Question

Explain why the enthalpies of vaporization of the following substances increase in the order CH4 < NH3 < H2O, even though all three substances have approximately the same molar mass.

Answer 1

Explanation:

More is the intermolecular forces present between the molecules of a substance more will be the boiling point of the substance.

Since, water is able to form hydrogen bonding and it also have dipole-dipole interaction as these are stronger than the forces present in ammonia. Hence, boiling point of water is greater than the boiling point of $NH_{3}$.

In $CH_{4}$, there are only dispersion forces present in it and these forces are weakest intermolecular forces.

As a result, the enthalpies of vaporization of the following substances increase in the order $CH_{4} < NH_{3} < H_{2}O$.