Hydrochloric Acid

Hydrochloric Acid

Manufacture

Large quantities of commercial HCl today are being obtained by the synthetic method which is based on the direct combination of hydrogen with chlorine in accordance with the equation:

H2 + Cl2 2HCl + 44,000 kcals

This reaction is very slow under ordinary conditions but proceeds very smooth in the presence of high temperature and sunlight. Both the gases, viz, H2 and Cl2 are obtained as by products during the manufacture of NaOH by the electrolysis of a solution of NaCl. Hence the process is quite economical. In this process pure electrolytic chlorine is burnt in an atmosphere of electrolytic hydrogen in large vertical silica tubes. The acid obtained by this process is quite pure.

Physical Properties

It is a colourless gas, heavier than air with pungent smell and acidic taste. It is extremely soluble in water and fumes strongly in moist air. 450 volumes of the gas dissolve in one volume of water. The gas can be easily liquefied and the liquid so produced boils at −83°C and freezes to a white crystalline solid at −113°C. It forms a constant boiling mixture with water when it contains 22.2 percent of the acid. The mixture boils at 110°C. Dilute solution of the acid cannot, therefore, be concentrated by boiling beyond 22.2 percent.

Chemical Properties

(i) Dissociation: HCl is quite stable but dissociates to give H2 and Cl2 at about 1500°C.

2HCl H2 + Cl2

(ii) Action on litmus paper: Hydrogen chloride does not affect litmus in the absence of moisture but in the moist state or in solution it turns blue litmus red. This is due to the fact that it dissolves in water to give hydrochloric acid.

(iii) Combustibility: It is neither combustible nor a supporter of combustion.

(iv) Action on NH3: It forms dense white fumes of ammonium chloride with ammonia.

HCl + NH3 NH4Cl

(v) Acidic nature: The solution of the gas is highly acidic and one of the strongest acid. It reacts with metals like iron, zinc sodium, potassium and calcium, forming the corresponding chlorides with the evolution of hydrogen. HCl gas also reacts with metals like zinc, aluminium, tin etc., when heated, forming anhydrous chlorides.

(vi) Action on oxides, hydroxides, carbonates and bicarbonates: HCl neutralizes oxides and hydroxides while carbonates and bicarbonates get decomposed with the evolution of CO2. For example,

MgO + 2HCl MgCl2 + H2O  

NaOH + HCl NaCl + H2O

2HCl + Na2CO3 2NaCl + H2O + CO2

HCl + NaHCO3 NaCl + H2O + CO2

(vii) Action of oxidizing agents: HCl gets oxidized to Cl2 by oxidizing agents like MnO2, KMnO4, K2Cr2O7 and PbO2.

(viii) Precipitation reactions: When HCl is added to the solution of a solute salt of Ag(I), Pb(II) and Hg(I), AgCl, PbCl2 and Hg2Cl2 are precipitated.

(ix) Formation of aqua regia: HCl forms aqua regia with HNO3 when mixed in the ratio
of 3 : 1.

3HCl + HNO3 NOCl + 2Cl + 2H2O

Uses

(i) Hydrochloric acid is used in the manufacture of chlorides and chlorine.

(ii) It is used in the textile and dyeing industries and also in tanning.

(iii) In the form of aqua regia it is used for dissolving metals like gold and platinum.

(iv) It is used as an important reagent in the laboratory.

(v) It is also used in medicine.

(vi) It is also used for cleaning iron sheets during tin plating, galvanization etc.

(vii) It is also used for the extraction of glue from animal tissues and bones.