Question

Why does H2S have a lower boiling point than H2Se? Why does H2O have a much higher boiling point than H2S?

Answer 1

The boiling point of compounds depends on how tightly they are bonded to each other. This tightness depends upon the strength of intermolecular van der Waals forces (the attractive forces between molecules). This force increases with molecular weight. Since H2Se has a higher molecular weight and bigger size than H2S, it should thus have the higher boiling point.

The two molecules H2O and H2S has the same geometrical form, the only difference is the smaller and higher electronegativity O atom. Due to its higher electronegativity the electrons of the H atoms spend more time around the oxygen, making slightly dipole the whole molecule which results in a strong H-bonding with other surrounded water molecules. These connected water molecules creating tangled water chains and required more energy to break apart. This type of effect is not enough to be present in case of H2S and hence its boiling point is less as compared to H2O.