Propane (C₃H₈) is a simple molecular substance that is a gas at room temperature. Octane (C₈H₁₈) is also a simple molecular substance but is a liquid at room temperature. Both substances contain only carbon–hydrogen bonds. Explain why octane has a higher boiling point than propane.

Edexcel A-Level Chemistry (9CH0) — 2.4 Forces between molecules · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Propane (C₃H₈) is a gas at room temperature with a boiling point of −42 °C. Octane (C₈H₁₈) is a liquid at room temperature with a boiling point of 126 °C. Both are simple molecular substances containing only carbon and hydrogen.

Model answer (5 marks)

Both substances have London dispersion forces.
Octane has a larger molecular size and more electrons than propane.
The larger electron cloud gives octane stronger dispersion forces.
Stronger forces require more energy to break.
Therefore octane has a higher boiling point than propane.

Examiner tips

  • Use the word ‘London dispersion forces’ first. Mention size/electron count before force strength. Show the logical chain: size → force → energy → boiling point.
  • common_mistakes
  • :
  • Confusing covalent bonds with intermolecular forces. Saying ‘octane has more bonds’ instead of ‘more electrons’.

Mark scheme (5 marks)

  1. Both substances have intermolecular forces (between molecules) / van der Waals forces / London dispersion forces
  2. Octane has more electrons / is a larger molecule / has a greater relative molecular mass than propane
  3. Octane has stronger intermolecular forces than propane
  4. More energy is needed to overcome / break the intermolecular forces in octane
  5. Therefore octane has a higher boiling point / a higher temperature is needed to boil octane

Key terms in this question

simple molecular substance · boiling point

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