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 ?

Explanation

From the information and data given in the question, we concluded that
(a) In `HCl, HBr` and `HI`, dipole-dipole and London forces are present because molecules possess permanent dipole. In `HF` dipole-dipole, Lodon forces and hydrogen bonding are present.
(b) Electronegativity of chlorine, bromine and iodine decreases in the order are present
`Cl gt Br gt I`
Therefore, dipole moment should decrease from `HCl` to `HI`. Thus, dipole-dipole interaction should decrease from `HCI` to `HI`. But boiling point increases on moving from `HCl` to `HI`. This means that London forces are predominant.
This is so because London forces increases as the number of electrons in a molecule increases and in this case number of electrons is increasing from `HCl` towards `HI`.
(c) Hydrogen fluoride has highest dipole moment attributes due to highest electronegativity of fluorine as well as presence of hydrogen bonding in `HF`. Therefore, `HF` has highest boiling point.