Explain why the boiling point of hydrogen fluoride (HF) is significantly higher than that of hydrogen chloride (HCl), despite HCl having a greater molar mass.

IB DP Chemistry Higher Level (2023 syllabus) — S2.2 The covalent model · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

HF molecules form hydrogen bonds between molecules, whereas HCl does not (only van der Waals/London dispersion and dipole–dipole forces). Hydrogen bonding arises because fluorine is highly electronegative and small, creating a large partial positive charge on H, which interacts with a lone pair on F of a neighbouring molecule. Hydrogen bonds are stronger than the van der Waals and dipole–dipole forces present in HCl. Therefore more energy (higher temperature) is required to separate HF molecules from the liquid phase, giving HF a higher boiling point despite its lower molar mass.

Examiner tips

  • Use the term "hydrogen bond" explicitly; examiners look for this keyword.
  • Explain why HF can form H‑bonds (electronegativity and lone pair) and contrast with HCl.
  • Show the chain of reasoning: stronger forces → higher boiling point.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Confusing London dispersion with hydrogen bonding; forgetting to mention HCl lacks H‑bonds.
  • Failing to state that fluorine’s high electronegativity creates a large partial positive H.

Mark scheme (4 marks)

  1. HF molecules form hydrogen bonds between molecules, whereas HCl does not (only van der Waals/London dispersion and dipole–dipole forces).
  2. Hydrogen bonding arises because fluorine is highly electronegative and small, creating a large partial positive charge on H, which interacts with a lone pair on F of a neighbouring molecule.
  3. Hydrogen bonds are stronger than the van der Waals and dipole–dipole forces present in HCl.
  4. Therefore more energy (higher temperature) is required to separate HF molecules from the liquid phase, giving HF a higher boiling point despite its lower molar mass.

Key terms in this question

boiling point

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