The vlaue of van der waals constant 'a' for nitrogen gas is `1.37L^(2)*atm*mol^(-2)`, but that for ammonia gas is `4.30L^(2)*atm*mol^(-2)`. What is the reason for this large difference ? Which one of these two gases would you expect to have higher critical temperature?

Explanation

Since `N_(2)` is a non-polar molecule, the only attractive forces that operate between the molecules in `N_(2)` gas are weak London forces. On the other hand, `NH_(3)` is a polar molecule. So, the molecules in `NH_(3)` gas experience dipole-dipole attractive forces in addition to london forcs. hence, the intermolecular forces of attraction in `NH_(3)` gas are much stronger than those in `N_(2)` gas, and this makes the value of 'a' for `NH_(3)` gas higher.
the critical temperature of a gas depends upon the strength of intermolecular forces of attraction in the gas. the stronger the intermolecular forces of attraction in a gas, the higher the critical temperature of the gas, since the intermolecular forces of attraction are stronger in `NH_(3)` gas than those in `H_(2)` gas, the critical temperature of `NH_(3)` will be higher than that of `H_(2)`.