Hydrochloric Acid (HCl)

Oxy Acids	Name of oxy – acid
H2N2O2	Hyponitrous acid
H2 NO2	Hydro-nitrous acid
HNO2	Nitrous acid
HNO3	Nitric acid
HNO4	Per nitric acid

Preparation of HCl

In laboratory, HCl gas can be dried by passing through concentrated sulphuric acid

NaCl + H₂SO₄  NaHSO₄ + HCl

NaHSO₄ + NaCl  Na₂SO₄ + HCl

Properties of HCl

• 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,

HCl (g) + H₂O ↔ H₃O⁺ (aq) + Cl⁻ (aq); K_a = 10⁷

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

• It reacts with NH₃ and gives white fumes of NH₄Cl.

NH₃ + HCl → NH₄Cl

• Noble metals like gold, platinum can dissolve in aqua-regia [three part conc HCl and one part of conc HNO₃].

Au + 4H⁺ + NO₃^- + 4Cl^- → AuCl₄^- + NO + 2H₂O

3Pt +16H⁺ + 4NO₃^- +18Cl^{- →} 3PtCl₆^2- + 4NO + 8H₂O

• 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 of HCl

It is used

Ø in the manufacture of chlorine, NH₄Cl and glucose (from corn starch),

Ø for extracting glue from bones and purifying bone black,

Ø in medicine and as a laboratory reagent.