(Bleach) disproportionation reactions of chlorine.

• Disproportionation is the name for a reaction where an element simultaneously oxidises and reduces.
• Chlorine with water: DISPROPORTIONATION

  Cl2 (g) + H2O (l) ⇌ 2H+ (aq) + Cl- (aq) + ClO- (aq)

  • chlorine oxidation state = -1
  • In chlorate (I) ions chlorines oxidation state = +1

  both oxidised and reduced
• Reaction with water in sunlight: (not disproportionation)

  Cl2 (g) + H2O (l) ----> 2H+ (aq) + 2Cl - + 1.5O2 (g)

  • chlorine is only reduced ?
• if you mix chlorine gas with cold, dilute sodium hydroxide at room temperature you get sodium chlorate (I) solution which is a household bleach
• making bleach:

  Cl2 (g) + 2 NaOH (aq) ----> NaCl (aq) + NaClO (aq) + H2O (l)

  • Cl2 oxidation state = 0 (chlorine bonded to chlorine)
  • NaCl chlorines oxidation state = -1
  • ClO chlorate ion chlorines oxidation state = +1

  disproportionation = chlorine is both oxidised and reduced
• water treatment: POSITIVES

  • chlorate (I) ions kill bacteria - so adding chlorine or compound containing chlorate ions makes water safe to drink or swim so is an important part of water treatment

  prevents growth of algae, eliminate bad taste and smell, removed discolouration
• water treatment: NEGATIVES

  • chlorine gas is very harmful when breathed in irritating the respiratory system
  • liquid chlorine on skin or eyes is an irritant or causes severe chemical burns
  • chlorine reacts with compounds to form chlorinated hydrocarbons (chloromethane) which are carcinogens

  increased cancer risk is small compared to risk of untreated water - cholera epidemic could kill thousands

  so we have to weigh up risks and benefits