Redox Reactions of the Halogens (Edexcel International A Level Chemistry)

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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) → CaBr(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 oxidising agent 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:

Group 17 - General Equation, downloadable AS & A Level Chemistry revision notes
  • The ionic equation is:

Group 17 - Ionic Equation, downloadable AS & A Level Chemistry revision notes
  • 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:

Group 17 - Oxidation Half-Equation, downloadable AS & A Level Chemistry revision notes

  • 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:

Group 17 - Reduction Half-Equation, downloadable AS & A Level Chemistry revision notes

Chlorine in hot alkali (70 oC)

  • The reaction that takes place is:

Group 17 - General Equation 2, downloadable AS & A Level Chemistry revision notes
  • The ionic equation is:

Group 17 - Ionic Equation 2_1, downloadable AS & A Level Chemistry revision notes
  • 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:

Group 17 - Oxidation Half-Equation 2, downloadable AS & A Level Chemistry revision notes

  • 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:

Group 17 - Reduction Half-Equation 2, downloadable AS & A Level Chemistry revision notes

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

 Group 17 - Chlorine and Water Reaction, downloadable AS & A Level Chemistry revision notes

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

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.