Question

(a) Bleaching of flowers by chlorine is stable but bleaching of \[{\rm{S}}{{\rm{O}}_2}\] is unstable. Why?
(b) Water is a liquid while hydrogen sulfide is a gas at normal temperature. Why?

Answer 1

Hint: (a) Consider the basic difference involved in the mechanism of bleaching action of sulphur dioxide and chlorine. The bleaching action of sulphur dioxide is due to the action of nascent hydrogen. The bleaching action of chlorine gas is due to the action of nascent oxygen.
(b) Consider the possibility of the formation of intermolecular hydrogen bonds. Formation of intermolecular hydrogen bonds leads to molecular association.

Complete step by step answer:
(a) In presence of moisture, sulphur dioxide gas produces nascent hydrogen.

\[{\rm{S}}{{\rm{O}}_2}{\rm{ + 2 }}{{\rm{H}}_2}{\rm{O }} \to {\rm{ }}{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4}{\rm{ + 2}}\left[ {\rm{H}} \right]\]

This nascent hydrogen reduces coloured material. Thus, coloured material becomes colourless by reduction with nascent hydrogen.

\[{\rm{Coloured\;material + }}\left[ {\rm{H}} \right] \to {\rm{ Colourless\;material}}\]

After some time, the oxygen from the air (atmosphere) reacts with colourless material. Colourless material gets oxidized back to the original coloured material. Thus, the bleaching of coloured material to colourless material by sulphur dioxide gas is unstable and can be easily reversed by reaction with atmospheric oxygen.

\[{\rm{Colourless\;material + atmospheric\;oxygen}} \to {\rm{ Coloured\;material}}\]

On the other hand, the bleaching action of chlorine is due to formation of nascent oxygen.
In presence of moisture, chlorine gas produces nascent oxygen.

\[{\rm{C}}{{\rm{l}}_2}{\rm{ + }}{{\rm{H}}_2}{\rm{O }} \to {\rm{ 2 HCl + }}\left[ {\rm{O}} \right]\]

This nascent hydrogen reduces coloured material. Thus, coloured material becomes colourless by reduction with nascent hydrogen.

\[{\rm{Coloured\;material + }}\left[ {\rm{H}} \right] \to {\rm{ Colourless\;material}}\]

(b) Intermolecular hydrogen bonds are present in water. These result in molecular association and high boiling point. Hence, water is a liquid at normal temperature. However, hydrogen sulphide cannot form intermolecular hydrogen bonds. Hence, hydrogen sulphide has low boiling point and is a gas at normal temperature.

Note: a) The nascent hydrogen is the hydrogen atom that is just prepared in the reaction. It has high energy and has high reactivity. It is unstable. Normal hydrogen is a molecule containing two hydrogen atoms. It is stable and has low energy.
(b) A hydrogen bond is formed when a hydrogen atom is joined to electronegative nitrogen, oxygen or fluorine atom. Thus, molecules such as water, methanol, methylamine, etc, can form hydrogen bonds as in these molecules, hydrogen atom is attached to electronegative nitrogen, oxygen or fluorine atom. However other molecules such as methane, chloroform, hydrogen sulphide and benzene cannot form hydrogen bonds as in these molecules, hydrogen atom is not attached to electronegative nitrogen, oxygen or fluorine atom.