Chapter 21, Problem 54A

Interpretation Introduction

Interpretation:

The effect of intermolecular attractions on the boiling and freezing point of the substance (water and methane) needs to be explained.

Concept introduction:

The strength of intermolecular forces present between the molecules influences the properties of the substance like boiling point, freezing point, etc. If the strength of the intermolecular forces is strong, then it results in higher boiling point and low vapor pressure of the liquid.

Explanation of Solution

The structure of water, ${\text{H}}_{\text{2}}\text{O}$ and methane, ${\text{CH}}_4$ is:

Methane is a non-polar molecule whereas water is a polar molecule. The force of attraction between polar molecules are greater than those of non-polar molecule so, methane being non-polar will possess less attraction between methane molecules but water molecules possess polar $\text{O-H}$ bond and the type of intermolecular forces in water is hydrogen bonding, type of intermolecular force due to dipole-dipole attraction formed between H atom which is covalently bonded to one molecule and to an electronegative atom such as F, O, or N which is covalently bonded to another atom.

Due to presence of an oxygen atom in water, the formation of intermolecular hydrogen bonding takes place in which O atom of one molecule of water is bonded to H atom of another molecule of water. The hydrogen bonding in ${\text{H}}_{\text{2}}\text{O}$ is as shown:

Hence, the water molecule has higher boiling and freezing point than methane.