Question

Although the hydrogen bonding in hydrogen fluoride is much stronger than in water, yet water has a much higher boiling point than hydrogen fluoride. Explain.

Answer 1

In a molecule of water, there are two hydrogen bond donors, while a hydrogen fluoride molecule only has one donor. There is only one donor in $HF$, but there are 2 hydrogen atoms in $H_{2}O$. The oxygen molecule in water has two lone pairs which are capable of accepting two hydrogen bonds. Therefore, a single water molecule can have up to 4 hydrogen bonds, i.e., 1 for each hydrogen and 2 for the oxygen. On the other hand, $HF$ can only undergo 2 hydrogen bonds per molecule.

Hence, it is clear that water has more hydrogen bonds per molecule and per molecular mass as compared to $HF$ which allows water to form a complex network of hydrogen bonding that creates a high surface tension and consequently a higher boiling point than $HF$.