Question

Boiling points of NH3 and HF are abnormally high as compared to those of hydrides of other elements in their respective groups ?

Answer 1

The relatively high boiling point of HF is due to the presence of intermolecular hydrogen bonds in hydrogen fluoride and in ammonia molecule, nitrogen, an electronegative atom is bonded to three hydrogen atoms. The nitrogen atom forms a negative site of the molecule whereas each of three H-atoms acquires a partial positive charge and hence a large amount of energy is needed to overcome the intermolecular hydrogen bonds and to separate the molecules. So melting point is abnormally high.

Answer 2

The boiling point of HF is greater than other hydrides of same group like HCl, HI, HBr etc because F tightly bounds H atom due to its strong force as compared to Cl, Br and Iodine. In Ammonia NH3 N2 and H2 forms strong bonding which results in high boiling point as compared to other hydrides in the group.