Hydrogen Chloride

Hydrogen Chloride

Preparation

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

Lead acetate

(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

             Nitrosyl chloride

Aqua regia can dissolve even noble metals like gold and platinum which are not otherwise dissolved by any single acid as:

Au + 3Cl ——> AuCl₃

   Auric chloride

  (soluble)

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.