The boiling point of pentane is higher than that of propane because?

Analyze the question: We are asked why pentane (C₅H₁₂) boils at a higher temperature (about 36 °C) than propane (C₃H₈, about -42 °C).

Concept: Boiling is a physical change. When a liquid boils, the molecules separate from one another, but the bonds inside each molecule stay intact. So what matters is the strength of the intermolecular forces, not the covalent bonds within the molecules.

Alkanes are non-polar, so the only forces between their molecules are weak van der Waals (London dispersion) forces. The strength of these forces increases with molecular size and surface area, because a larger electron cloud is more easily polarised.

Solution:

• Pentane has 5 carbon atoms and 12 hydrogen atoms, so it has a larger molar mass and more electrons than propane (3 carbons, 8 hydrogens).
• A larger electron cloud means stronger London dispersion forces between pentane molecules.
• More energy (a higher temperature) is needed to overcome these forces, so pentane boils at a higher temperature.

Checking the options:

• A. Wrong. Covalent bonds are not broken during boiling. Also, C-C bonds are not stronger than C-H bonds in general.
• B. Wrong. Covalent bonds are not broken when a liquid boils, so the number of them does not matter.
• C. Wrong. How easily a substance burns is a chemical property and has nothing to do with boiling point.
• D. Correct. Pentane's intermolecular forces are stronger than propane's.

Tip: In "boiling point" questions, remember that boiling or melting breaks intermolecular forces, not covalent bonds. Within a homologous series, a bigger molecule means a higher boiling point.