Heat and Thermodynamics

5.2 Van der Waals′ Equation of State

The behaviour of real gases can be explained when we consider the assumptions of the kinetic theory which are ideal, but in the real case they are to be modified. Two important assumptions of the kinetic theory— (i) the molecules are treated as mass points and (ii) there is no force between the molecules— are not valid. The molecules have finite dimensions however small they may be and the molecules are not devoid of molecular force. These two are responsible for the volume correction and pressure correction. We shall consider two methods of volume correction:

1. Simple method: Let the molecules be regarded as hard elastic spheres of diameter σ. Considering a molecule, the centre of no other molecule can come ...