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Study of Compounds – Hydrogen Chloride

Hydrogen Chloride

Molecular formula: HCl
Molecular mass: 36.5 amu
Bond: Covalent

General Preparation of HCl gas
i. By synthesis
Moist hydrogen gas combines with chlorine in the presence of diffused sunlight.

ii. By heating metallic chloride with conc. sulphuric acid

Laboratory Preparation of Hydrogen Chloride
Hydrogen chloride gas is prepared by heating a metallic chloride (NaCl) with conc. sulphuric acid (H₂SO₄). 
Reactions:

Collection

• Hydrogen chloride gas is collected by the upward displacement of air as it is 1.28 times heavier than air.
• It is not collected over water because it is highly soluble in water.

Physical Properties

• Colourless, pungent, choking odour, slight sour taste.
• It is 1.28 times heavier than water and highly soluble in water.
• Liquefies at temperature of about 10°C at 40 atmospheric pressure.
• Boiling point is −83°C, and freezing point is −113°C.

Chemical Properties of HCl

1. Combustibility: The gas is neither combustible nor a supporter of combustion.
2. Thermal dissociation: On heating above 500°C, it dissociates into hydrogen and chlorine.
   
3. With metals: Metals which come before hydrogen in the electrochemical series form chlorides with the liberation of hydrogen.
   Zn + 2HCl → ZnCl₂ + H_2(g)
4. Reaction with ammonia: It combines with ammonia to form dense white fumes of ammonium chloride.
   NH_{3 (g)} + HCl _(g) → NH₄Cl

Hydrochloric Acid

Properties of Hydrochloric Acid
Physical Properties

• Colourless, slightly pungent with sharp sour taste.
• Corrosive in nature and causes blisters on the skin.
• Density is 1.2 gm/cc with boiling point of 110°C.

Chemical Properties

• Monobasic in nature
  HCl dissociates in aqueous solution to produce one hydrogen ion [H+] per molecule of the acid.
  HCl + H₂O → H₃O⁺ + Cl^-
• Acidic nature
  The presence of hydrogen ion [H⁺] in HCl imparts acidic properties to an aqueous solution of hydrochloric acid.
• Action on metals
  Ca + 2HCl → CaCl₂ + H₂
  Mg + 2HCl → MgCl₂ + H₂
• Action on oxides and hydroxides 
  MgO + 2HCl → MgCl₂ + H₂O
  Ca(OH)₂ + 2HCl → CaCl₂ + H₂O
• With salts of weaker acids 
  Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
  NaHCO₃ + HCl → NaCl + H₂O + CO₂
  Na₂SO₃ + 2HCl → 2NaCl + H₂O + CO₂
  NaHSO₃ + HCl → NaCl + H₂O + SO₂
  Na₂S + 2HCl → 2NaCl + H₂S
• Action on thiosulphates
  Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂ + S 
• Reaction with nitrates
  AgNO₃ + HCl → AgCl + HNO₃

Oxidation of Hydrochloric Acid

• MnO₂ + 4HCl MnCl₂ + 2H₂O + Cl₂
• K₂Cr₂O₇ + 14HCl 2KCl + 2CrCl₃ + 7H₂O + 3Cl₂
• 2KMnO₄ + 16HCl 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂
• Pb₃O₄ + 8HCl 3 PbCl₂ + 4H₂O + Cl₂

Formation of Aqua Regia

Aqua regia is a mixture of one part of conc. nitric acid and three parts of conc. hydrochloric acid.
HNO₃ + 3HCl → NOCl + 2H₂O + 2[Cl]
The nascent chlorine released reacts with noble metals such as gold and platinum to give their soluble chlorides.
Au + 3[Cl] → AuCl₃
Pt + 4[Cl] → PtCl₄

Uses of Hydrochloric Acid

a. In the manufacture of dyes, drugs, paints and silver chloride.
b. For purifying bone black, because HCl dissolves the calcium phosphate present in bones.
c. To remove rust from iron sheets.