group 7

• Fluorine (F2)


  Very pale yellow gas, highly reactive
• Chlorine (Cl2)


  Greenish, reactive gas, poisonous in high concentrations
• Bromine (Br2)


  Red liquid, gives off dense brown/orange poisonous fumes
• Iodine (I2)


  Shiny grey solid, sublimes to purple gas
• Displacement reactions of halide ions by halogens

  1. A halogen that is a strong oxidising agent will displace a halogen that has a lower oxidising power from one of its compounds

  2. The oxidising strength decreases down the group

  3. Oxidising agents are electron acceptors

  4. Chlorine will displace both bromide and iodide ions; bromine will displace iodide ions
• The colour of the solution in the test tube shows which free halogen is present in solution
• Chlorine


  Very pale green solution (often colourless)
• Bromine


  Yellow solution
• Iodine


  Brown solution (sometimes black solid present)
• Reactions of halide ions with silver nitrate

  1. The test solution is made acidic with nitric acid, and then silver nitrate solution is added dropwise

  2. Fluorides produce no precipitate

  3. Chlorides produce a white precipitate

  4. Bromides produce a cream precipitate

  5. Iodides produce a pale yellow precipitate

  6. The silver halide precipitates can be treated with ammonia solution to help differentiate between them
• Silver chloride dissolves in dilute ammonia to form a colourless solution
• Silver bromide dissolves in concentrated ammonia to form a colourless solution
• Silver iodide does not react with ammonia
• Reaction of halide salts with concentrated sulfuric acid

  1. F- and Cl- ions are not strong enough reducing agents to reduce the S in H2SO4, only acid-base reactions occur

  2. Br- ions are stronger reducing agents than Cl- and F- and after the initial acid-base reaction, the bromide ions reduce the sulfur in H2SO4 from +6 to + 4 in SO2

  3. I- ions are the strongest halide reducing agents. They can reduce the sulfur from +6 in H2SO4 to + 4 in SO2, to 0 in S and -2 in H2S
• Overall equation for reaction of NaBr with H2SO4: 2NaBr + 3H2SO4 → 2NaHSO4 + SO2 + Br2 + 2H2O
• Disproportionation reactions of chlorine

  1. Cl2 (g) + H2O (l) ⇌ HClO (aq) + HCl (aq)

  2. 2Cl2 + 2H2O → 4H+ + 4Cl- + O2
• Chlorine is used in water treatment to kill bacteria
• Reaction of chlorine with cold dilute NaOH solution

  Cl2 (aq) + 2 NaOH (aq) → NaCl (aq) + NaClO (aq) + H2O (l)
• Chlorates/sulfates


  In IUPAC convention the various forms of sulfur and chlorine compounds where oxygen is combined are all called sulfates and chlorates with relevant oxidation number given in roman numerals