Ncert Exercises & Solutions

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.

1. Which type of in termolecular forces are present in the molecules HF, HCl, HBr and HI?

2. 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.

3. Why is boiling point of hydrogen fluoride highest while that of hydrogen chloride lowest?

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, London forces and hydrogen bonding are present.

(b) Electronegativity of chlorine, bromine and iodine decreases in the order are present

Cl>Br>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.

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