The ideal gas law is based on the assumption that the gases are made up of point masses that undergo perfectly elastic collisions. However, real gases deviate from those assumptions at low temperatures or high pressures. Consider a container where the pressure is increased. The volume of the container decreases, when the pressure is increased. The volume occupied by the gas particles is no longer negligible compared to the volume of the container and the volume of the gas particles need to be considered.
At low temperatures, the gas particles have lower kinetic energy and do not move fast. The gas particles are affected by the intermolecular forces that act on them,which leads to inelastic collisions between them. This, leads to lesser collisions with the container and a lower pressure than that is expected from the ideal gas.
The van der Waal’s equation modifies the ideal gas law. This law predicts the properties of real gases by describing particles of non-zero volume governed by pairwise attractive forces.
The van der Waals equation of state is written as :
where P is pressure, V is volume, R is universal gas constant and T is absolute temperature.
Fig. shows a curve plotted between PV and P in which behaviour of two gases nitrogen and hydrogen has been shown.
Vander waal’s introduces the necessary corrections in the equation PV = RT.
