Chapter 14, Problem 61A

(a)

Interpretation Introduction

Interpretation: The molecule which contains a stronger polar bond is to be chosen.

Concept introduction: Molecules having polar bonds are called polar molecules.

Answer to Problem 61A

Electronegativity of halogen atom decreases from F to I. Hence, the polar character decreases. Therefore, $HF$ forms stronger polar bond than $HCl$ . There is a stronger hydrogen bonding interactions present in case of $HF$ than $HCl$ . Due to strong hydrogen bonding interactions, HF has the highest boiling point.

Explanation of Solution

Fluorine is the most electronegative element than chlorine.

Let us consider the case of hydrogen fluoride molecule. Fluorine is more electronegative than hydrogen. So the force of attraction on the shared pair, exerted by fluorine is more than that by hydrogen. Hence, it will be shifted towards fluorine, i.e. electron density of the electron cloud is more concentrated on fluorine side than on the hydrogen side. Consequently, fluorine becomes slightly negatively charged and hydrogen becomes slightly positively charged.

HF has the strongest hydrogen bonding interactions due to small size fluorine atoms.

(b)

Interpretation Introduction

Interpretation: The Dipole-dipole interactions are to be compared.

Concept introduction: Dipole-dipole interactions are in between two polar molecules.

Answer to Problem 61A

In case of $HF$ , there is a strongest H-bonding interactions present in the molecule than the dipole-dipole interactions present in case of $HCl$

Explanation of Solution

The positively charged H-atom of one molecule make electrostatic force of attraction with the most negatively charged fluorine atom and form H-bond. Hence, the strongest intermolecular force for Hydrogen fluoride is hydrogen- bonding interaction.

(c)

Interpretation Introduction

Interpretation: The liquid which is boiled first is to be explained.

Concept introduction: The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.

The boiling point is defined as the temperature at atmospheric pressure when liquid starts boiling.

Answer to Problem 61A

$HF$ has high boiling point due to the strongest hydrogen bonding interactions present between them.

Explanation of Solution

In H-F, due to the presence of hydrogen bonding in the molecule, boiling point is high. There is no H-bonding present in case of $HCl$ . So, the boiling point is less. Hence, $HCl$ has the lowest boiling point.