Question

Give the chemical name and formula of bleaching powder. What happens when it is exposed to air? Write a chemical equation and mention two uses of bleaching powder.

Answer 1

Hint: $\text{C}{{\text{l}}_{\text{2}}}$ acts as a permanent bleaching agent because its bleaching action is due to oxidation.
$\begin{align}
  & \text{C}{{\text{l}}_{\text{2}}}\text{+}\,\,{{\text{H}}_{\text{2}}}\text{O}\,\to \,\,\,\text{2HCl}\,\text{+ }\,\text{ }\!\![\!\!\text{ O }\!\!]\!\!\text{ } \\
 & \text{organic colouring compound +}\,\text{ }\!\![\!\!\text{ O }\!\!]\!\!\text{ }\to \,\text{colourless matter} \\
\end{align}$
While $\text{S}{{\text{O}}_{\text{2}}}$ acts as a temporary bleaching agent because its bleaching action is due to reduction.

Complete Solution :
The chemical name of bleaching powder is calcium hypochlorite.it is manufactured by the action of chlorine on slaked lime ($\text{Ca(OH}{{\text{)}}_{\text{2}}}$. The chemical formula of bleaching powder is $\text{CaOC}{{\text{l}}_{\text{2}}}\text{.}{{\text{H}}_{\text{2}}}\text{O}$.
\[\text{2Ca(OH}{{\text{)}}_{\text{2}}}\text{+ }\,2\text{C}{{\text{l}}_{\text{2}}}\to \,\,\text{CaOC}{{\text{l}}_{\text{2}}}\text{+ }\,2{{\text{H}}_{\text{2}}}\text{O}\,\text{+}\,\text{CaC}{{\text{l}}_{\text{2}}}\]

The composition of bleaching powder is $\text{Ca(OCl}{{\text{)}}_{\text{2}}}\text{.CaC}{{\text{l}}_{\text{2}}}\text{.Ca(OH}{{\text{)}}_{\text{2}}}\text{.2}{{\text{H}}_{\text{2}}}\text{O}$ . Industrially bleaching powder is manufactured in Bachmann’s plant or Hosenclever plant.
- Bleaching powder is a yellowish powder with a strong smell of chlorine. When it expose to air, it absorbs the moisture and reacts with carbon dioxide $\text{(C}{{\text{O}}_{\text{2}}})$ and loses chlorine gas $\text{(C}{{\text{l}}_{\text{2}}})$. The obtained chlorine is known as ‘available chlorine’ which reacts with water and loses its yellow colour due to the formation of $\text{HCl}$ and $\text{HOCl}$. $\text{HOCl}$ on dissociation gives nascent oxygen $\text{(O)}$ which is responsible for the oxidizing and bleaching property of bleaching powder.
\[\begin{align}
  & \text{CaoC}{{\text{l}}_{\text{2}}}\,\text{+}\,\text{C}{{\text{O}}_{\text{2}}}\to \,\,\text{CaC}{{\text{O}}_{\text{3}}}\,\text{+}\,\,\text{C}{{\text{l}}_{\text{2}}} \\
 & \text{C}{{\text{l}}_{\text{2}}}\,\,\text{+}\,\,{{\text{H}}_{\text{2}}}\text{O}\,\,\,\,\,\to \,\,\text{HCl}\,\text{+}\,\text{HOCl} \\
 & \text{HOCl}\,\,\to \,\,\text{HCl}\,\text{+}\,\,\text{ }\!\![\!\!\text{ O }\!\!]\!\!\text{ } \\
\end{align}\]

Uses of bleaching powder –
It is used as disinfectant and germicide, so it is used mainly for the sterilization of drinking water.
It is used in the manufacturing of chloroform.
It is used for making unshrinkable wool and bleaching cotton, wood pulp.

Note: In the presence of a slight amount of dilute acid the evolution of nascent oxygen makes it an oxidizing and bleaching agent.
- Chlorine acts as an oxidizing and bleaching agent in the presence of moisture. Due to incomplete octet and its high electronegativity chlorine has a high tendency to accept electrons, so it acts as a strong oxidizing agent.