Van der Waals equation was derived by Johannes Diderik van der Waals in the year 1873. The van der Waals equation of state was one of the first to perform markedly better than the ideal gas law which states that gases consist of point masses that undergo perfectly elastic collisions. However, this law fails to explain the behavior of real gases. Therefore, the Van der Waals equation was devised and it helps us define the physical state of a real gas. This equation takes into consideration the molecular size and molecular interaction forces (attractive and repulsive forces). Sometimes, it is also referred to as the Van der Waals equation of state.
Van der Waals equation is an equation relating the relationship between the pressure, volume, temperature, and amount of real gases. For a real gas containing ‘n’ moles, the equation is written as:
Where, P, V, T, n are the pressure, volume, temperature, and moles of the gas. ‘a’ and ‘b’ constants are specific to each gas.
Read also: Peng Robinson Equation of State
The constants a and b have positive values and are specific to each gas. The term involving the constant a corrects for intermolecular attraction. Attractive forces between molecules decrease the pressure of a real gas, slowing the molecules and reducing collisions with the walls.
- The higher the value of a the greater the attraction between molecules and the more easily the gas will compress.
- The b term represents the excluded volume of the gas or the volume occupied by the gas particles.
The van der Waals equation becomes the Ideal Gas Law as these two correction terms approach zero. The van der Waals model offers a reasonable approximation for real gases at moderately high pressures.