20. Hydrogen Chloride

The p-Block Elements (XII) | Chemistry

The p-Block Elements (XII)

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7.20 Hydrogen Chloride

 NEETprep Audio Note:  

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   NEETprep Audio Note (English):
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    Glauber prepared this acid in 1648 by heating common salt with concentrated sulphuric acid. Davy in 1810 showed that it is a compound of hydrogen and chlorine.

Preparation

In laboratory, it is prepared by heating sodium chloride with concentrated sulphuric acid.

NaCl + H2SO4  NaHSO4 + HCl

NaHSO4 + NaCl  Na2SO4 + HCl

HCl gas can be dried by passing through concentrated sulphuric acid.

Properties

 

It is a colourless and pungent smelling gas. It is easily liquefied to a colourless liquid (b.p.189 K) and freezes to a white crystalline solid (f.p. 159 K). It is extremely soluble in water and ionises as follows:

Its aqueous solution is called hydrochloric acid. High value of dissociation constant (Ka) indicates that it is a strong acid in water. It reacts with NH3 and gives white fumes of NH4Cl.

NH3 + HCl → NH4Cl

 NEETprep Audio Note:  

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   When three parts of concentrated HCl and one part of concentrated HNO3 are mixed,aqua regia is formed which is used for dissolving noble metals, e.g., gold, platinum.


Hydrochloric acid decomposes salts of weaker acids, e.g., carbonates, hydrogencarbonates, sulphites, etc.

Na2CO3 + 2HCl → 2NaCl + H2O + CO2

NaHCO3 + HCl → NaCl + H2O + CO2

Na2SO3 + 2HCl → 2NaCl + H2O + SO2

Uses: It is used (i) in the manufacture of chlorine, NH4Cl and glucose (from corn starch), (ii) for extracting glue from bones and purifying bone black, (iii) in medicine and as a laboratory reagent.

 
Example 7.18

When HCl reacts with finely powdered iron, it forms ferrous chloride and not ferric chloride. Why? NEETprep Audio Note (English): 

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Solution

Its reaction with iron produces H2.

Liberation of hydrogen prevents the formation of ferric chloride.