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Kinetic Molecular Theory of Gases

This theoretical model was developed by the contribution of Maxwell, Boltzmann. Clausius, etc., which is known as kinetic molecular theory of gases or microscopic model of gases.

The various postulates of the theory are:

1. All gases are made up of very large number of extremely small particles called molecules.

2. The molecules are separated from one another by large spaces so that the actual volume occupied by the molecules is negligible as compared to the total volume of the gas.

3. The molecules are not at rest but possess rapid random motion. During their motion, they collide with one another and also against the walls of the container.

4. The pressure of the gas is due to bombardment of the gas molecules against the walls of the container.

5. The collisions of the molecules with each other and with the walls of the container are perfectly elastic, i.e., there is no loss or gain of kinetic energy. However, there may be redistribution of energy during such collisions.

6. There are no attractive or repulsive forces between the molecules of the gas. They are completely independent of each other.

7. At any instant, different molecules possess different velocities and hence, different energies. However, the average kinetic energy of the molecules is directly proportional to the absolute temperature.

 

JUSTIFICATION FOR THE POSTULATES

There is an ample evidence in favour of the postulates of kinetic theory as described below:

(i) The first postulate is in accordance with the particle nature of matter.

(ii) The high compressibility of gases provides ample proof for the existence of large empty space between the gas molecules. For example, at S.T.P. the empty space in 1 mole of N2 gas has been calculated to be 99.91%.

(iii) Dust particles or smoke particles are found to be in a constant zig-zag movement called Brownian movement. This is evident due to the fact that they are being tossed about by the fast moving molecules of the gas present in air (see Activity 14.1).

(iv) As a result of their movement, the molecules go on colliding with the walls of the container. As a result the walls of the container experience outward force. This force per unit area of the walls is called pressure of the gas.

(v) During the course of their rapid movement, the molecules of the gas collide with one another and also with the walls of the container. If these collisions were non-elastic, there would have been loss of energy and ultimately the molecules would come to rest and gases should gradually settle down, but this does not happen.

(vi) The indefinite expansion of the gases and their tendency to occupy the whole available space implies that the movement of molecules occurs away from one another. This is possible if the attractive forces between the molecules are negligible.

(vii) On increasing the temperature of the gas, the molecular motion becomes rapid resulting in the increase in the average kinetic energy of gas molecules.

GAS LAWS AND BEHAVIOUR OF GASES

The extensive research on the physical properties of gases carried out by different scientists for several centuries led to the formulation of certain generalisations. These generalisations are known as gas laws. We shall now, study some of the gas laws in details.