Question

Use the information and data given below to answer the question (a) to (c),
Stronger intermolecular forces result in higher boiling point.
Strength of London forces increases with the number of electrons in the molecule.
Boiling point of `HF, HCl, HBr` and `HI` are 293 K, 189 K, 206 K and 238 K respectively.
(a) which type of intermolecular forces are present in the molecules `HF, HCl, HBr` and `HI` ?
(b) Looking at the trend of boiling points of `HCl, HBr` and `HI`, explain out of dipole-dipole interaction and London interaction, which one is predominant here.
(c) Why is boiling point of hydrogen fluoride highest while that of hydrogen chloride lowest ?

Answer 1

(a) In the molecules of all these substance of all these substances, the intermolecular forces are dipole-dipole interactions since all of them are polar in nature. In the molecules of `Hf`, intermolecular hydrogen bonding is also present. This is quite evident from its boiling point `(293 K)` which is the maximum.
(b) In the three molecules that are listed, the electrogrativies of the halogens involved are in the order `Cl gt Br gt l`. This means that dipole- dipole interactions are expected to decrease in the order : `HCl gt HBr ft HI`. But the boiling points increase in the reverse order : `HCl (189 K), HBr (206 K)` and `HI (238 K)` respectively. This clearly shown that the magnitude of the London forces increases in the order `HCl lt HBr lt HI`. Thus the London force or intereactions dominate over dipolar forces.
(c) The boiling point of hydrogen fluoride is the highest because of maximum dipole-dipole intereactions (HF is maximum polar) and also because of intermolecular hydrogen bonding in the molecules (Section `5.2.`)
On the other hand, in hydrogen chloride (HCl), there is no intermolecular hydrogen bonding and the London forces of interaction are also minimum as stated above. Therefore, its boiling point is the lowest.