Hydrogen Chloride and Hydrochloric Acid
Hydrogen chloride (HCl) :
Formula : HCl
Nature : Covalent.
Molecular Mass : 36·5u [i.e. HCl = 1 + 35·5 = 36·5]
Present in gastric juices.
Present in volcanic gases.
Laboratory preparation of HCl gas :
Hydrogen chloride gas is prepared in laboratory by heating conc. H2SO4 with NaCl. NaCl + H2SO4
NaCl + NaHSO4
Sodium hydrogen sulphate
Na2SO4 + HCl
Physical Properties of HCl :
It is a colourless pungent smelling gas with acidic taste and which fumes strongly in air. (ii)
It is 1.25 times heavier than air (V. D. = 18·25) and is extremely soluble in water, at room temperature 1 volume of water dissolves 450 volumes of the gas.
Hydrogen gas can be liquified into a colourless liquid at 40 atm. pressure and 10°C temperature. Boiling point of liquid HCl is –83°C.
Freezing point of liquid HCl is –113°C.
The high solubility of HCl can be demonstrated by means of a Fountain Experiment. Fountain Experiment : A round bottom flask filled with dry HCl gas is taken and fixed in a stand as shown in fig. On the mouth of the flask is fixed a double holed stopper. In one hole a long glass jet tube is inserted and in the other a dropper with few drops of water. Lower end of the jet tube is dipped in the beaker containing blue litmus solution.
The rubber bulb of the dropper is pressed, so drops enter the flask and some HCl gas gets dissolved in it. This creates a vacuum in the flask which makes blue litmus solution rise into the jet tube. A red colour is thus observed in the flask.
Chemical Properties of HCl :
It is neither combustible nor supporter of combustion.
Due to its acidic nature, it turns moist litmus paper red.
It rapidly dissolve in water, to form hydronium ions.
HCl + H2O
H3O+ + Cl–
When hydrogen chloride gas is passed through silver nitrate solution, it forms white precipitate of silver chloride.
AgCl ↓ + HNO3
AgNO3 + HCl
Reaction with NH3 : Dry HCl gas combines with ammonia (NH3) gas to give dense white fumes of ammonium chloride.
NH3(g) + HCl(g)
Reaction of HCl gas with ammonia can be used as a test for HCl. Chemical Reactions of Hydrogen chloride :
Dilute hydrochloric acid reacts with metallic oxides on gentle warming, forming their respective chloride.
MgO + 2HCl MgCl2 + H2O
Concentrated hydrochloric acid is oxidized by powerful oxidising agents such as PbO2, MnO2, K2Cr2O7, Pb3O4. It liberates chlorine gas.
PbO2 + 4HCl
PbCl2 + 2H2O + Cl2(g) ↑
Pb3O4 + 8HCl
3PbCl2 + 4H2O + Cl2(g) ↑
Hydrochloric acid :
It is a solution of hydrogen chloride in water. The concentrated acid contains 25% of HCl by weight and is colourless, fuming, corrosive liquid. Its salts are known as chlorides. Uses of hydrogen chloride :
It is used in manufacturing of glucose from starch.
It is used for the extraction of glue from animal tissue and bones. (iii)
It is used in the manufacture of dyes, drugs, paints and is widely used in photography.