Group 7: Halogens

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Could you explain what happens in section 6.4 of the Nuffield course, since the Nuffield Chemistry students' book doesn't actually follow up the experiments? (These are tests to observe the reactions of halide ions.)
 

Igloo writes
Section 6.4 forms an extended part of Topic 6, and I can only give brief answers to your question. The results of the experiments are very much part of the specification, so make sure that you learn and understand what is going on.
 
Part 1: The silver halides
(a) The chloride gives a white precipitate of silver chloride; this precipitate reacts with the dilute ammonia solution to form a soluble complex, so the precipitate is seen to dissolve with a relatively small amount of the aqueous ammonia.
 
(b) The bromide gives a cream precipitate of silver bromide; similar results are obtained as with the chloride, but concentrated ammonia needs to be added before the precipitate disappears entirely.
 
(c) The iodide gives a yellow precipitate of silver iodide; this solid does not easily form the complex, so the precipitate remains, even with excess 8M ammonia.
 
All three precipitates darken when exposed to light, particularly the silver chloride. This is because powdered metallic silver has been formed during the decomposition, e.g. 2AgCl(s) -> 2Ag(s) + Cl₂
 
Part 2: Action of concentrated H₂SO₄ on potassium halides
Potassium chloride gives misty colourless fumes of hydrogen chloride.
 
Potassium bromide and potassium iodide initially give hydrogen bromide and hydrogen iodide, respectively, but these gases are easily oxidised by the excess H₂SO₄, giving the free gaseous halogen Br₂ (brown fumes) or I₂ (purple fumes), SO₂(g), H₂S(g). You should know how to test for SO₂ and H₂S.
 
I’ll leave you to construct and balance the relevant equations.
 
Part 3: Action of concentrated phosphoric acid on potassium halides
All three potassium halides give the hydrogen halides, i.e. HCl, HBr or HI (all three are colourless gases).
 
Phosphoric acid isn’t an oxidising agent so no redox occurs as with concentrated H₂SO₄
 
Reactions of hydrogen halides
(a) All three hydrogen halides react and dissolve in the water to form acids, e.g. hydrochloric acid. If any gas remains in the test-tube, this will be the air that was once mixed with the hydrogen halide.
 
(b) All three gases react with gaseous ammonia to form a dense white smoke containing particles of the relevant ammonium halide: e.g. NH₃(g) + HBr(g) -> NH₄Br(s)
 
(c) Hydrogen iodide decomposes to form purple fumes of gaseous iodine [2HI(g) -> H₂(g) + I₂(g)], but higher temperatures are needed to decompose the other two hydrogen halides, so the gas in the tube remains colourless for these two compounds.

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updated: 19 January 2005