Chapter 14, Problem 2STP

Interpretation Introduction

Interpretation: The reason for difference in strength of intermolecular forces between ammonia and methane needs to be explained.

Concept introduction: When polar molecules with dipole moments are put together, they orient themselves in such a way that molecules with dipole moments can attract each other so that the positive and negative portions of the molecules come close to each other.

The forces that exist among non-polar molecules are called London dispersion forces. Atoms can develop a temporary dipole moment due the distortion of symmetrical charge distribution as the electrons move around the nucleus. This instantaneous dipole generated in turn induces a similar dipole in a neighboring atom.

Answer to Problem 2STP

Option “A” is correct.

Explanation of Solution

Ammonia due to electronegativity difference between $N$ and $H$ developes a dipole moment where $N$ acquires partial negative charge and $H$ acquires partial positive charge. Due to the pyramidal structure the net dipole directed towards the lone pair.

  

On the other hand due tetrahedral structure of methane ( $C{H}_4$ ) the dipole generated in $C-H$ are in opposite direction and hence the cancel each other. Hence $C{H}_4$ is non-polar molecule and only force that operates in it is London dispersion forces.

  

Conclusion

Option “A” is correct.