Question

Write the Van der Waals equation for a real gas. Explain the correction term for pressure and volume.

Answer 1

Vander Waals equation for a real gas is given by

`("P" + ("an"^2)/("V"^2)) ("V" - "nb") = "nRT"`

Pressure Correction:

The pressure of a gas is directly proportional to the force created by the bombardment of molecules on the walls of the container. The speed of a molecule moving towards the wall of the container is reduced by the attractive forces exerted by its neighbours. Hence, the measured gas pressure is lower than the ideal pressure of the gas. Hence, van der Waals introduced a correction term to this effect.

Where n is the number of moles of gas and V is the volume of the container

`"p" ∝ "n"^2/"V"^2`

`"P" = "an"^2/"V"^2`

Where a is proportionality constant and depends on the nature of gas

Therefore, P = `"P" + "an"^2/"V"^2`

Volume Correction:

As every individual molecule of a gas occupies a certain volume, the actual volume is less than the volume of the container, V. Van der Waals introduced a correction factor V to this effect. Let us calculate the correction term by considering gas molecules as spheres.

V = excluded volume

Excluded volume for two molecules = `4/3` π(2r)³ = 8V_m

where V_m is a volume of a single molecule.

Excluded volume for single molecule = `(8"V"_m")/2` = 4V_m

Excluded volume for n molecule = n(4V_m) = nb

Where b is van der Waals constant which is equal to 4V_m

V’ = nb

V_ideal = V – nb

Replacing the corrected pressure and volume in the ideal gas equation PV = nRT we get the van der Waals equation of state for real gases as below,

`("P" + ("an"^2)/("V"^2)) ("V" - "nb") = "nRT"`

The constants a and b are van der Waals constants and their values vary with the nature of the gas.