# 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

## 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

- [boiling point](https://www.gradenine.co.uk/glossary/boiling-point)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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