CBSE Class 11-science Answered

The van der Waals equation contains a pair of constants (a and b) that change from gas to gas. The value of ‘a’ accounts for the intermolecular attractive forces between gas molecules. The magnitude of ‘a’ is indicative of the strength of the intermolecular attractive force. Molecules experiencing the weakest attractive forces will have the smallest ‘a’ constant while those with the strongest attractive forces will have the largest values.

Eg: Consider N₂ gas and NH₃ gas. N₂ gas has non-polar bonds between their components atoms which results in weaker attractions between molecules. In case of NH₃ gas hydrogen bonds between molecules which are stronger than non-polar bonds. So NH₃ gas has higher ‘a’ constant value (4.170) than N₂ gas (1.390).

The factor ‘b’ accounts for the volume occupied by the gas molecules. Small molecular volume results in small b values and a large molecular volume corresponds to a large 'b' constant.

Eg: CH₄ gas has higher molecular volume as compare to H₂ gas so CH₄ gas will show higher ‘b’ constant value (0.04278) while H₂ gas will show lower ‘b’ constant value (0.02661).