If two molecules interact only via dispersion forces, which molecule will tend to have the higher boiling point?

If two molecules interact only via dispersion forces, which molecule will tend to have the higher boiling point?

• A. Smaller
• B. Lighter
• C. Branched
• D. Larger

Answer: (D)

When molecules rely only on dispersion forces, the strength of those interactions drives the boiling point. London dispersion forces come from temporary fluctuations in the electron cloud, and they grow stronger as the molecule becomes larger and more polarizable. A bigger molecule has more electrons and a more easily distorted electron distribution, which makes the dispersion forces between molecules stronger. Stronger intermolecular forces mean more energy is required to separate the molecules into a gas, so the boiling point is higher. Smaller and lighter molecules have weaker dispersion forces for the same type of interaction, so they boil at lower temperatures. Branched structures can reduce the surface area available for dispersion interactions, also tending to lower the boiling point relative to a larger, more extended molecule of similar composition. The overarching trend here is that increased size leads to stronger dispersion forces and a higher boiling point, making the larger molecule the correct choice.

When molecules rely only on dispersion forces, the strength of those interactions drives the boiling point. London dispersion forces come from temporary fluctuations in the electron cloud, and they grow stronger as the molecule becomes larger and more polarizable. A bigger molecule has more electrons and a more easily distorted electron distribution, which makes the dispersion forces between molecules stronger. Stronger intermolecular forces mean more energy is required to separate the molecules into a gas, so the boiling point is higher.

Smaller and lighter molecules have weaker dispersion forces for the same type of interaction, so they boil at lower temperatures. Branched structures can reduce the surface area available for dispersion interactions, also tending to lower the boiling point relative to a larger, more extended molecule of similar composition. The overarching trend here is that increased size leads to stronger dispersion forces and a higher boiling point, making the larger molecule the correct choice.