Question

What is the role of the constants a and b in the van der waals equation in terms of the kinetic molecular theory?

Answer 1

Ideal gas equation is written as PV= nRT and this derviation is based on the assumption that

1)The actual volume of the gas molecules is negligible as compared with the total volume of the gas.

2) The gas molecules do not exert any appreciable attraction on each other.

The ideal gas law is obeyed only at high temperature and low pressure where the volume of gas molecules can be neglected compared to the volume of gas and at low pressure where the gas molecules are far apart to have any force of atrtraction.

Vanderwaal equation is written as (P+an²/V²)*(V-nb) =nRT

Ideal gas is one where there is no interaction between molecules of gas. The molecules in an ideal gas travel in straight line till they collide with the container or other molecules. In non-ideal gas, there is force of attraction between molecules.The term a in Vanderwaal equation takes care of this.

When the pressure is very high, the volume of gas is so small, one has to consider the volume of molecules of gas as well. This reduces the free space available for molecules to move. This is taken care of by the term b in Vanderwaal equation.