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9701 · 11.3

Some reactions of the halide ions — practice questions

Practice and worked examples for 9701 Some reactions of the halide ions. Short previews only — attempt the full question in MarkScheme against the official scheme.

Worked example 1

An unknown solution containing a sodium halide is tested. Addition of acidified silver nitrate solution produces a cream precipitate. This precipitate is found to be insoluble in dilute aqueous ammonia, but it dissolves when concentrated aqueous ammonia is added. Identify the halide ion and write the two ionic equations for the reactions that occurred.

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  1. Identification: A cream precipitate suggests the presence of bromide ions, Br⁻. This is confirmed by its solubility in concentrated ammonia but not dilute ammonia.
  2. Precipitation Reaction: The silver ions from silver nitrate react with the bromide ions to form a solid precipitate of silver bromide. Ionic Equation: Ag+(aq)+Br−(aq)→AgBr(s)Ag^+(aq) + Br^-(aq) \rightarrow AgBr(s)
  3. Complex Formation: The silver bromide precipitate reacts with concentrated ammonia to form the soluble diamminesilver(I) complex ion. Ionic Equation: AgBr(s)+2NH3(aq)→[Ag(NH3)2]+(aq)+Br−(aq)AgBr(s) + 2NH_3(aq) \rightarrow [Ag(NH_3)_2]^+(aq) + Br^-(aq)

Worked example 2

Describe the expected observations when a small amount of concentrated sulfuric acid is added to solid potassium iodide. For any two different redox reactions occurring, write a balanced chemical equation.

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  1. Observations:
    • Misty fumes of hydrogen iodide (HI).
    • A choking gas (sulfur dioxide, SO₂).
    • Purple vapour and a grey-black solid (iodine, I₂).
    • A yellow solid (sulfur, S).
    • A gas with the smell of bad eggs (hydrogen sulfide, H₂S).
  2. Redox Equations (any two from the following):

    • Formation of Iodine and Sulfur Dioxide: 2KI(s)+2H2SO4(l)→I2(s)+SO2(g)+2H2O(l)+K2SO4(aq)2KI(s) + 2H_2SO_4(l) \rightarrow I_2(s) + SO_2(g) + 2H_2O(l) + K_2SO_4(aq) (Or the ionic half-equations: 2I−→I2+2e−2I^- \rightarrow I_2 + 2e^-; H2SO4+2H++2e−→SO2+2H2OH_2SO_4 + 2H^+ + 2e^- \rightarrow SO_2 + 2H_2O)

    • Formation of Iodine and Hydrogen Sulfide: 8KI(s)+5H2SO4(l)→4I2(s)+H2S(g)+4H2O(l)+4K2SO4(aq)8KI(s) + 5H_2SO_4(l) \rightarrow 4I_2(s) + H_2S(g) + 4H_2O(l) + 4K_2SO_4(aq) (Or the ionic half-equations: 2I−→I2+2e−2I^- \rightarrow I_2 + 2e^-; H2SO4+8H++8e−→H2S+4H2OH_2SO_4 + 8H^+ + 8e^- \rightarrow H_2S + 4H_2O)

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