Why do longer hydrocarbon chains have higher boiling points than shorter hydrocarbon chains?

• A. Stronger intermolecular forces
• B. More reactive
• C. Higher density
• D. More volatile

Answer: (A)

Boiling points for these hydrocarbons are governed by how strongly their molecules attract each other. In hydrocarbons, the main attraction is London dispersion forces, which get stronger as molecules become larger and have more electrons. Longer chains have a bigger surface area and more electrons, so the temporary dipoles that create these forces are stronger. As a result, more energy is required to separate the molecules and turn the liquid into a gas, increasing the boiling point.

Other ideas don’t explain the trend as well. Reactivity isn’t tied to boiling point here, and density differences aren’t the driving factor for when a substance boils. Longer chains are actually less volatile because the stronger intermolecular forces hold them in the liquid state more tightly, requiring more heat to vaporize.