Revision Guide: Important Chlorine Compounds

Introduction to the Major Compounds of Chlorine

Chlorine (Cl), a halogen in Group 17, is highly reactive and forms numerous compounds. Its compounds exhibit diverse chemical properties due to chlorine’s variable oxidation states (e.g., -1, +1, +3, +5, +7). This guide focuses on three key inorganic chlorine compounds frequently tested in high school examinations.

Hydrogen Chloride (HCl)

• Chemical Formula: HCl
• Common Name: Hydrogen Chloride (gaseous state), Hydrochloric Acid (aqueous solution)

Laboratory Preparation

Hydrogen chloride gas is typically prepared by the reaction of concentrated sulfuric acid with sodium chloride.

• At low temperature (~200 K): NaCl(s) + H₂SO₄(conc.) → NaHSO₄(s) + HCl(g)
• At higher temperature (~420 K): 2NaCl(s) + H₂SO₄(conc.) → Na₂SO₄(s) + 2HCl(g)

The gaseous HCl is dried by passing it through concentrated sulfuric acid.

Properties

• Physical: Colourless, pungent-smelling gas; fumes in moist air; extremely soluble in water (forming hydrochloric acid); non-flammable.
• Chemical:
  • Acidic Nature: Dissolves in water to form a strong monoprotic acid. HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
  • Reaction with Metals: Reacts with metals above hydrogen in the reactivity series to produce hydrogen gas. Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
  • Reaction with Metal Oxides/Hydroxides: Forms salt and water. CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
  • Reaction with Carbonates/Bicarbonates: Produces carbon dioxide gas. Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
  • Reaction with Ammonia (Test for HCl): Forms dense white fumes of ammonium chloride. NH₃(g) + HCl(g) → NH₄Cl(s)
  • Aqua Regia: A mixture of 1 part concentrated nitric acid and 3 parts concentrated hydrochloric acid, capable of dissolving noble metals like gold and platinum. HNO₃(conc.) + 3HCl(conc.) → NOCl(g) + 2H₂O(l) + 2[Cl] Au(s) + 3[Cl] + HNO₃(conc.) → AuCl₃(aq) + HNO₂ (simplified for gold dissolution)

Bleaching Powder (Calcium Oxychloride, CaOCl₂)

• Chemical Formula: CaOCl₂ (often represented as Ca(OCl)Cl to show its mixed salt nature)
• Common Name: Bleaching Powder, Chloride of Lime

Preparation Process

Bleaching powder is manufactured by passing chlorine gas over dry slaked lime (calcium hydroxide) at about 313 K.

Ca(OH)₂(s) + Cl₂(g) → CaOCl₂(s) + H₂O(l)

Properties

• Physical: Yellowish-white amorphous powder with a strong smell of chlorine. Sparingly soluble in water, forming a milky suspension due to insoluble impurities (e.g., Ca(OH)₂).
• Chemical:
  • Action of Dilute Acids: Reacts with dilute acids to liberate chlorine gas, which is the active bleaching agent. CaOCl₂(s) + H₂SO₄(dil.) → CaSO₄(s) + H₂O(l) + Cl₂(g) CaOCl₂(s) + 2HCl(dil.) → CaCl₂(aq) + H₂O(l) + Cl₂(g)
  • Bleaching Action: Chlorine reacts with water to form hypochlorous acid (HOCl) and hydrochloric acid. Hypochlorous acid is unstable and decomposes to give nascent oxygen, which is responsible for the bleaching action (oxidation). Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HOCl(aq) HOCl(aq) → HCl(aq) + [O] (Nascent oxygen) Coloured substance + [O] → Colourless substance
  • Oxidizing Agent: Acts as an oxidizing agent due to the release of chlorine or nascent oxygen. 2FeSO₄(aq) + H₂SO₄(aq) + Cl₂(g) → Fe₂(SO₄)₃(aq) + 2HCl(aq) (Cl₂ produced from bleaching powder + acid)
  • Decomposition: Decomposes slowly on exposure to air and carbon dioxide: CaOCl₂(s) + CO₂(g) → CaCO₃(s) + Cl₂(g)
  • Germicidal Action: The chlorine released acts as a powerful germicide and disinfectant.

Uses

• For bleaching cotton, linen, and wood pulp in the textile and paper industries.
• As an oxidizing agent in many chemical industries.
• For disinfecting drinking water and swimming pools.
• As a germicide.

Sodium Chloride (NaCl)

• Chemical Formula: NaCl
• Common Name: Common Salt, Table Salt, Rock Salt

Brief Guide

Sodium chloride is the most abundant and fundamental source of chlorine and its compounds.

• Occurrence: Found abundantly in seawater (approx. 2.7-2.9% by mass) and as solid deposits of rock salt (halite) in various parts of the world.
• Preparation:
  • From Seawater: Large-scale production involves the evaporation of seawater in shallow lagoons. The crude salt obtained contains impurities like CaSO₄, MgCl₂, and CaCl₂.
  • Purification: Impure common salt is dissolved in a minimum amount of water, and then HCl gas is passed through the saturated solution. Due to the common ion effect (Cl⁻), pure NaCl precipitates out, while the more soluble impurities remain in the solution.
• Properties: White crystalline solid, highly soluble in water, neutral pH.
• Uses:
  • An essential component of the human diet.
  • Used as a preservative for food (e.g., meat, fish).
  • Crucial raw material for the industrial preparation of a vast array of chemicals, including:
    • Sodium hydroxide (NaOH) by the electrolysis of brine (concentrated aqueous NaCl solution).
    • Chlorine gas (Cl₂) by the electrolysis of brine.
    • Hydrogen gas (H₂) also by the electrolysis of brine.
    • Sodium carbonate (Na₂CO₃) via the Solvay process.
    • Hydrochloric acid (HCl).
    • Sodium metal (Na) by electrolysis of molten NaCl.

Comparative Properties

Compound	Formula	Nature (Acid/Base/Salt)	Solubility in Water	Important Use(s)	Key Reaction(s)
Hydrogen Chloride	HCl	Strong Acid	Very High	Pickling metals, lab reagent, synthesis of chlorides	Forms white fumes with NH₃; reacts with metals above H
Bleaching Powder	CaOCl₂	Mixed salt	Moderate (decomposes)	Bleaching agent, disinfectant, oxidizing agent	Releases Cl₂ with dilute acids; bleaching action by [O]
Sodium Chloride	NaCl	Neutral Salt	High	Food additive, industrial raw material	Electrolysis to produce NaOH, Cl₂, H₂