Molecular interactions and the properties of real gases

The classical kinetic theoty of gases treats a system of non-interacting particles, but in real gases there is a short-range interaction which has an effect on the physical properties of gases. The most simple description of this interaction uses the Lennard-Jones potential which postulates a central force between molecules, giving an energy of interaction as a function of the inter-nuclear distance, r. 

Chapman and Enskog (see Chapman and Cowling, 1951) made a semi-empirical study of tire physical properties of gases using the Lennard-Jones 

The values of s/k are less dran 600 K for most of the simple molecules which are found in high temperature systems, and hence the collision integral may be assumed to have a value of unity in tlrese systems. 

An approximate value for dc in the equation for tire Lennard-Jones potential, quoted above, may be obtained from the van der Waals constant, b, since 

This dispersion interaction must be added to the dipole-dipole interactions between molecules, such as HCl, NH3 and H2O which have a permanent dipole, fi. The magnitude of die dipole moment depends on tire differences in electronegativity of the atoms in the molecule. Here again, the energy of interaction varies as (orientation effect). 

---