KINETIC THEORY OF GASES

Macroscopic physical quantities of a gas like pressure, temperature, etc. can be understood from the interrelationships between its microscopic quantities. This is discussed in the kinetic theory of gases based on the following postulates.

Molecular model of ideal gas: Postulates:



( 1 ) A gas is made up of microscopic particles called molecules which may be monoatomic or polyatomic. If only one element is present in a gas, all its molecules are same and chemically stable.

( 2 ) The molecules of a gas can be considered as perfectly rigid spheres or particles devoid of internal Structure.

( 3 ) The molecules are in continuous random motion colliding with each other and with the walls of the container.

( 4 ) The molecules of a gas follow Newton’s laws of motion.

( 5 ) The number of molecules in a gas is very large. This assumption justifies randomness of their motion.

( 6 ) The total volume of all the molecules of a gas is negligible as compared to the volume of the vessel containing the gas.

( 7 ) Intermolecular forces act only when two molecules come close to each other or collide.

( 8 ) The collision between the molecules and between the molecules and the wall of the container are elastic. The impact time of collision is negligible as compared to the time between successive collisions. Kinetic energy is conserved in an elastic collision. During the impact time of collisions, kinetic energy before collision is momentarily converted into potential energy but is again reconverted into the same amount of kinetic energy
after the collision. Hence kinetic energy of the gas can be considered to be its total mechanical energy.