The Reactions of Chlorine (CIE AS Chemistry)

• A disproportionation reaction is a reaction in which the same species is both oxidised and reduced
• The reaction of chlorine with dilute alkali is an example of a disproportionation reaction
• In these reactions, the chlorine gets oxidised and reduced at the same time
• Different reactions take place at different temperatures of the dilute alkali

Chlorine in cold alkali (15 ^oC)

• The reaction that takes place is:

Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H₂O (l)

• The ionic equation is:

Cl₂ (aq) + 2OH^– (aq) → Cl^– (aq) + ClO^– (aq) + H₂O (l)

• The ionic equations show that the chlorine undergoes disproportionation, i.e. gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in oxidation number from 0 in Cl₂ (aq) to +1 in ClO^– (aq)
  • The half-equation for the oxidation reaction is:

½Cl₂ (aq) + 2OH^– (aq) → ClO^– (aq) + H₂O (l) + e^–

• Chlorine gets reduced as there is a decrease in oxidation number from 0 in Cl₂ (aq) to –1 in Cl^– (aq)
  • The half-equation for the reduction reaction is:

½Cl₂ (aq) + e^– → Cl^– (aq)

Chlorine in hot alkali (70 ^oC)

• The reaction that takes place is:

3Cl₂ (aq) + 6NaOH (aq) → 5NaCl (aq) + NaClO₃ (aq) + 3H₂O (l)

• The ionic equation is:

3Cl₂ (aq) + 6OH^– (aq) → 5Cl^– (aq) + ClO₃^– (aq) + H₂O (l)

• The ionic equations show that the chlorine undergoes disproportionation, i.e. gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in oxidation number from 0 in Cl₂ (aq) to +5 in ClO₃^– (aq)
  • The half-equation for the oxidation reaction is:

½Cl₂ (aq) + 6OH^– (aq) → ClO₃^– (aq) + 3H₂O (l) + e^–

• Chlorine gets reduced as there is a decrease in oxidation number from 0 in Cl₂ (aq) to –1 in Cl^– (aq)
  • The simplified half-equation for the reduction reaction is:

½Cl₂ (aq) + e^– → Cl^– (aq)

• Chlorine can be used to clean water and make it drinkable
• The reaction of chlorine in water is a disproportionation reaction in which the chlorine gets both oxidised and reduced

Cl₂ (aq) + H₂O (l) → HCl (aq) + HClO (aq)

• The ionic equations show that the chlorine undergoes disproportionation, i.e. gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in oxidation number from 0 in Cl₂ (aq) to +1 in ClO^– (aq)
  • The half-equation for the oxidation reaction is:

½Cl₂ (aq) + 2OH^– (aq) → ClO^– (aq) + H₂O (l) + e^–

• Chlorine gets reduced as there is a decrease in oxidation number from 0 in Cl₂ (aq) to –1 in Cl^– (aq)
  • The half-equation for the reduction reaction is:

½Cl₂ (aq) + e^– → Cl^– (aq)

• Chloric(I) acid (HClO) sterilises water by killing bacteria
• Chloric acid can further dissociate in water to form ClO^- (aq):

HClO (aq) → H⁺ (aq) + ClO^- (aq)

• ClO^-(aq) also acts as a sterilising agent cleaning the water