9.3 Halogens

Chemistry – Class 11

Comprehensive Class 11 Chemistry Nepal notes on Halogens, covering general characteristics, chemical properties, uses of Cl₂, Br₂, I₂, tests, and comparative study of haloacids (HCl, HBr, HI). Includes diagrams for effective learning.

No MCQ questions available for this chapter.

Unit 9.3: Halogens (5 Teaching Hours)

  1. General Characteristics of Halogens

    • Group 17 Elements
      • Halogens (F, Cl, Br, I, At) have seven valence electrons (ns²np⁵), needing one electron for stable octet.
      • Exist as diatomic molecules (Cl₂, Br₂, I₂). Physical state: F₂, Cl₂ (gases); Br₂ (liquid); I₂ (solid) at room temperature.
      • Electronegativity decreases down the group (F > Cl > Br > I). Strong oxidizing agents; reactivity decreases (F most reactive, I least).
      • Form ionic/covalent compounds with metals/non-metals.

      • Figure 1: Periodic Trends in Halogens (Diagram showing trends in electronegativity, atomic size, and reactivity).

  2. Comparative Study on Preparation of Halogens (Cl₂, Br₂, I₂)

    • Preparation
      • No diagrams or descriptions required as per curriculum.
      • Chlorine (Cl₂): Prepared by oxidation of chloride ions (e.g., heating NaCl with MnO₂ and conc. H₂SO₄).
      • Bromine (Br₂): Prepared by oxidation of bromide ions (e.g., heating NaBr with MnO₂ and conc. H₂SO₄).
      • Iodine (I₂): Prepared by oxidation of iodide ions (e.g., heating NaI with MnO₂ and conc. H₂SO₄ or using Cl₂ to displace I₂ from KI).

  3. Chemical Properties of Halogens (Cl₂, Br₂, I₂)

    • With Water
      • Cl₂: Disproportionates to form HCl and HClO: Cl₂ + H₂O → HCl + HClO.
      • Br₂: Less reactive, forms HBr and HBrO to a lesser extent.
      • I₂: Sparingly soluble, does not react significantly with water.
    • With Alkali
      • Cl₂: Cold dilute NaOH forms hypochlorite: Cl₂ + 2NaOH → NaCl + NaOCl + H₂O. Hot conc. NaOH forms chlorate: 3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O.
      • Br₂: Similar, forms NaBr, NaOBr (cold), or NaBrO₃ (hot).
      • I₂: Forms NaI, NaIO (cold), or NaIO₃ (hot), less reactive.
    • With Ammonia
      • Cl₂: Forms nitrogen trichloride (explosive): 8NH₃ + 3Cl₂ → 6NH₄Cl + NCl₃.
      • Br₂: Forms nitrogen and ammonium bromide: 8NH₃ + 3Br₂ → 6NH₄Br + N₂.
      • I₂: Forms nitrogen triiodide (explosive when dry): 5NH₃ + 3I₂ → 3NH₄I + NI₃.
    • Oxidizing Character
      • Halogens oxidize other substances; strength decreases down the group (Cl₂ > Br₂ > I₂).
      • Cl₂ oxidizes Fe²⁺ to Fe³⁺, Br⁻ to Br₂, I⁻ to I₂.
      • Br₂ oxidizes I⁻ to I₂ but cannot oxidize Cl⁻.
      • I₂ is the weakest oxidizer.
    • Bleaching Action
      • Cl₂: Bleaches by oxidation (nascent oxygen via HClO): Cl₂ + H₂O → 2HCl + [O]. Permanent bleaching (e.g., textiles, paper).
      • Br₂: Weaker bleaching action, less effective.
      • I₂: No significant bleaching action.

      • Figure 2: Bleaching Action of Chlorine (Diagram showing Cl₂ reacting with water to produce nascent oxygen).

  4. Uses of Halogens (Cl₂, Br₂, I₂)

    • Chlorine (Cl₂)
      • Water purification, disinfectants, bleach production, PVC manufacture, synthesis of HCl.
    • Bromine (Br₂)
      • Flame retardants, pesticides, pharmaceuticals, photography (AgBr).
    • Iodine (I₂)
      • Antiseptics (tincture of iodine), thyroid hormone synthesis, dyes, photography (AgI).

  5. Test for Cl₂, Br₂, and I₂

    • Chlorine (Cl₂)
      • Pungent smell, turns moist litmus paper red then bleaches it, turns starch-iodide paper blue (displaces I₂).
    • Bromine (Br₂)
      • Reddish-brown vapor, turns starch paper orange-yellow, displaces I₂ from KI solution.
    • Iodine (I₂)
      • Violet vapor, turns starch paper blue-black (forms starch-iodine complex).

      • Figure 3: Tests for Halogens (Diagram showing litmus, starch, and starch-iodide paper tests).

  6. Comparative Study on Preparation, Properties, and Uses of Haloacids (HCl, HBr, HI)

    • Preparation
      • No diagrams or descriptions required as
      • HCl: Prepared by heating NaCl with conc. H₂SO₄.
      • HBr: Prepared by heating NaBr with conc. H₃PO₄ (H₂SO₄ oxidizes Br⁻ to Br₂).
      • HI: Prepared by heating NaI with conc. H₃PO₄ (H₂SO₄ oxidizes I⁻ to I₂).
    • Properties
      • Reducing Strength: Increases down the group (HI > HBr > HCl). HI is strongest reducing agent due to weakest H–I bond, easily oxidized to I₂ (e.g., by HNO₃ or air). HBr moderately reducing, HCl least.
      • Acidic Nature: All strong acids, acidity decreases slightly (HCl > HBr > HI) due to decreasing bond dissociation energy.
      • Solubility: All highly soluble in water, forming acidic solutions. HCl forms constant boiling mixture (azeotrope) at 20.2%.
    • Uses
      • HCl: Cleaning metal surfaces, production of chlorides, organic synthesis, lab reagent.
      • HBr: Synthesis of alkyl bromides, pharmaceuticals, analytical chemistry.
      • HI: Synthesis of alkyl iodides, reducing agent in organic reactions, pharmaceuticals.

      • Figure 4: Properties of Haloacids (Diagram comparing bond strength and reducing nature of HCl, HBr, HI).
← Previous Next →