The high boiling point of hydrogen fluoride can be explained in terms of?

1. Analyze the question The question asks why hydrogen fluoride (HF) has an unusually high boiling point compared with other hydrogen halides such as HCl, HBr and HI. This is about intermolecular forces, not the bonds inside the molecule.

2. Explain the concept

• Fluorine is the most electronegative element, so the H-F bond is very polar. Hydrogen carries a strong partial positive charge (δ+) and fluorine a strong partial negative charge (δ−).
• When hydrogen is bonded to a very electronegative, small atom (F, O or N), the δ+ hydrogen of one molecule is strongly attracted to a lone pair on the fluorine of a neighbouring molecule. This is hydrogen bonding.
• Hydrogen bonds are much stronger than ordinary dipole-dipole forces or van der Waals forces, so much more energy is needed to separate the molecules.

3. Solution

• A. Covalent bonds: these hold H and F together within one molecule. They are not broken when HF boils, so they do not explain the boiling point.
• B. Dative bonds: not relevant to HF's physical state.
• C. Dipole-dipole: present in HF, but it is not the main reason for the unusually high boiling point. Other polar hydrogen halides like HCl have dipole-dipole forces as well and boil much lower.
• D. Ionic bonds: HF is a covalent molecule, not ionic.
• E. Hydrogen bonds: this explains why HF (b.p. about 19.5 °C) boils far higher than HCl (about -85 °C), even though HCl has a larger mass.

4. Answer The correct option is E, hydrogen bonds.

5. Exam tip Remember "FON": when H is bonded to F, O or N, think hydrogen bonding. Whenever a question mentions an unusually high boiling point for HF, H₂O or NH₃, hydrogen bonding is almost always the answer.