Lesson 11 of 12 · 10 min
Hydrogen bonding
NCERT § "Hydrogen Bonding"
Water boils at 373 K. Hydrogen sulphide, H₂S, a heavier molecule of exactly the same bent shape, boils at 213 K, far below freezing. And the ice cubes Riya drops in her glass float on their own liquid. One weak attraction explains both.
The lesson in notes
In short
An H atom attached to F, O or N is left with a partial positive charge, so it is pulled towards a lone pair on a nearby F, O or N atom; that pull is a hydrogen bond, drawn as a dotted line.
A hydrogen bond is much weaker than a covalent bond.
Intermolecular hydrogen bonds link different molecules, as in HF and water; they raise boiling points and account for the unusually high boiling point of water.
Intramolecular hydrogen bonds form within one molecule, as in o-nitrophenol, where the H of –OH bonds to an O of the nearby –NO₂ group.
HCl does not show significant hydrogen bonding; chlorine is less electronegative and larger than F, O or N.
HF forms zig-zag chains held by F–H···F hydrogen bonds, and water molecules are linked through O–H···O hydrogen bonds.