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Group 17: Chlorine - Hydrogen Chloride

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Group 17: Chlorine - Hydrogen Chloride - Lesson Summary

In 1648, Glauber became the first to prepare hydrogen chloride by heating a mixture of common salt and concentrated sulphuric acid. In 1810, Davy showed that it is a compound of hydrogen and chlorine.

                                                                     Δ
       NaCl         +         H2SO4                      →        NaHSO4           + HCl
  Common salt          Conc. Sulphuric                      Sodium                   Hydrogen
(Sodium Chloride)              acid                                 bisulphate               Chloride

Hydrogen chloride is prepared in the laboratory by treating sodium chloride with concentrated sulphuric acid. The reaction mixture, consisting of concentrated sulphuric acid and sodium chloride, is first heated to 420 K.

                                                                    420 K
       NaCl         +         H2SO4                          →        NaHSO4           + HCl
  Common salt          Conc. Sulphuric                               Sodium                   Hydrogen
(Sodium Chloride)              acid                                            bisulphate               Chloride

Sodium bisulphate is obtained, which is insoluble. Hence, sodium bisulphate is mixed with more sodium chloride and further heated to a high temperature of about 823 k to give HCl gas and soluble sodium sulphate.

                                                           823 K
     NaHSO4              +    NaCl                →         Na2SO4             + HCl
Sodium bisulphate      Sodium Chloride         Sodium sulphate       Hydrogen chloride

The HCl gas is dried by passing it through concentrated sulphuric acid. HCl is not dried either over phosphorus pentoxide or quick lime because it reacts with them both.


Properties of HCl: 

  • Hydrogen chloride is a colourless gas with a pungent odour.
  • It liquefies to a colourless liquid at 189 K, and freezes to a white solid at 159K.
  • It is highly soluble in water.
  • An aqueous solution of hydrogen chloride is called hydrochloric acid.
  • An aqueous solution of hydrogen chloride undergoes ionisation to produce hydronium ions and chloride ions.

The larger value of the dissociation constant (Ka) indicates that hydrochloric acid is a strong acid.

HCl has the ability to dissociate almost 100 per cent to produce hydronium ions; it is considered a strong acid.

Hydrochloric acid reacts with metals and alkalis to form chlorides

EX: It reacts with zinc to form zinc chloride, and with sodium hydroxide to form sodium chloride.

Hydrochloric acid reacts with iron to form ferrous chloride.

     Fe       +  2HCl        →           FeCl2                  +H2
     Iron                               Ferrous chloride

Hydrochloric acid reacts with ammonia to form dense white fumes of ammonium chloride.

    NH3      +       HCl         →       NH4Cl
Ammonium                          Ammonium Chloride

Noble metals dissolve in a concentrated solution of nitric acid and hydrochloric acid taken in the ratio of 1:3. This solution is called aqua regia.

Hydrochloric acid decomposes salts of weaker acids, such as sodium carbonate, sodium bicarbonate and sodium sulphite.

Hydrochloric acid decomposes sodium carbonate and sodium bicarbonate to sodium chloride, carbon dioxide and water, while it decomposes sodium sulphite to sodium chloride, sulphur dioxide and water.

Sodium carbonate

     Na2CO3                +   2HCl  →        2NaCl                + CO2↑                   + H2O
Sodium carbonate                         Sodium chloride         carbon dioxide            Water

Sodium bicarbonate

     NaHCO3                +   HCl  →        NaCl                       + CO2↑                   + H2O
Sodium bicarbonate                         Sodium chloride         carbon dioxide            Water

Sodium sulphate

     Na2SO3                +   2HCl  →        2NaCl                      + SO2↑                   + H2O
Sodium sulphite                               Sodium chloride         Sulphur dioxide            Water

Uses of hydrochloric acid:

  • It is used in the manufacture of chlorine, and chlorides like ammonium chloride. It is also used in the manufacture of glucose from corn starch.
  • It is used as a laboratory reagent and in medicines.
  • A saturated solution of zinc chloride in dilute hydrochloric acid is used to remove the impurities on a metal surface before soldering or electroplating.

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