Question

The variation of the boiling points of the hydrogen halides is in the order HF > HI > HBr > HCl. What explains the higher boiling point of hydrogen fluoride?
A) The bond energy of HF molecules is greater than in other hydrogen halides.
B) The effect of nuclear shielding is much reduced in fluorine which polarises the HF molecule.
C) The electronegativity of fluorine is much higher than for other elements in the group.
D) There is strong hydrogen bonding between HF molecules.

Answer 1

Hint : The boiling point of a given molecule depends upon the intermolecular interactions. Higher the strength of these interactions, higher is the value of boiling point of molecules. So, we can determine the reason behind the higher boiling of hydrogen fluoride.

Complete answer :

First, we are given the order of variation of the boiling points in molecules i.e. hydrogen halides. F, Cl, Br, and I belong to the group of halogens.
Talking about the options, first is related to the bond energy. The bond energy is required to break the bond. In the hydrogen halides, HF requires a large amount of energy due to the presence of strong hydrogen bonding.
Now, if we talk about the nuclear shielding, and the electronegativity, in HF there is the attraction of electrons, so it shows the high electronegativity, and it is a very polar molecule.
We can conclude that the boiling point of HF molecule is higher due to the presence of strong hydrogen bonding between HF molecules.

Hence, the correct option is (D).

Note : Don’t get confused between the boiling points of HF, and HI. The confusion can occur why HF has a higher boiling point than the HI. HI has the larger size, and greater van der Waals forces but there is the presence of hydrogen intermolecular interactions in HF. The hydrogen-bonding interactions are greater than van der Waals forces. So, HF has a higher boiling point than HI.