AQA AS Chemistry

Revision Notes

2.3.3 Testing for Halide Ions

Testing for Halides

Silver ions & ammonia

  • Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding a silver nitrate solution followed by ammonia solution
  • The halide ions will react with the silver nitrate solution as follows:

AgNO3(aq) + X(aq) → AgX(s) + NO3(aq)
(general equation)

Ag+(aq) + X(aq) → AgX(s)
(ionic equation)
Where X is the halide ion

  • If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)

Group 17 - Halide Test, downloadable AS & A Level Chemistry revision notes

A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution

  • Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
  • If the precipitate dissolves in dilute ammonia the unknown halide is chloride
  • If the precipitate does not dissolve in dilute but in concentrated ammonia the unknown halide is bromide
  • If the precipitate does not dissolve in dilute nor concentrated ammonia the unknown halide is iodide

Group 17 - Dissolving of Silver Halide, downloadable AS & A Level Chemistry revision notes

Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide doesn’t

Reaction of halide ions with silver nitrate & ammonia solutions table

Group 17 - Table 1_Reactions of Halide Ions, downloadable AS & A Level Chemistry revision notes

Concentrated sulfuric acid

  • Chloride, bromide and iodide ions react with concentrated sulfuric acid to produce toxic gases
  • These reactions should therefore be carried out in a fume cupboard
  • The general reaction of the halide ions with concentrated sulfuric acid is:

H2SO4(l) + X(aq) → HX(g) + HSO4(aq)
(general equation)
Where X is the halide ion

Reaction of chloride ions with concentrated sulfuric Acid

  • Concentrated sulfuric acid is dropwise added to sodium chloride crystals to produce hydrogen chloride gas

 

Group 17 - Apparatus Set Up, downloadable AS & A Level Chemistry revision notes

Apparatus set up for the reaction of sodium chloride with concentrated sulfuric acid

 

  • The reaction that takes place is:

H2SO4(l) + NaCl(s) → HCl(g) + NaHSO4(s)      

  • The HCl gas produces is seen as white fumes

Reaction of bromide ions with concentrated sulfuric acid

  • The thermal stability of the hydrogen halides decreases down the group
  • The reaction of sodium bromide and concentrated sulfuric acid is:

H2SO4(l) + NaBr(s) → HBr(g) + NaHSO4(s)     

  • The concentrated sulfuric acid oxidises HBr which decomposes into bromine and hydrogen gas and sulfuric acid itself is reduced to sulfur dioxide gas:

2HBr(g) + H2SO4(l) → Br2(g) + SO2(g) + 2H2O(l)

  • The bromine is seen as a reddish-brown gas

Reaction of iodide ions with concentrated sulfuric acid

  • The reaction of sodium iodide and concentrated sulfuric acid is:

H2SO4(l) + NaI(s) → HI(g) + NaHSO4(s)          

  • Hydrogen iodide decomposes the easiest
  • Sulfuric acid oxidises the hydrogen iodide to several extents:
  • The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur dioxide gas:

2HI(g) + H2SO4(l) → I2(g) + SO2(g) + 2H2O(l)

  • Iodine is seen as a violet/purple vapour
  • The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur:

6HI(g) + H2SO4(l) → 3I2(g) + S(s) + 4H2O(l)

  • Sulfur is seen as a yellow solid
  • The concentrated sulfuric acid oxidises HI and is itself reduced to hydrogen sulfide:

8HI(g) + H2SO4(l) → 4I2(g) + H2S(s) + 4H2O(l)

  • Hydrogen sulfide has a strong smell of bad eggs

Halide ion reactions with concentrated sulfuric acid table

Group 17 Table 2_Reactions of Halide Ions, downloadable AS & A Level Chemistry revision notes

Exam Tip

It gets easier to oxidise the hydrogen halides going down Group 17: the halides become stronger reducing agents.

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