Laboratory Preparation of Hydrogen Chloride
Hydrogen chloride is prepared by heating sodium chloride with concentrated sulphuric acid.
NaCl + H2SO4 —–> NAHSO4 + HCl
NaCl + NaHSO4 ——> Na2SO4 + HCl
HCl gas is dried by passing through concentrated sulphuric acid.
On Industrial scale hydrochloric acid is manufactured by heating a mixture of concentrated sulphuric acid and sodium chloride in a Muffle furnace.
Properties of Hydrogen chloride
Physical properties of Hydrogen Chloride
(i) It is a colourless gas.
(ii) It has pungent suffocating smell.
(iii) It is highly soluble in water and its aqueous solution is known as hydrochloric acid.
(iv) The gas fumes in moist air.
(v) It is heavier than air (vapour density is 18.25). It is easily liquefied to a colourless liquid (b.p. 189 K) and freezes to a white crystalline solid (f.p. 159 K).
(vi) Its b.p. is 189 K and m.p. is 159 K.
Chemical properties of Hydrogen Chloride
(1) Action with litmus: Dry HCl gas has no action on litmus. However, its aqueous solution turns blue litmus red indicating its acidic character.
(2) Oxidation : It is oxidised by atmospheric oxygen in the presence of cuprous chloride, Cu2Cl2 to produce chlorine gas.
4HCl + O2 ——> 2H2O + 2Cl2
(3) Acidic properties : Its aqueous solution behaves as a strong acid as it is almost fully ionised in water.
H2O + 4HCl ⇔ H3O+ + Cl¯
Its aqueous solution is called hydrochloric acid. It shows all the typical properties of the acids as:
(i) It reacts with metals like sodium, potassium, magnesium, zinc and iron to liberate hydrogen gas.
2Na + 2HCl ——> 2NaCl + H2
Zn + 2HCl ——> ZnCl2 + H2
(ii) It reacts with bases like oxides, hydroxides and carbonates of metals to form their chlorides as :
ZnO + 2HCl—–> ZnCl2 + H2O
NaOH + HCl ——-> NaCl + H2O
K2CO3 +2HCl ——> 2KCl + H2O + CO2
(iii) It neutralises ammonia to form white dense fumes of ammonium chloride, NH4Cl.
NH3 + HCl —–> NH4Cl
(4) As a reducing agent : It acts as reducing agent because it can be oxidised to chlorine by strong oxidising agents like manganese dioxide, potassium permanganate and potassium dichromate as :
MnO2+ 4HCl —-> MnCl2 + 2H2O + Cl2
2KMnO4 + 16HCl —–> 2KCl + 2MnCl2 + 8H2O + 5Cl2
K2Cr2O7 + 14HCl —–> 2 KCl + 2 CrCl3 + 7 H2O + 3Cl2
(5) Action with fluorine : Fluorine can displace chlorine from hydrogen chloride.
2HCl + F2 ——> 2HF
However, bromine and iodine have no action on hydrogen chloride.
(6) Action with salts of weak acids : Hydrochloric acid decomposes salts of weak acids like carbonates, bicarbonates, sulphides, sulphites, thiosulphates or nitrites.
K2CO3 + 2HCl —–> 2KCl + H2O + CO2 ↑
NaHCO3 +HCl —–> NaCl + H2O + CO2 ↑
Na2S + 2HCl —–> 2 NaCl + H2S
Na2SO3 + 2 HCl ——> 2 NaCl + H2O
Na2S2O3 + 2 HCl —–> 2 NaCl + S + H2O + SO2 ↑
2 NaNO3 + 2 HCl —–> 2 NaCl + H2O + NO2 ↑+ NO↑
(7) Action with some metal salts
(i) With lead acetate solution : Hydrochloric acid reacts with lead acetate solution to form a white precipitate of lead chloride which is soluble in hot water.
Pb(CH3COO)2 + 2HCl ——> PbCl2 + 2CH3COOH
(ii) With silver nitrate solution: Hydrochloric acid reacts with silver nitrate solution to form a white precipitate of silver chloride which is soluble in ammonium hydroxide due to the formation of a complex, [Ag(NH3)2]Cl.
AgNO3 —-> AgCl + HNO3
AgCl + 2 NH4OH ——> [Ag(NH3)2]Cl + 2 H2O
(iii) With mercurous nitrate solution: Hydrochloric acid reacts with mercurous nitrate solution to form a white precipitate of mercurous chloride, HgCl2 which is neither soluble in hot water nor in ammonia.
Hg2(NO3)2 + 2HCl —–> Hg2Cl2 + 2HNO3
(8) Formation of aqua regia: A mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid is known as aqua regia. In aqua regia hydrochloric acid (which is a reducing agent) is oxidisied by nitric acid (which is an oxidising agent) to form nascent chlorine.
3HCl + HNO3 —–>NOCl + 2H2O + 2Cl
Aqua regia can dissolve even noble metals like gold and platinum which are not otherwise dissolved by any single acid as:
Au + 3Cl ——> AuCl3
Au + 4H++ NO3¯ + 4 Cl¯ ———> AuCl4¯ + NO + 2 H2O
3Pt + 16 H+ + 4NO3‾ + 18 Cl¯ ——-> 3 [PtCl6]2¯ + 4 NO + 8 H2O
Uses Of Hydrogen Chloride
(1) In dyeing calicoprinting and tanning industries
(2) In pickling before tinning , galvanising and electroplating.
(3) In the manufacture of glucose by the hydrolysis of starch.
(4) In the manufacture of chlorides and chlorine.
(5) In the preparation of aqua regia and which is used to dissolve noble metal like gold.
(6) As an important reagent in laboratory.
(7) It is used in medicines.