the gas, which may be a molecule – like CO2 – or an atom – like He.)

We will walk through the origins and implications of these statements and see how they affect our gas behaviour.