2.5.3 Redox Reactions of the Halogens

Reactions with Group 1 & 2 metals

• The halogens react with some metals to form ionic compounds which are metal halide salts
• In all reactions where halogens are reacting with metals, the metals are being oxidised
• Reaction of sodium and chlorine 
  
  • 2Na (s) + Cl₂ (g) → 2NaCl (s) 
  • Na is being oxidised, the oxidation number is changing from 0 to +1

• Calcium is a group 2 metal:
  • Ca (s) + Br₂ (l) → CaBr₂ (s) 
  • Ca is being oxidised, the oxidation number is changing from 0 to +2

• Therefore the halogens are acting as oxidising agents 

Reactions with Iron(II)

• Chlorine and bromine can oxidise iron(II) to iron(III)

Cl₂ (g) + 2Fe²⁺ (aq) → 2Cl^- (aq) + 2Fe³⁺ (aq)

Br₂ (g) + 2Fe²⁺ (aq) → 2Br^- (aq) + 2Fe³⁺ (aq)

• However, iodine is not a strong enough reducing argent to oxidise iron(II) to iron(III)
• Iodine is actually oxidised from iodide ions to iodine by iron(III)

2I^- (aq) + 2Fe³⁺ (aq) → I₂ (aq) + 2Fe²⁺ (aq)

Disproportionation reaction

• 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:

• The ionic equation is:

• The ionic equation shows that the chlorine gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO^-(aq)
  • The half-equation for the oxidation reaction is:

• Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl^-(aq)
  • The half-equation for the reduction reaction is:

Chlorine in hot alkali (70 ^oC)

• The reaction that takes place is:

• The ionic equation is:

• The ionic equation shows that the chlorine gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in ox. no. from 0 to +5 in ClO₃^-(aq)
  • The half-equation for the oxidation reaction is:

• Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl^-(aq)
  • The half-equation for the reduction reaction is:

Drinking water

• 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

 

The disproportionation reaction of chlorine with water in which chlorine gets reduced to HCl and oxidised to HClO

• 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