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2.5.3Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Redox Reactions of the Halogens — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 35 of 53 Unit quiz

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Part 1 of 4Halogens as Oxidising Agents

2.5.3 Redox Reactions of the Halogens
With Group 1 metals
  • 2Na(s) + Cl₂(g) → 2NaCl(s), where sodium is oxidised from 0 to +1.
With Group 2 metals
  • Ca(s) + Br₂(l) → CaBr₂(s), where calcium is oxidised from 0 to +2.
With iron
  • Chlorine oxidises iron to the higher state: 2Fe(s) + 3Cl₂(g) → 2FeCl₃(s).
With iron(II) ions
  • Cl₂(aq) + 2Fe²⁺(aq) → 2Cl⁻(aq) + 2Fe³⁺(aq), and bromine behaves in the same way.
Iodine
  • Iodine is too weak to oxidise Fe²⁺; instead Fe³⁺ oxidises iodide: 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq).
Fluorine
  • Fluorine is such a strong oxidising agent that it reacts with a very wide range of elements.

Part 2 of 4Disproportionation of Chlorine

Chlorine is simultaneously oxidised and reduced when it reacts with water or aqueous alkali.

Disproportionation of Chlorine
Cold dilute alkali
  • Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l), producing bleach containing chlorate(I).
Hot alkali
  • 3Cl₂(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO₃(aq) + 3H₂O(l), producing chlorate(V).
Water
  • Cl₂(g) + H₂O(l) ⇌ HCl(aq) + HClO(aq), producing chloric(I) acid.
Oxidation numbers
  • Chlorine goes from 0 to -1 and +1 in cold alkali, and from 0 to -1 and +5 in hot alkali.
Bleach
  • The chlorate(I) in household bleach is an oxidising agent that kills bacteria and removes colour from stains.

Part 3 of 4Water Treatment

1
Chlorine gas is added to drinking water supplies and dissolves, reacting with the water.
2
The reaction forms chloric(I) acid, HClO, and hydrochloric acid, HCl.
3
HClO partly dissociates into H⁺ and the chlorate(I) ion, ClO⁻, and both HClO and ClO⁻ kill bacteria.
4
Treating water this way makes it safe to drink by sterilising it.
5
There are risks, as chlorine is toxic and can form harmful chlorinated organic compounds, but the benefit outweighs them.
6
Only a low concentration of chlorine is needed, which keeps the water free of harmful bacteria without making it unsafe to drink.

Part 4 of 4Exam Checklist & Tips

2.5.3 – Key Points for the Exam

  • Disproportionation A reaction in which the same element is both oxidised and reduced.
  • Chlorate(I) The ion ClO⁻, present in bleach, in which chlorine is +1.
  • Chloric(I) acid HClO, formed when chlorine dissolves in water.
  • Sterilise To kill bacteria and other microorganisms.

Common Mistakes

  • Writing the wrong product for hot and cold alkali, so that chlorate(V) appears in the cold reaction.
  • Stating that iodine oxidises iron(II) ions.
  • Forgetting that chlorine reacts with water reversibly, so the equation needs ⇌.
  • Assigning chlorine in HClO an oxidation number of -1 instead of +1.

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