Reactions of halides and silver nitrateEdexcel A-Level Chemistry: Revision notes
Section 1
Solid halides with concentrated sulfuric acid
Warming a solid Group 1 halide with concentrated sulfuric acid (in a fume cupboard) first gives a hydrogen halide in an acid–base reaction, seen as steamy fumes:
NaX(s) + H₂SO₄(l) → NaHSO₄(s) + HX(g)
What happens next depends on the reducing ability of the halide ion, which increases down the group: Cl⁻ < Br⁻ < I⁻.
- Chloride: steamy fumes of HCl only. No redox; sulfur stays +6.
- Bromide: HBr reduces H₂SO₄ to SO₂ (+6 to +4); bromide is oxidised to orange-brown Br₂. 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O
- Iodide: HI is a much stronger reducing agent and reduces H₂SO₄ to SO₂, to yellow sulfur (0) and to H₂S (−2, rotten egg smell), forming purple iodine fumes or a black solid.
- 2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O
- 6HI + H₂SO₄ → 3I₂ + S + 4H₂O
- 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O
Writing that HCl reduces sulfuric acid. Chloride is too weak a reducing agent, so only steamy HCl is seen.
Section 2
Why reducing ability increases down the group
A halide ion acts as a reducing agent by losing its outer electron: X⁻ → ½X₂ + e⁻.
Down the group the halide ion is larger, its outer electron is further from the nucleus and there is more shielding, so the electron is lost more easily. Reducing ability: I⁻ > Br⁻ > Cl⁻.
This is the reverse of oxidising power of the halogens (Cl₂ > Br₂ > I₂).
Trend questions need three ideas: larger ion, outer electron further from the nucleus, more shielding.
Section 3
Silver nitrate test for halide ions
Add dilute nitric acid first (to remove carbonate or hydroxide ions, which would also precipitate with silver), then aqueous silver nitrate:
Ag⁺(aq) + X⁻(aq) → AgX(s)
- Chloride: white precipitate of AgCl
- Bromide: cream precipitate of AgBr
- Iodide: yellow precipitate of AgI
The colours are close, so confirm with aqueous ammonia:
- AgCl dissolves in dilute ammonia: AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq)
- AgBr dissolves only in concentrated ammonia
- AgI is insoluble in concentrated ammonia
Acidifying with hydrochloric acid before the silver nitrate test; it adds chloride ions and gives a false white precipitate. Use nitric acid.
Section 4
Hydrogen halides with ammonia and with water
Hydrogen halides are colourless gases that fume in moist air.
- With ammonia, a white solid forms (white smoke if both are gases): NH₃(g) + HCl(g) → NH₄Cl(s). Hydrogen bromide and hydrogen iodide give ammonium bromide and ammonium iodide in the same way.
- With water they dissolve and ionise completely to give strong acids: HCl(g) + aq → H⁺(aq) + Cl⁻(aq). Hydrochloric acid is the usual example.
Must know
- NaX + H₂SO₄ gives HX (steamy fumes); NaCl stops there
- Bromide gives Br₂ (orange-brown) and SO₂; iodide gives I₂, S and H₂S
- Reducing ability of halide ions increases down the group
- AgNO₃ after HNO₃: white, cream, yellow; dissolves in dilute NH₃ (Cl), concentrated NH₃ (Br), not at all (I)
- NH₃ + HCl gives white smoke of NH₄Cl; HX in water gives strong acids
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Reactions of halides and silver nitrate
- A technician adds concentrated sulfuric acid, in a fume cupboard, to separate samples of solid sodium chloride, sodium bromide and sodium iodide.Hydrogen iodide reduces sulfuric acid to hydrogen sulfide, H₂S. Deduce the change in oxidation number of sulfur and write a balanced equation for the reaction, in which iodine is also formed.2 marks
- A technician has three unlabelled aqueous solutions, each containing one of sodium chloride, sodium bromide and sodium iodide. She has dilute nitric acid, aqueous silver nitrate, dilute aqueous ammonia and concentrated aqueous ammonia.The precipitate formed in one solution is white and dissolves completely in dilute aqueous ammonia. Identify the halide ion present and give the formula of the ion that makes the precipitate dissolve.2 marks
- A technician adds concentrated sulfuric acid to solid sodium chloride in a fume cupboard. She tests the steamy fumes with a glass rod dipped in concentrated ammonia solution and by bubbling them into water. She then repeats the experiment with solid sodium bromide, which also gives orange-brown fumes and a colourless gas with a choking smell.Explain the three observations made with sodium chloride, using equations where appropriate.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).