Chapter 14, Problem 2A

Interpretation Introduction

Interpretation: The relation between the dipole-dipole attraction and distance between polar molecules needs to be explained. Also, whether dipole-dipole attraction short ranges or long ranges need to be determined.

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 by lining up so that the positive and negative portion of the molecules are close to each other.

For example, in acetone due to electronegativity difference between carbon and oxygen, a dipole moment is generated where oxygen acquires partial negative charge and carbon acquires partial positive charge. So, negative end of oxygen of one acetone molecule is attracted towards positive end of carbon of another molecule of acetone.

Answer to Problem 2A

With increase in distance between polar molecules dipole-dipole force decreases. Dipole-dipole force is short range.

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.

The attractive force is effective over a very short distance and its strength is about $2kcal/mol$ .

Conclusion

With increase in distance between polar molecules dipole-dipole force decreases. Dipole-dipole force is short range.