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Topic 3.2.3 Group 7 Exam Practice

AQA A-Level Chemistry • Inorganic Chemistry • Group 7

Exam Practice

3.2.3 Group 7 Exam Practice

Test your understanding of halogen trends, halide reactions, silver halides, and water treatment with exam-style questions.

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📋 Structured Questions

Complete each question on paper, then check your answers against the mark scheme.

Question 1: Reactions of Halides with Concentrated Sulfuric Acid

9 marks

(a) Describe what is observed when concentrated sulfuric acid is added to solid sodium chloride. Write a balanced chemical equation for the reaction and state the role of the sulfuric acid. [3]

(b) When concentrated sulfuric acid is added to solid sodium bromide, a redox reaction occurs alongside the initial acid-base reaction. Explain why this redox reaction occurs with sodium bromide but not with sodium chloride. [3]

(c) Write a balanced chemical equation for the redox reaction that occurs between hydrogen bromide and concentrated sulfuric acid, identifying the oxidation state changes of sulfur and bromine. [3]

Show Mark Scheme

(a)

  • Observation: Steamy / misty white fumes [1]
  • NaCl(s) + H2SO4(l) → NaHSO4(s) + HCl(g) [1]
  • Role of sulfuric acid: Acid / proton donor [1]

(b)

  • Bromide ions are stronger reducing agents than chloride ions [1]
  • Bromide ions are larger / have more shells / experience more shielding than chloride ions [1]
  • The outer electron of a bromide ion is further from the nucleus, so it is attracted less strongly and lost more easily [1]

(c)

  • 2HBr(g) + H2SO4(l) → Br2(g) + SO2(g) + 2H2O(l) (or ionic equivalent) [1]
  • Sulfur is reduced: oxidation state changes from +6 (in H2SO4) to +4 (in SO2) [1]
  • Bromine is oxidised: oxidation state changes from -1 (in HBr) to 0 (in Br2) [1]
Examiner tip: Under AQA, you must be clear on the differences in reducing power. Iodide is strong enough to reduce sulfur from +6 all the way to -2 (in H2S, rotten egg smell), yellow sulfur (0), or SO2 (+4). Bromide is only strong enough to reduce sulfur to +4 (in SO2). Chloride and fluoride cannot reduce sulfur at all.

Question 2: Testing for Halide Ions

6 marks

(a) Describe the procedure a student should use to test an unknown aqueous solution for halide ions, including the reagents used. Explain why nitric acid is added. [3]

(b) Explain how the student can distinguish between silver chloride, silver bromide, and silver iodide precipitates using ammonia solution. [3]

Show Mark Scheme

(a)

  • Add dilute nitric acid (HNO3) to the solution [1]
  • Add silver nitrate (AgNO3) solution dropwise [1]
  • Nitric acid is added to react with and remove carbonate/sulfite impurities, which would otherwise react with silver ions to form silver carbonate/sulfite precipitates [1]

(b)

  • Silver chloride (white precipitate) dissolves in dilute ammonia solution [1]
  • Silver bromide (cream precipitate) is insoluble in dilute ammonia, but dissolves in concentrated ammonia solution [1]
  • Silver iodide (yellow precipitate) is insoluble in both dilute and concentrated ammonia solutions [1]
Examiner tip: Cream and white precipitates can look very similar under laboratory lighting. The ammonia solubility test is crucial for confirming the identity of AgCl vs AgBr. Make sure to specify the concentration of the ammonia used (dilute vs concentrated).

Question 3: Chlorine and Water Treatment

6 marks

(a) Write a chemical equation for the reaction of chlorine gas with water in the absence of sunlight. Explain, in terms of oxidation states, why this is a disproportionation reaction. [3]

(b) State one benefit and one risk associated with adding chlorine to drinking water. [2]

(c) Write a chemical equation for the reaction of chlorine with cold, dilute sodium hydroxide. [1]

Show Mark Scheme

(a)

  • Cl2 + H2O → HCl + HClO (reversible arrow acceptable) [1]
  • In Cl2, chlorine has an oxidation state of 0 [1]
  • Chlorine is both reduced to -1 (in HCl) and oxidised to +1 (in HClO) in the same reaction [1]

(b)

  • Benefit: Kills bacteria / pathogens, preventing waterborne diseases like cholera [1]
  • Risk: Chlorine is toxic / can react with organic matter in water to form chlorinated hydrocarbons (such as trihalomethanes), which are carcinogenic [1]

(c)

  • Cl2 + 2NaOH → NaCl + NaClO + H2O [1]
Examiner tip: Disproportionation reactions are high-frequency exam questions. When explaining them, always define the term (an element is simultaneously oxidised and reduced) and support your answer with the exact oxidation state values from before and after the reaction.
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