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Reactions of Halogens

Chemistry: AQA A Level, topic 3.3.2

Reactive Trends of the Halogens

The redox abilities of the halogens change within the group and these can be used to identify the halide ions.

Oxidation reactions

Oxidation reactions

  • Oxidation is the loss of electrons (remember OIL RIG).
  • Fluorine is the most oxidising element because it is the most electronegative.
  • An oxidising element is itself reduced.
  • The trend in oxidising ability follows the trend in electronegativity - it decreases down the group.
Reduction reactions

Reduction reactions

  • Reduction is the opposite of oxidation - it is the gain of electrons.
  • So the trend in reducing ability is the reverse of the trend in oxidising ability.
  • Iodide ions are the most reducing.
  • Fluoride ions are not reducing at all.
Halide displacement reactions

Halide displacement reactions

  • If an elemental halogen is added to a halide solution, the heavier halide will be displaced.
  • This is because the lighter halide has a greater preference to be the halide ion because it is more electronegative.
  • E.g. If you add Cl2(g) to a solution of NaBr, the chlorine will displace the bromine as a gas.
Reactions with sulfuric acid

Reactions with sulfuric acid

  • Halides react differently with sulfuric acid depending on their reducing ability:
  • Reaction with a fluoride or chloride produces HCl or HF.
  • Reaction with a bromide produces some Br2 and some SO2.
  • Reaction with an iodide produces I2 immediately and will reduce the sulfur to H2S - a toxic molecule that smells like eggs.

Identification of Halides

We can determine which halide ion is present through specific reactions.

Reaction with silver nitrate

Reaction with silver nitrate

  • If we add silver nitrate to a solution of halide ions, the halides will react with the silver cations:
  • Ag+(aq) + X−(aq) → AgX(s)
Test results

Test results

  • The product of the silver nitrate reaction is a solid.
  • We call this solid the precipitate.
  • The colour of this solid depends on what halide is involved:
    • AgCl → white precipitate.
    • AgBr → cream precipitate.
    • AgI → yellow precipitate.
Further testing

Further testing

  • Using the silver halide product, we can test that their ability to redissolve in ammonia is as expected.
    • AgCl should redissolve.
    • AgBr should redissolve slowly and needs a lot of ammonia.
    • AgI should NOT dissolve.

Test yourself on Reactions of Halogens

  • If an elemental halogen is added to a halide solution, which element will end up in the halide solution?

  • What do the reactions of Cl-, Br- and I- with silver nitrate have in common?

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