Halogens

• Fluorine


  • Yellow gas
  • Boiling point/-188°C
  • Melting point/-220°C
• Halogens


  • Chlorine
  • Bromine
  • Iodine
• Halogen physical properties at room temperature


  • Chlorine - gas
  • Bromine - red-brown liquid
  • Iodine - grey-black solid (purple vapour when heated)
• Atomic radius


  Increases down the group
• First ionisation energy


  Decreases down the group
• Electronegativity

  Decreases down the group
• Solubility of halogens


  Decreases down the group in water, but all soluble in non-polar solvents like hexane
• Chlorine water and iodine water are solutions of the halogens in water
• Halogens reacting with water

  1. Cl₂ + H₂O → HOCl + HCl

  2. Oxidation and reduction occur (disproportionation)
• Halogens reacting with cold dilute NaOH

  2NaOH + Cl₂ → NaCl + NaClO + H₂O
• NaClO


  Sodium hypochlorite or sodium chlorate(I)
• Halogens reacting with hot concentrated NaOH

  3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O
• Iodine does not react with cold dilute alkali, but will react with hot concentrated alkali
• Chlorine undergoes disproportionation in the reactions with alkali
• The yellow-green gas dissolves to form a colourless solution in the reactions with alkali
• Oxidising ability of halogens


  Decreases down the group from fluorine to iodine
• Oxidising agents


  Cause oxidation by accepting electrons
• Ease of gaining electrons

  Determines effectiveness as an oxidising agent
• Fluorine gains electrons more easily than chlorine due to less shielding and closer proximity to nucleus
• Displacement reactions of halogens


  1. Fluorine displaces all other halogens

  2. Chlorine displaces bromine and iodide

  3. Bromine displaces iodide
• Reducing ability of halides


  Increases down the group from fluoride to iodide
• Reducing agents


  Cause reduction by donating electrons
• Ease of losing electrons


  Determines effectiveness as a reducing agent
• Iodide ions lose electrons more easily than bromide ions as the electron is further from the nucleus
• Bromine is produced in the solution and bromine turns the solution brown
• Reducing ability of the halides


  The redoxing power of the halide increases as atomic number increases from fluoride to chloride to bromide to iodide
• Reducing agents


  Cause a reduction by causing a molecule or ion to gain electrons, they lose electrons and become oxidised
• Halide ions


  Lose electrons from their outer energy level, the electron lost has a weaker attraction to the nucleus because it is further from the nucleus compared to other halide ions
• Fluoride and chloride ions have little reducing ability because they are not as able to donate electrons as bromide and iodide ions
• Reaction of solid halide compounds with concentrated sulfuric acid


  1. Equation: NaF + H₂SO₄ → NaHSO₄ + HF

  2. Observations: Misty fumes (HF), heat released, gas produced, solid disappears, pungent smell (HF)

  3. Names of products: Sodium hydrogen sulfate and hydrogen fluoride
• Reaction of chloride with concentrated sulfuric acid


  1. Equation: NaCl + H₂SO₄ → NaHSO₄ + HCl

  2. Observations: Misty fumes (HCl), heat released, gas produced, solid disappears, pungent smell (HCl)

  3. Names of products: Sodium hydrogen sulfate and hydrogen chloride
• Reaction of bromide with concentrated sulfuric acid


  1. Equation: NaBr + H₂SO₄ → NaHSO₄ + HBr

  2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

  2. Observations: Misty fumes (HBr), heat released, gas produced, solid disappears, red-brown vapour (Br₂), pungent smell (HBr/SO₂/Br₂)

  3. Names of products: Sodium hydrogen sulfate, hydrogen bromide, bromine, sulfur dioxide and water
• Reaction of iodide with concentrated sulfuric acid


  1. Equation: NaI + H₂SO₄ → NaHSO₄ + HI

  2HI + H₂SO₄ → SO₂ + I₂ + 2H₂O

  8HI + H₂SO₄ → H₂S + 4I₂ + 4H₂O

  2. Observations: Misty fumes, heat released, gas produced, solid disappears, purple vapour and grey-black solid (I₂), pungent smell (HI/SO₂/I₂), rotten eggs smell (H₂S), yellow solid (S)

  3. Names of products: Sodium hydrogen sulfate, hydrogen iodide, iodine, sulfur dioxide, sulfur, hydrogen sulfide, water
• Reaction of solid metal halides with concentrated phosphoric acid


  1. Equation: NaF + H₃PO₄ → NaH₂PO₄ + HF

  NaCl + H₃PO₄ → NaH₂PO₄ + HCl

  KBr + H₃PO₄ → KH₂PO₄ + HBr

  KI + H₃PO₄ → KH₂PO₄ + HI

  2. Observation: Misty fumes of the hydrogen halide
• Chlorine or ozone may be added to drinking water at source to kill microorganisms
• Ozone


  An allotrope of molecular oxygen, O₃, can be generated by passing a high voltage electric discharge through oxygen or on a smaller scale using ultraviolet light
• Reaction of chlorine with water


  Cl₂ + H₂O → HOCl + HCl
• Advantages and disadvantages of using chlorine and ozone in the treatment of drinking water


  • Chlorine advantages: Cheaper than ozone, provides residual protection, more soluble in water than ozone

  Chlorine disadvantages: Cannot kill some microorganisms, leaves chemicals in the water, unpleasant taste

  Ozone advantages: Kills more different types of microorganisms than chlorine, breakdown product is oxygen, removes metal and organic particles, no unpleasant taste

  Ozone disadvantages: More expensive than chlorine, does not provide residual protection, less soluble in water than chlorine so requires special mixing techniques