Question

Use the information and data given below to answer the questions (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, 189K, 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) All the given molecules viz. HF,HCl, HBR and HI have permanent dipoles. Hence, all of them posses dipole-dipole and London forces. Hf in addition to dipole-dipole and London forces also has hydrogen bonding.
(b) Electronegativity of Cl, Br and I is in the order : `Cl gt Br gt I`. Therefore, polar character and hence dipole-dipole interactions should be in the order `HCl gt HBr gt HI`. But boiling points are found to be in the order `HCl lt HBr lt HI`. This shows that London forces are predominant. This is because London forces increases as the number of electrons in the molecule increases. In this case, the number of electrons increases from HCl to HI.
(c ) Due to very high elctronegativity of F, HF is most polar and also there is hydrogen bonding present it. Hence, it has the highest boiling point.