JEE Main 2020 · 3 September · morning · Single correct · +4 / −1
If the boiling point of H₂O is 373 K, the boiling point of H₂S will be :
- (a) greater than 300 K but less than 373 K
- (b) equal to 373 K
- (c) more than 373 K
- (d) less than 300 K
Answer and solution
Answer (d) less than 300 K
Solution
Chemistry ma’am · 3 steps
Water boils high only because of hydrogen bonds. Without them, H₂S boils below room temperature.
- Why water is special. Oxygen is small and very electronegative, so each O-H bond is strongly polar. Water molecules hold each other through strong hydrogen bonds, and a lot of energy is needed to pull them apart.
- What changes in H2S. Sulphur is larger and much less electronegative than oxygen. The S-H bond is only weakly polar, so H₂S forms almost no hydrogen bonds. Only weak dipole–dipole and dispersion forces hold its molecules together.
Molecule Main intermolecular force Boiling point H₂O strong hydrogen bonding 373 K H₂S weak dipole–dipole and dispersion about 213 K (a gas at room temperature) - So. H₂S is a gas at room temperature (about 300 K), so its boiling point must be below 300 K. The answer is (d).
The idea to keep. In the hydrides of a group, the first member (with N, O or F) boils abnormally high because of hydrogen bonding. The heavier hydrides follow molar mass, but they all boil far lower.
Answer from the NTA final answer key. Solution by PrepWiser.