AS · Practice questions · Reactions of the halide ions

Test and reduce.

Six original Cambridge-style questions on the reactions of the halide ions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Bigger ion, stronger reducing agent.

Reducing power: I⁻ > Br⁻ > Cl⁻.
Conc. H₂SO₄: Cl⁻ HCl only; Br⁻ to SO₂; I⁻ to H₂S.
AgNO₃: white, cream, yellow.
Ammonia: dilute dissolves AgCl; conc. dissolves AgBr; AgI never.

Recall

[3 marks]

Describe the test for halide ions in solution, including the results for chloride, bromide and iodide.

Add dilute nitric acid, then aqueous silver nitrate. ✓
Chloride: white precipitate; bromide: cream; iodide: yellow. ✓
Confirm with ammonia: AgCl dissolves in dilute NH₃, AgBr only in concentrated NH₃, AgI in neither. ✓

Recall

[2 marks]

Write equations for the reactions of concentrated sulfuric acid with (a) sodium chloride and (b) hydrogen bromide.

(a) NaCl + H₂SO₄ → NaHSO₄ + HCl. ✓
(b) 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O. ✓

Explain

[3 marks]

Explain why concentrated sulfuric acid is reduced by bromide and iodide ions but not by chloride ions.

Reducing power increases down the group as the ions get larger. ✓
The outer electrons of Br⁻ and I⁻ are further from the nucleus and more shielded, so they are lost more easily. ✓
Cl⁻ holds its electrons too strongly to reduce the sulfuric acid. ✓

Analysis

[2 marks]

Write an equation for the reduction of concentrated sulfuric acid to hydrogen sulfide by hydrogen iodide, and give the change in oxidation number of sulfur.

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O. ✓
Sulfur changes from +6 to −2. ✓

Calculation

[2 marks]

Excess AgNO₃(aq) is added to a solution containing 0.585 g of NaCl (M_r 58.5). Calculate the mass of AgCl (M_r 143.4) formed.

n(NaCl) = 0.585 ÷ 58.5 = 0.0100 mol = n(AgCl). ✓
Mass = 0.0100 × 143.4 = 1.43 g. ✓

Explain

[2 marks]

Explain why dilute nitric acid is added before silver nitrate in the halide test.

It reacts with and removes ions such as carbonate that would also form precipitates with Ag⁺. ✓
Nitric acid is used because it adds no halide ions. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.