Chapter 14, Problem 5A

Interpretation Introduction

Interpretation: The reason for dipole-dipole force to be not important in vapor state needs to be explained.

Concept introduction: When polar molecules with dipole moments are put together, the orientation in the molecules takes place such that the positive and negative portion of the molecules are close to each other.

Answer to Problem 5A

Due to large distance between molecules dipole-dipole force in not important in vapor state.

Explanation of Solution

The dipole-dipole attraction decreases with the increase in distance between dipoles because attractive force between the dipoles decreases. Dipole-dipole interaction energy is proportional to $\frac{1}{r^3}$ where $r$ is the distance between two molecules.

In vapor state kinetic energy of the molecules are very high as compared to liquid state. As a result distance between the dipoles is maximum and dipole-dipole force is minimum.

Conclusion

Due to large distance between molecules dipole-dipole force in not important in vapor state.