Ethanol (C₂H₅OH) is a simple molecular substance that is liquid at room temperature. It has a boiling point of 78 °C. Pentane (C₅H₁₂) is also a simple molecular substance but has a boiling point of 36 °C. Explain why both ethanol and pentane have relatively low boiling points, and suggest why ethanol has a higher boiling point than pentane despite having fewer electrons.

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).

Ethanol has the molecular formula C₂H₅OH and a boiling point of 78 °C. Pentane has the molecular formula C₅H₁₂ and a boiling point of 36 °C. Both are simple molecular substances.

Model answer (5 marks)

Simple molecular substances have only weak London dispersion forces between molecules.

Because these forces are weak, only a small amount of energy is required to separate the molecules, so both ethanol and pentane boil at relatively low temperatures.

Ethanol contains an –OH group and can form hydrogen bonds between its molecules. Hydrogen bonds are stronger than the London dispersion forces present in pentane.

Therefore, more energy is required to break the intermolecular forces in ethanol, giving it a higher boiling point than pentane despite having fewer electrons.

Examiner tips

  • Use the term "London dispersion forces" for simple molecules. Show the chain of reasoning: weak forces → low energy needed → low boiling point. Mention hydrogen bonding for ethanol. Explain why stronger forces raise the boiling point.
  • common_mistakes
  • :
  • Confusing hydrogen bonding with dipole–dipole forces. Forgetting to state that both substances are simple molecules. Using vague terms like "weak forces" without naming London dispersion forces.

Mark scheme (5 marks)

  1. Simple molecular substances have weak intermolecular forces between molecules
  2. Only a small amount of energy is needed to overcome these weak intermolecular forces
  3. Therefore the boiling point is low / the substance boils at a low temperature
  4. Ethanol can form hydrogen bonds between its molecules (due to the OH group), which are stronger than the intermolecular forces in pentane
  5. More energy is needed to overcome the stronger intermolecular forces in ethanol, so its boiling point is higher than pentane's

Key terms in this question

simple molecular substance · boiling point

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