AQA A-Level Chemistry AS Paper 1, June 2022: Question 21

1 mark · Easy difficulty · Multiple Choice

Identify which species is not a possible product of the reactions between chlorine and water.

Practise this question

Question

Question 21 asks: 'Which species is not a possible product of the reactions between chlorine and water?' followed by four multiple-choice options with lozenges: A is Cl⁻, B is ClO⁻, C is O₂, and D is OH⁻.
Question text

21 Which species is not a possible product of the reactions between chlorine and water?

[1 mark]

A Cl–

B ClO–

C O2

D OH–

Mark scheme

Show the mark scheme Mark scheme for question 21 shows the correct answer is D (AO1), awarding 1 mark, corresponding to OH⁻.

21 D (AO1) 1 OH–

How to answer it

Reactions of Chlorine with Water

📌 What this question tests

This question assesses your recall and understanding of AQA Group 7 (Halogens) chemistry, specifically:

  • The disproportionation reaction of chlorine in water without sunlight.
  • The alternative reaction of chlorine in water in the presence of bright sunlight/UV radiation.
  • Identifying products, dissociated ions, and acidic species in aqueous halogen equilibria.

Question 21 Walkthrough

Identifying impossible reaction products of Cl₂ + H₂O

✅ Correct Answer

D — OH⁻

Mark Scheme: Option D [1 mark] (AO1 recall)

Both reactions of chlorine with water form acidic solutions rich in H⁺ ions. Hydroxide ions (OH⁻) are not generated as products; rather, any added base reacts with the acids formed.

💡 Key Knowledge: The Two Water Reactions

  • 1. In dark / standard conditions (Disproportionation):
    Cl₂(aq) + H₂O(l) ⇌ HClO(aq) + HCl(aq)
    In water, these acids partially/fully dissociate:
    HClO ⇌ H⁺ + ClO⁻ (chlorate(I) ion)
    HCl → H⁺ + Cl⁻ (chloride ion)
    Eliminates options A and B!
  • 2. In sunlight / UV light:
    2Cl₂(aq) + 2H₂O(l) → 4HCl(aq) + O₂(g)
    Chloric(I) acid decomposes under UV radiation to release oxygen gas.
    Eliminates option C!

🧠 Exam Technique: Negative Phrasing

The prompt specifically asks which species is not a possible product:

  • Always circle or underline the word "not" in multiple-choice stems.
  • Note the plural: "reactions" (with an 's') hints that you must consider both the standard reversible reaction and the sunlight-driven photochemical decomposition.
  • Use pH logic as a quick sanity check: adding chlorine to water forms hydrochloric and chloric(I) acids (pH < 7), so a basic species like OH⁻ cannot be a product.

❌ Common Errors & Misconceptions

  • Selecting C (O₂): The most common student error. Many candidates forget the sunlight reaction (2Cl₂ + 2H₂O → 4H⁺ + 4Cl⁻ + O₂) and mistakenly believe oxygen cannot be produced.
  • Confusing with cold, dilute NaOH: In alkali, chlorine forms NaCl, NaClO, and H₂O:
    Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O . Here, OH⁻ is a reactant, never a product.
  • Overlooking ionisation: Some forget that HClO and HCl exist in equilibrium with their ions ( ClO⁻ and Cl⁻ ), incorrectly thinking only intact molecular acids are formed.

Topics

Inorganic Chemistry · 3.2.3 Group 7(17), The Halogens

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.