Question
Explain the two types of hydrogen bonding with examples. Why does HF have a much higher boiling point (19.5°C) than HCl (-85°C) despite having a lower molecular weight?
(NCERT Class 11, Chapter 4)
Solution — Step by Step
Hydrogen bonding is a special type of dipole-dipole interaction. It occurs when hydrogen is bonded to a highly electronegative atom (F, O, or N) and interacts with a lone pair on another F, O, or N atom.
The bond is represented as X-H…Y, where X and Y are F, O, or N.
Intermolecular hydrogen bonding — between different molecules. Examples: , HF, , alcohols, carboxylic acids. This affects physical properties like boiling point and solubility.
Intramolecular hydrogen bonding — within the same molecule. Examples: o-nitrophenol, salicylaldehyde. This reduces intermolecular association, so these compounds have lower boiling points than their p-isomers (which form intermolecular H-bonds).
Fluorine is the most electronegative element (3.98). The H-F bond is highly polar, creating a strong partial positive charge on H and a strong partial negative charge on F. The resulting H-bonds between HF molecules are strong (~29 kJ/mol).
HCl, on the other hand, has Cl as the electronegative atom. Chlorine is less electronegative (3.16) and too large — its lone pairs are more diffuse, making H-bonding very weak or absent. HCl is held together mainly by weaker van der Waals forces.
Despite HCl being heavier (larger London forces), the strong hydrogen bonding in HF dominates, giving HF a boiling point that is over 100°C higher than HCl.
Why This Works
Boiling point depends on the strength of intermolecular forces. For similar-sized molecules, hydrogen bonding is much stronger (10-40 kJ/mol) than van der Waals forces (1-10 kJ/mol). Only F, O, and N are electronegative and small enough to form effective H-bonds — their lone pairs are concentrated in a small volume, allowing strong electrostatic interaction with the H atom.
The anomalous boiling points of , HF, and (all higher than expected from their group trends) are entirely due to hydrogen bonding. Water’s unusually high boiling point (100°C vs at -60°C) is perhaps the most important consequence of H-bonding in all of chemistry and biology.
Alternative Method
You can rank H-bond strength by looking at electronegativity differences and atomic sizes. The order of H-bond strength: F-H…F > O-H…O > N-H…N. This is why HF forms unusually strong H-bonds (it even forms polymeric chains and the ion).
For NEET, remember the boiling point anomaly trend: — NO! The actual order is . Water is anomalously high (H-bonding), and after that, boiling points increase with molecular weight due to London forces.
Common Mistake
Students sometimes claim HCl also forms hydrogen bonds because “H is bonded to an electronegative atom.” While Cl is electronegative, it is not electronegative enough AND too large to form significant H-bonds. Only F, O, and N form hydrogen bonds. This is a strict rule — do not extend it to Cl, S, or P in JEE/NEET contexts.