Hydrogen Chloride: Properties and Preparation

Hydrogen Chloride

• Formula: HCl

• Molecular Mass: 36.5

• Bond Type: Covalent

• Occurrence:

  • Found in volcanic emissions.
  • Present in 0.2-0.4% concentration in mammalian gastric juice, aiding digestion.

• Preparation by Direct Combination:

  By Synthesis:

  • Moist hydrogen gas reacts with chlorine gas in the presence of diffused sunlight.
  • H₂(g) + Cl₂(g)--> 2HCl (g)
  • Reaction is explosive in direct sunlight, negligible in the dark
  • Requires a catalyst (like activated carbon) to proceed in the dark
  • Also forms when hydrogen burns in the presence of chlorine gas

  By heating Metallic Chloride with conc. H2SO4:

• Laboratory Preparation:

  • Concentrated sulfuric acid reacts with sodium chloride to produce hydrogen chloride gas.
  • Reaction Temperature: Below and above 200°C
  • Collected by downward displacement of air. (Heavier than air)
  • Purified by passing through concentrated sulfuric acid to remove water.

Physical Properties of Hydrogen Chloride Gas

• Colour: Colourless
• Smell: Pungent, choking
• Taste: Sour (acidic)
• Physiological Nature: Corrosive, irritating to nose, throat, and lungs
• Density: Heavier than air
• Boiling Point: -83°C
• Melting Point: -113°C
• Solubility: Highly soluble in water (forms Hydrochloric Acid)

Chemical Properties of Hydrogen Chloride Gas

• Combustibility: Neither combustible nor a supporter of combustion; does not burn or support combustion of other substances.
• Reaction with Metals: Reacts with metals above hydrogen in the activity series, forming metallic chlorides and liberating hydrogen gas.
• Reaction with Oxides and Hydroxides: Reacts with metal oxides and hydroxides to form respective salts and water.
• Reaction with Ammonia: Combines with ammonia to form dense white fumes of ammonium chloride (NH₄Cl).
• Reaction with Thiosulphates: Reacts with thiosulphates to produce sulphur dioxide and precipitated yellow sulphur.
• Reaction with Nitrates (generally not a reaction): With some exceptions, it does not react with nitrates. However, certain nitrates, such as lead nitrate and mercury(I) nitrate, react with dilute hydrochloric acid to produce respective chlorides and nitric acid.
• Oxidation of Hydrogen Chloride: Oxidizing agents like manganese dioxide, lead dioxide, and red lead, oxidize concentrated hydrogen chloride to chlorine.
• Formation of Aqua Regia: Mixture of concentrated nitric acid and concentrated hydrochloric acid. It is used to dissolve gold and platinum.

Hydrochloric Acid (Aqueous Solution)

• Formation: Hydrogen chloride gas dissolved in water.

• Properties:

  • Strong acid
  • Turns blue litmus red
  • Conducts electricity (in aqueous solutions)
  • Reacts with metals to produce hydrogen gas and respective salt.
  • Reacts with oxides/hydroxides to form salts and water.
  • Reacts with salts of weaker acids (e.g., carbonates, hydrogen carbonates, sulphites, sulphides)
  • Concentrated hydrochloric acid is not prepared by boiling aqueous HCl to dryness.

• Uses:

  • Laboratory reagent
  • Industrial processes (e.g., cleaning metals, manufacturing chemicals, in various industrial processes like pickling of steel and tanning)
  • Medical applications (treat cases of decreased gastric activity)

• Important Notes:-The anhydrous state of HCl (g) doesn't alter litmus paper. -The solution of HCl (in water) turns litmus red. -The preparation of concentrated hydrochloric acid involves controlled boiling for specific concentrations.