Group 7

• Colour of the halogens


  Gets darker down the group: F: yellow; Cl₂: yellowish-green; Br₂: reddish-brown, I₂: grey-black
• Fluorine


  • Reacts with water
• Chlorine


  • Reacts slightly with water
• Chlorine water and bromine water


  Solutions of Cl₂ and Br₂ respectively
• Iodine solution

  Iodine dissolved in aqueous potassium iodide, appears brown
• Atomic radius


  Increases down the group as the number of electron shells increases and there is increased shielding of the outer shell electrons by the inner shells
• Density

  Increases down the group
• Melting points


  Increase down the group as the increasing number of electrons makes the van der Waals forces between the molecules stronger, so it takes more energy to break these intermolecular forces
• Boiling points


  Show a similar trend, F₂ and Cl₂ being gases at room temperature, bromine a volatile liquid and iodine a solid
• Halogens


  Less reactive going down the group
• Displacement reaction between halogens


  More reactive halogen can displace a less reactive halogen from an aqueous halide solution
• Aqueous chlorine


  Displaces bromine from an aqueous solution of potassium bromide
• Iodine will not displace bromine from potassium bromide because iodine is not as good an oxidising agent as bromine
• E values


  Used to explain the relative reactivity of halogens as oxidising agents
• As E values get more positive (less negative), the halogens on the left become better oxidising agents and accept electrons more readily
• As E values get less positive (more negative), the halides on the right become better reducing agents and release electrons more readily
• Cl2 accepts electrons more readily than I-, and I- ions release electrons more readily than Cl-
• The reaction with the more negative E value


  Goes in the reverse direction
• Bromine water


  Used to test for C=C (carbon-carbon double) bonds in organic compounds
• Testing for C=C bonds with bromine water

  1. Shake unsaturated compound with bromine water

  2. Solution changes colour from orange (bromine) to colourless
• Halogens


  • Exist as diatomic molecules
  • Become less volatile and darker in colour going down the group
  • Are good oxidising agents, with oxidising ability decreasing down the group
• Sodium thiosulphate


  Na₂S₂O₃, can be used to determine the concentration of iodine by titration and to analyse samples of bleach
• S₂O₃²⁻ ions are better reducing agents than I⁻ ions


  I₂ molecules are better oxidising agents than S₂O₃²⁻ ions
• Reaction of sodium thiosulphate with iodine


  2Na₂S₂O₃(aq) + I₂(aq) → Na₂S₄O₆(aq) + 2NaI(aq)
• Bromine and chlorine are stronger oxidising agents than iodine


  They cause further oxidation of sodium thiosulphate to sulphate ions
• Estimation of chlorine in bleaches


  1. Dilute the bleach

  2. Add excess acidified potassium iodide solution to liberate iodine

  3. Titrate the liberated iodine with sodium thiosulphate of a known concentration using starch indicator

  4. End point is when the blue-black colour of the starch-iodine indicator turns colourless
• Hydrogen halides


  • Formed when hydrogen combines directly with the halogens
  • Reactions are slower down the halogen group
• Reaction of hydrogen with fluorine

  H₂(g) + F₂(g) → 2HF(g)
• Hydrogen fluoride


  Boils at 19.5°C, so its state under normal laboratory conditions may be either liquid or gas
• Other hydrogen halides


  Gases at room temperature
• Higher boiling point of HF compared with other hydrogen halides


  Due to its extensive hydrogen bonding
• Hydrogen halides


  Acidic, form acids when they dissolve in water
• Reaction of hydrogen halides in water


  HC + HOH(aq) → H3O+(aq) + Cl-(aq)
• Thermal stability of hydrogen halides


  Decreases as the halogen atom increases in size
• Reaction of hydrogen iodide with hot platinum wire


  Purple vapour of iodine first seen, then turns directly to a grey-black solid on the side of the tube
• Ease of thermal decomposition of hydrogen halides


  Related to the bond energies of the hydrogen-halogen bond
• Bond energies


  • H-F: 562 kJ/mol
  • H-Cl: 431 kJ/mol
  • H-Br: 366 kJ/mol
  • H-I: 299 kJ/mol
• Larger the halogen atom


  Greater the distance between the hydrogen and halogen nuclei, smaller the attractive force between the nuclei and the bonding electrons, less energy to break the hydrogen-halogen bond
• Solid halides react with concentrated sulphuric acid

  1. Form hydrogen halides

  2. Fumes of hydrogen halide are the first product
• Reactions of solid halides with concentrated sulphuric acid

  • NaCl + H₂SO₄ → HCl + NaHSO₄
  • NaBr + H₂SO₄ → HBr + NaHSO₄
  • NaI + H₂SO₄ → HI + NaHSO₄