Question

NH₃ has an exceptionally high melting point and boiling point as compared to those of the hydrides of the remaining element of group 15 - Explain.

Answer 1

1. NH₃ has an exceptionally high melting point and boiling point due to hydrogen bonding between NH₃ molecules.
2. Each molecule can form a maximum of 4 hydrogen bonds but on average 1 hydrogen bond per molecule as there is only one lone pair on NH₃ available for hydrogen bonding.
3. Hydrogen bonding is a strong intermolecular attraction as H on NH₃ acts as a proton due to partial positive on it the whole N has the partial negative charge. Thus when the very polarized H comes close to an N atom in another NH₃ molecule, a very strong hydrogen bond is formed.
4. Due to many strong intermolecular interactions compared to weaker permanent dipole-dipole interactions between other XH₃ molecules in group 15, a large amount of energy is required to overcome the forces, giving it the highest boiling point and highest melting point.