Pentane (C₅H₁₂) and decane (C₁₀H₂₂) are both simple molecular substances. Pentane has a boiling point of 36 °C whilst decane has a boiling point of 174 °C. Explain why both substances have relatively low boiling points compared to ionic compounds, and why decane has a higher boiling point than pentane.

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).

Pentane (C₅H₁₂) boiling point: 36 °C. Decane (C₁₀H₂₂) boiling point: 174 °C. Both are simple molecular hydrocarbons.

Model answer (5 marks)

Simple molecular substances such as pentane and decane have only London dispersion forces between their molecules. These forces are weak, so only a small amount of energy is required to overcome them and vapourise the liquid. Consequently their boiling points are low compared with ionic compounds, which possess strong ionic bonds that require much more energy to break.

Decane has a longer carbon chain and more electrons than pentane, giving it a larger surface area. This increases the magnitude of London dispersion forces in decane, so more energy is needed to overcome these stronger intermolecular forces. Therefore decane has a higher boiling point than pentane.

Examiner tips

  • Use the term "London dispersion forces" to show you know the force type.
  • Explain that weaker forces mean lower boiling points.
  • Show the chain‑length argument for stronger forces in decane.
  • Mention the need for more energy to break stronger forces.

Common mistakes

  • Confusing London forces with dipole–dipole or hydrogen bonding.
  • Failing to state that ionic compounds have much stronger bonds.
  • Not linking the larger size of decane to stronger 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 intermolecular forces
  3. Hence the boiling point is low / molecules are easily separated from the liquid
  4. Decane has more electrons / is a larger molecule than pentane, so has stronger intermolecular forces
  5. More energy is needed to overcome the stronger intermolecular forces in decane, so its boiling point is higher

Key terms in this question

simple molecular substance · boiling point

Related

More Forces between molecules questions

▶ Try answering this question with AI marking (free) →