Question

How do intermolecular interaction create deviation from ideality for gas?

Answer 1

Intermolecular forces describe the attraction and repulsion between particles. They include dipole-dipole forces, ion-dipole forces, dipole-induced dipole forces ,instantaneous dipole-induced dipole forces, London etc.

consider a molecule in the interior of a gas. It is surrounded by other molecules uniformly so that there is no resultant attractive force on this molecule. However, when it approaches the wall of the vessel, the uniform distribution of molecules around it is disturbed and it is now attracted by molecules from other side only. The molecule will thus experience a net inward pull. Hence it will strike the wall with lower velocity and will exert a lower pressure than it would have done if there was no forces of attraction. The pressure P will thus be less than the ideal pressure.

To correct for intermolecular forces between gas particles, J.D. van der Waals introduced a new term. By adding the term n²a/V² to pressure, van der Waals corrected for the slight reduction in pressure due to the interaction between gas particles:

P_ideal = P + an²/V²

In the term above, (a) is a constant specific to each gas and V is the volume. van der Waals also corrected the volume term by subtracting out (b) the excluded volume of the gas. The full van der Waals equation of state is written as:

(P + an²/V²)(V-nb) =nRT

Intermolecular attraction become increasingly important in low temperatures, when translational molecular motion slows down, almost to a halt. However, at high temperatures, or even normal, everyday temperatures, the intermolecular forces are very small and tend to be considered negligible.