# 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

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

## 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

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

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-boiling-point-of-410be7c4) · Published by Druglandscape Ltd.