3. Hydrogen chloride

HYDROGEN CHLORIDE (HCl)

Preparation :

By heating a halide with concentrated acid (Laboratory method) :

NaCl + H₂SO₄  NaHSO₄ + HCl 

NaHSO₄ + NaCl  Na₂SO₄ + HCl

This method is called as salt cake method as it involves the formation of  NaHSO₄ (salt cake).

HCl cannot be dried over P₂O₅ (P₄O₁₀) or quick lime since they react with gas chemically.

CaO + 2HCl  ® CaCl₂  + H₂O  

P₄O₁₀  + 3HCl ® POCl₃  + 3HPO₃ 

HCl is, hence dried by passing through concentrated H₂SO₄ .

Properties :

(i) It is a colourless, pungent smelling gases with acidic tastes.

(ii) It is easily liqueflied to a colourless liquid (b.p. 189 K) and freezes to a white crystalline solid (f.p. 159 K).

(iii)    It is quite soluble in water.

HCl ionises as below : HCl(g) + H₂O (l) ® H₃O+ (aq) + Cl^– (aq) ;  Ka = 107

It aqueous solution is called hydrochloric acid. High value of dissociation constant (Ka) indicates that it is a strong acid in water.

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.

Au + 4 H⁺ + NO3^– + 4 Cl^– ® [AuCl₄]^– + NO + 2 H₂O

3 Pt + 16 H⁺ + 4 NO^3­– + 18 Cl^– 3 [PtCl₆]^2– + 4 NO + 8 H₂O

It reacts with ammonia forming white fumes of NH₄Cl

NH₃ + HCl ® NH₄Cl

(iv)     It decomposes salt of weaker acids.

Na₂CO₃ + 2HCl ® 2NaCl + H₂O + CO₂

NaHCO₃ + HCl ® NaCl + H₂O + CO₂

Na₂SO₃ + 2HCl  ® 2NaCl + H₂O+ SO₂

Uses :

1. HCl is used in preparation of Cl₂, chlorides, aqua regia, glucose,  (from corn starch),

2. It is used in medicines, laboratory as reagents, cleaning metal surfaces before soldering or electroplating.

3. It is used for extracting glue from bones and purifying bone black.