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

IB DP Chemistry Standard 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).

Hydrogen fluoride has a boiling point of 19.5 °C, whereas hydrogen chloride has a boiling point of −85.1 °C.

Model answer (4 marks)

HF molecules form hydrogen bonds between molecules, whereas HCl does not (or only forms weaker intermolecular forces). Hydrogen bonding in HF arises because fluorine is highly electronegative and has lone pairs, allowing interaction with the δ⁺ hydrogen of a neighbouring molecule. HCl molecules are held together only by permanent dipole–dipole interactions and London/van der Waals dispersion forces, which are weaker than hydrogen bonds. More energy is required to overcome the stronger hydrogen bonds in HF, so HF has a higher boiling point despite its lower molar mass.

Examiner tips

  • Use the word ‘hydrogen bonding’ explicitly; examiners reward clear identification of the force. Include the reason – electronegativity and lone pairs – to show understanding. Contrast with dipole–dipole and London forces. Mention energy required to break bonds to justify the higher boiling point.

Common mistakes

  • Confusing dipole–dipole with hydrogen bonding; forgetting to mention fluorine’s electronegativity. Writing that HCl has a higher molar mass is irrelevant to the explanation.

Mark scheme (4 marks)

  1. HF molecules form hydrogen bonds between molecules, whereas HCl does not (or only forms weaker intermolecular forces).
  2. Hydrogen bonding in HF arises because fluorine is highly electronegative and has lone pairs, allowing interaction with the δ+ hydrogen of a neighbouring molecule.
  3. HCl molecules are held together only by (permanent) dipole–dipole interactions and London/van der Waals dispersion forces, which are weaker than hydrogen bonds.
  4. More energy is required to overcome the stronger hydrogen bonds in HF, so HF has a higher boiling point despite its lower molar mass.

Key terms in this question

boiling point

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