Chapter 14, Problem 61A

Interpretation Introduction

Interpretation: The explanation for the difference between the boiling point of methane and ammonia needs to be provided.

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 a higher boiling point and low vapor pressure of the liquid.

Explanation of Solution

The structures of ammonia, ${\text{NH}}_3$ and methane, ${\text{CH}}_4$ are :

  

Methane is a non-polar molecule, whereas ammonia 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 ammonia molecules possess polar $\text{N-H}$ bond.Moreover,the type of intermolecular forces in ammonia is hydrogen bonding, a 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 the presence of a nitrogen atom in ammonia, the formation of intermolecular hydrogen bonding takes place in which the N atom of one molecule of ammonia is bonded to the H atom of another molecule of ammonia. The hydrogen bonding in ${\text{NH}}_3$ is as shown:

  

Hence, due to hydrogen bonding in ammonia, the ammonia molecule has a higher boiling point than methane.