Question

The molecular weight of ${{\text{H}}_{\text{2}}}{\text{S}}$ is more than that of ${{\text{H}}_{\text{2}}}{\text{O}}$, but ${{\text{H}}_{\text{2}}}{\text{S}}$ is a gas and ${{\text{H}}_{\text{2}}}{\text{O}}$ is a liquid. Explain.

Answer 1

We are given that ${{\text{H}}_{\text{2}}}{\text{S}}$ is a gas and ${{\text{H}}_{\text{2}}}{\text{O}}$ is a liquid i.e. the physical states of ${{\text{H}}_{\text{2}}}{\text{S}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$ are different. The difference in physical states of a substance is due to the different types of forces that exist between the molecules and also because of the strength of these forces.

Complete step-by-step answer:
We are given that ${{\text{H}}_{\text{2}}}{\text{S}}$ is a gas and ${{\text{H}}_{\text{2}}}{\text{O}}$ is a liquid i.e. the physical states of ${{\text{H}}_{\text{2}}}{\text{S}}$ and ${{\text{H}}_{\text{2}}}{\text{O}}$ are different.

The bonding between an electronegative atom and a hydrogen atom bonded to another electronegative atom like oxygen, nitrogen or fluorine is known as hydrogen bonding.In ${{\text{H}}_{\text{2}}}{\text{O}}$, the electronegativity difference between hydrogen and oxygen atom is very high. Thus, the covalent bond between hydrogen and oxygen gets polarised. Thus, the hydrogen atoms have partial positive charge and get attracted to the lone pairs on the oxygen atom in ${{\text{H}}_{\text{2}}}{\text{O}}$ molecule.

In ${{\text{H}}_{\text{2}}}{\text{S}}$, the electronegativity difference between hydrogen and sulphur atoms is low. Thus, the covalent bond between hydrogen and sulphur does not get polarised. Thus, the lone pairs on the sulphur atom in ${{\text{H}}_{\text{2}}}{\text{S}}$ molecule get delocalised. Thus, the negative charge of lone pairs gets distributed and the interactions are lowered.

Thus, the hydrogen bonding in ${{\text{H}}_{\text{2}}}{\text{O}}$ causes the association of molecules due to the intermolecular forces of attraction. In ${{\text{H}}_{\text{2}}}{\text{S}}$, the intermolecular forces of attraction are very weak.

We know that in a liquid state the intermolecular forces of attraction are stronger. Thus, ${{\text{H}}_{\text{2}}}{\text{O}}$ is liquid. and in a gaseous state the intermolecular forces of attraction are weaker. Thus, ${{\text{H}}_{\text{2}}}{\text{S}}$ is a gas.

Note: The physical state of a substance does not depend on its molecular weight. Even though the molecular weight of ${{\text{H}}_{\text{2}}}{\text{S}}$ is more than that of ${{\text{H}}_{\text{2}}}{\text{O}}$, ${{\text{H}}_{\text{2}}}{\text{S}}$ is a gas and ${{\text{H}}_{\text{2}}}{\text{O}}$ is a liquid. This is because the physical state depends on the intermolecular forces of attraction and not on the molecular weight.