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Group 17: Chlorine

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Group 17: Chlorine - Lesson Summary

Chlorine was first prepared by Scheele in 1774 by the action of hydrochloric acid on manganese dioxide. In 1810, Davy established its elementary nature and suggested the name chlorine on account of its colour.

The name 'chlorine' is derived from the Greek word 'chlorous,' means greenish yellow. Chlorine is a greenish-yellow gas.


Preparation methods:
Chlorine can be prepared by heating the manganese dioxide with concentrated hydrochloric acid.

Chlorine also prepared by the action of hydrochloric acid on bleaching powder (or) potassium permanganate (or) lead dioxide.







   16HCl           + 2KMnO4              →     2KCl             +   2MnCl2       + 8H2O         + 5Cl2
Hydrochloric       Potassium                  Potassium         Manganese       Water        Chlorine
     acid               permanganate              chloride             dichloride

   4HCl               + PbO2           → PbCl2             + 2H2O             + Cl2
Hydrochloric          Lead                Lead                  water            Chloride
      acid                 dioxide             dichloride


Electrolytic process:
Chlorine is obtained by the electrolysis of brine in a Nelson cell. This is the cheapest method and gives the purest chlorine.

Deacon's process: 
In this process, chlorine is obtained by the oxidation of hydrochloric acid in the presence of cuprous chloride at 723K and 1atmospheric pressure.


Physical properties:

  • It is a greenish-yellow gas with a pungent smell.
  • It is poisonous in nature.
  • It has a melting point of 171.6K and a boiling point of 239.11K.
  • It can be easily liquefied.
  • It is 2-5 times as heavier than air.
  • It is fairly soluble in water.


Chemical properties:

Chlorine dissolves in water to give a strongly smelling, yellow solution called chlorine water. Chlorine water loses its yellow colour on standing in sunlight, due to the formation of a mixture of hydrochloric acid and hypochlorous acid. Hypochlorous acid, being unstable, decomposes and gives nascent oxygen. The nascent oxygen so formed is responsible for the oxidising and bleaching properties of chlorine.
                                    Sun Light
     Cl2       + H2O             →               HCl               + HOCl
 Chlorine     Water                          Hydrochloric          Hypochlorous
                                                               acid                        acid
 
                                            Decompose
     2HOCl         +    2HCl             →                2[O]
Hydrochlorous     hydrochloric                        Nascent
           acid                 acid                                Oxygen

It combines directly with all non-metals except nitrogen, oxygen and carbon.

Chlorine reacts readily with most metals to form their corresponding chlorides.

It has great affinity for hydrogen. It combines with hydrogen in the presence of light with an explosion to form hydrochloric acid.

                                                    Light
          H2          +              Cl2           →         2HCl
      Hydrogen                   Chlorine               Hydrochloric
                                                                                 acid

It decomposes several hydrogen compounds to form hydrochloric acid.

It is a good oxidising agent, it oxidises ferrous to ferric, sulphites to sulphates, sulphur dioxide to sulphuric acid and iodine to iodic acid.

Moist chlorine due to the liberation of nascent oxygen, acts as a powerful bleaching agent. Moist chlorine bleaches vegetable or organic matter.

It forms bleaching powder with slaked lime.

          2Ca(OH)2          + 2Cl2       →        Ca(OCl)2       +        CaCl2        +    2H2O
  Calcium hydroxide      Cholrine        Calcium oxychloride        Calcium             Water
       (Slaked Lime)                             (Bleaching Powder)        Chloride

It reacts with saturated hydrocarbons to give substitution products and addition products with unsaturated hydrocarbons.

Ex: On reaction with methane in the presence of sunlight (or) ultraviolet light it produces chloromethane and ethene to 1, 2-dichloroethane.

Uses of chlorine:

Chlorine is used:

  • As a bleaching agent in the wood pulp, cotton and textile industries.
  • To sterilise drinking water.
  • As a germicide and disinfectant in swimming pools.
  • In the extraction of gold and platinum.
  • In the preparation of poisonous gases such as phosgene, tear gas and mustard gas.

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