# Question #a7be8

Dec 21, 2015

Here's why that is the case.

#### Explanation:

As you know, the boiling point of a given substance depends on the strength of the intermolecular forces of attraction that exist between its molecules.

When comparing the boiling points of two substances, you are actually comparing the relative strength of these intermolecular forces of attraction.

In this particular case, sulfur dioxide, ${\text{SO}}_{2}$, has a higher boiling point than ammonia, ${\text{NH}}_{3}$, so right from the start you can conclude that stronger intermolecular forces of attraction exist between ${\text{SO}}_{2}$ molecules than between ${\text{NH}}_{3}$ molecules.

Now, do not be thrown off by the fact that ammonia molecules can form hydrogen bonds, which are particularly strong intermolecular forces.

The fact that a substance can hydrogen bond and another can't does not automatically imply a higher boiling point for the former.

So, ammonia molecules exhibit

• hydrogen bonds $\to$ considerably weaker than for water molecules, for example, because ammonia is lone-pair deficient
• dipole - dipole interactions
• London dispersion forces

Sulfur dioxide molecules exhibit

• dipole - dipole interactions
• London dispersion forces

This tells you that the dipole - dipole interactions and the London dispersion forces exhibited by ${\text{SO}}_{2}$ molecules can overpower the three intermolecular forces of attraction exhibited by ${\text{NH}}_{3}$ molecules.

As it turns out, sulfur dioxide molecules has a bigger dipole moment than ammonia molecules, $\text{1.63 D}$ vs. $\text{1.47 D}$. This means that its dipole - dipole interactions will be stronger than those found in ammonia.

Moreover, ${\text{SO}}_{2}$ molecules are bigger than ammonia molecules, which of course implies that they have more electrons and larger electron clouds.

As a result, they will exhibit stronger London dispersion forces.

This is why sulfur dioxide has a boiling point of about $- {10}^{\circ} \text{C}$, and ammonia has a boiling point of about $- {33.4}^{\circ} \text{C}$.

So, as a conclusion, boiling points depend solely on the strength of the intermolecular forces of attraction that exist between a substance's molecules.

This is how you can explain the difference in the boiling point of any two substances.