AQA A-Level Chemistry Paper 3, 2021: Question 4

7 marks · Hard difficulty · Practical Techniques & Data Analysis

Group 7 Chemistry and Halide Ion Testing This question covers reactions and tests involving halide ions, focusing on reactions of sodium bromide with sulfuric acid and the identification of halide ions using silver nitrate and ammonia. It assesses understanding of displacement reactions, colour changes, ionic equations, and the rationale for using specific reagents in qualitative analysis.

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Question

AQA A-Level Chemistry Paper 3, 2021: Question 4
Question text

04 This question is about Group 7 chemistry.

04.1 Give an equation for the reaction of solid sodium bromide with

concentrated sulfuric acid to form bromine.

State one observation made during this reaction.

[2 marks]

Equation

Observation

04.2 A solution that is thought to contain chloride ions and iodide ions is tested.

1. Dilute nitric acid is added to the solution.

2. Aqueous silver nitrate is added to the solution.

3. A pale yellow precipitate forms.

4. Excess dilute aqueous ammonia is added to the mixture.

5. Some of the precipitate dissolves and a darker yellow precipitate remains.

Give a reason for the use of each reagent.

Explain the observations.

Give ionic equations for any reactions.

[5 marks]

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 4

Question Answers Additional comments/Guidelines Mark

4.1 M1 2H2SO4 + 2NaBr → Na2SO4 + SO2 + Br2 + 2H2O allow ionic and equation forming NaHSO4 1

3H2SO4 + 2NaBr → 2NaHSO4 + SO2 + Br2 + 2H2O

2H+ + 2Br- + H SO → SO + Br + 2H O

24 2 2 2

not equation from HBr unless formation of HBr

shown in separate equation

M2 orange/brown fumes/solution not liquid / yellow solid / bad eggs smell / white 1

ppt

ignore choking gas/fumes / steamy/white fumes

4.2 HNO3 removes (hydroxide/carbonate) ions that may give other ppts 1

with AgNO3

AgNO3 produces ppts with chloride/iodide/halide not chlorine/iodine/halogen 1

Ag+(aq) + Cl−(aq) → AgCl(s) OR allow Ag+(aq) + X−(aq) → AgX(s) 1

Ag+(aq) + I−(aq) → AgI(s) state symbols not required but not if wrong

NH3 dissolves AgCl (leaving yellow AgI) allow chloride/iodide salt/ppt 1

+ − allow with Ag+(aq) 1

AgCl(s) + 2NH3(aq) → Ag(NH3)2 (aq) + Cl (aq)

How to answer it

Study Guide: Group 7 Chemistry & Halide Ion Testing

Q1. Sodium Bromide and Concentrated Sulfuric Acid

Key reaction:

2NaBr + 3H2SO4 → 2NaHSO4 + SO2 + Br2 + 2H2O

Observation: Orange/brown fumes due to bromine gas.

Avoid vague phrases like “yellow solid” or “bad egg smell”. These relate to other reactions!
Think in redox terms: Br– is oxidised to Br2, H2SO4 is reduced to SO2.

Q2. Testing for Chloride and Iodide Ions

  • HNO3: removes carbonate/hydroxide ions that might interfere with AgNO3
  • AgNO3: forms precipitate with halide ions (Cl–, I–)
  • Observation: Pale yellow ppt forms → AgI. On adding NH3, some dissolves (AgCl) but AgI remains.

Equations:

Ag+(aq) + Cl–(aq) → AgCl(s)
Ag+(aq) + I–(aq) → AgI(s)
AgCl(s) + 2NH3(aq) → [Ag(NH3)2]+(aq) + Cl–(aq)
AgCl dissolves in ammonia. AgI does not. This is the basis for distinguishing between halides.
Don’t confuse the halogen names (e.g. “iodine”) with halide ions (e.g. “iodide”).

Topics

Inorganic Chemistry · Required Practicals · 3.1.7 Oxidation, Reduction, and Redox Equations · 3.2.3 Group 7(17), the Halogens · Required Practical 4: Carry out simple test-tube reactions to identify Cations and Anions

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.