Q. 17

The reaction

Cl2 (g) + 2OH- (aq) ClO- (aq) + Cl- (aq) + H2O (l)

represents the process of bleaching. Identify and name the species that bleaches the substances due to its oxidising action.

Answer :

Bleaching is a process in which the colors of the treated substance disappear and are whitened by using reactants called as bleaching agents. Cl2 was discovered by Swedish chemist Carl Wilhelm Scheele in 1774, which is used as a bleaching agent in pulp industry. Claude Louis Berthelot was the first to use Cl as a bleaching agent.

Cl2 is produced in-situ along with sodium hydroxide using electrolysis of aqueous NaCl



The oxidation state of Cl2 is zero, while that in the hypochlorite ion is +1, thus undergoing oxidation. This method is used to remove color or whiten the wood pulp in paper industry. This chlorine generated oxidizes the chromopohores present in wood pulp, which are the main color substances that can reflect, transmit or absorb wavelengths in the visible spectrum present in living things.


Note: In the above mentioned reaction, Cl2 is an oxidizing agent which in presence of hydroxide forms a hypochlorite which is harmful substance. Along with it, 2,3,7,8 – tetrachlorodibenzo-p-dioxin (TCDD), which is the most harmful substance produced in the process, so now the practice of Totally Chlorine Free (TCF) bleaching methods are used i.e. by using ClO2.


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