4.11 - Halogens in Oxidation State +1 and +5

• disproportionation reaction


  reaction where a single substance is both oxidised & reduced

  happens when halogens react with hot or cold alkalis
• general equation for reaction with cold aIkali

  X2 + 2NaOH → NaOX + NaX + H20
• general equation for reaction with hot alkali

  3X2 + 6NaOH → NaXO3 + 5NaX + 3H2O
• reaction of iodine & bromine with alkalis

  disproportionate at lower temperature than chlorine because BrO- and IO- ions are less stable
• reaction of potassium hydroxide with hot iodine solution

  produces solution containing potassium iodate (V) and potassium iodide
• chlorine oxoanions


  form when chlorine reacts with water & alkalis
• Oxoanions


  an ion with general formula XxOyz-
• what happens when chlorine dissolves in water

  reacts reversibly forming mixture of weak chloric (l) acid & strong hydrochloric acid in a disproportionation reaction

  H2O+ Cl2 → HCIO + HCI
• reaction of bromine with water

  similar to chlorine but to a lesser extent
• reaction of iodine with water


  insoluble in water

  hardly reacts
• bleach


  sodium chlorate (I)

  NaClO
• how bleach is made

  by dissolving Chlorine in sodium hydroxide solution in disproportionation reaction

  2NaOH + CI2 → NaCIO + NaCl + H2O
• uses of bleach


  water treatment

  bleaching paper & textiles

  cleaning agents
• how water treatment works

  when you dissolve chlorine in water it undergoes a disproportionation reaction producing hydrochloric acid hypochlorous acid

  Cl2 + H2O → HCI + HClO

  hypochlorous acid ionises to make chlorate (l) ions (hypochlorite ions)

  HCIO + H2O → CIO- + H3O+ (reversible reaction)

  hypochlorite ions can kill bacteria so water is safe to drink or swim in
• why is chloric (l) acid an effective disinfectant

  molecule can pass through cell walls of bacteria

  once inside it breaks cell wall open & hill organism by Oxidising & chlorinating molecules which make up its structure
• chloric (l) acid


  weak acid

  concentration of un-ionised HCIO in solution depends on pH
• pH range of swimming pools

  7.2-7.8