Halogens: Overview

Characteristics of Halogens

• Fluorine (F₂): Very pale yellow gas, highly reactive.
• Chlorine (Cl₂): Greenish, reactive gas, poisonous at high concentrations.
• Bromine (Br₂): Red liquid, emits dense brown/orange poisonous fumes.
• Iodine (I₂): Shiny grey solid, sublimes to purple gas.

Trends in Melting and Boiling Points

• Increases down the group due to:
  • Larger molecules with more electrons.
  • Stronger van der Waals forces between molecules.
  • Higher energy required to break intermolecular forces.

Trends in Electronegativity

• Electronegativity decreases down the group.
  • Atomic radii increase due to more electron shells.
  • Weaker attraction between nucleus and bonding electrons.

Reactions Involving Halogens

Displacement Reactions of Halide Ions

• Strong oxidizing halogen displaces a weaker one from its compounds.
• Oxidizing strength decreases down the group.
• Chlorine displaces both bromide and iodide ions; Bromine displaces iodide ions.

Observations:

• Chlorine: Very pale green solution.
• Bromine: Yellow solution.
• Iodine: Brown solution.

Example Reactions:

• Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
• Cl₂ + 2I⁻ → 2Cl⁻ + I₂
• Br₂ + 2I⁻ → 2Br⁻ + I₂

Reactions with Silver Nitrate:

• Used to identify halide ions:
  • Fluorides: No precipitate.
  • Chlorides: White precipitate (AgCl).
  • Bromides: Cream precipitate (AgBr).
  • Iodides: Pale yellow precipitate (AgI).
• Use ammonia to differentiate silver halide precipitates:
  • Silver chloride dissolves in dilute ammonia.
  • Silver bromide dissolves in concentrated ammonia.
  • Silver iodide does not dissolve.

Reactions with Concentrated Sulfuric Acid

• Reducing power of halides increases down group 7.
• F⁻ and Cl⁻: Only acid-base reactions occur.
  • NaF + H₂SO₄ → NaHSO₄ + HF
  • NaCl + H₂SO₄ → NaHSO₄ + HCl
• Br⁻ and I⁻: Redox reactions occur.
  • NaBr + H₂SO₄ → NaHSO₄ + HBr
  • NaI + H₂SO₄ → NaHSO₄ + HI

Observations:

• HF & HCl: White steamy fumes.
• HBr: White steamy fumes, orange bromine fumes.
• HI: White steamy fumes, black solid iodine, purple fumes, sulfur dioxide, sulfur, and hydrogen sulfide.

Chlorine Disproportionation Reactions

Chlorine with Water

• Cl₂ + H₂O → HClO + HCl
• Universal indicator turns red due to acidity, then colorless as HClO bleaches.

Reaction in Sunlight

• 2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂

Chlorine with Cold Dilute NaOH

• Cl₂ + 2NaOH → NaCl + NaClO + H₂O
• Mixture used as bleach and disinfectant.

Naming Chlorates/Sulfates

• Use IUPAC conventions with oxidation numbers:
  • NaClO: Sodium chlorate(I)
  • NaClO₃: Sodium chlorate(V)
  • K₂SO₄: Potassium sulfate(VI)
  • K₂SO₃: Potassium sulfate(IV)