plumiDisproportionation and chlorine in water: OCR A‑Level Chemistry A questions
OCR A-Level Chemistry A past-paper questions on 3.1.3.3 Disproportionation and chlorine in water, 2016–2025, with mark schemes and examiner’s reports where available.
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Q22(a)
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All 18 questions on Disproportionation and chlorine in water, as a list
- June 2025, AS Paper 1, Q22(a) (3 marks): An aqueous solution of bromine reacts with cold aqueous sodium hydroxide in a disproportionation reaction. The equation is shown below. Br2(aq) + 2NaOH(aq) NaBr
- June 2025, AS Paper 1, Q22(b)(ii) (1 mark): Chlorine is used in water treatment. State one disadvantage of treating water with chlorine. [1]
- June 2024, Paper 1, Q6 (1 mark): Which equation does not represent a disproportionation reaction? A Cl 2 + H2O HCl O + HCl B Cl 2 + 2NaOH NaCl O + NaCl + H2O C 4KCl O3 KCl + 3KCl O4 D 4HCl + Mn
- June 2024, Paper 1, Q20(a)(i) (1 mark): Chlorine is used to treat water in large-scale water treatment plants. Suggest why chlorine is added to water in large-scale water treatment plants. [1]
- June 2024, AS Paper 1, Q8 (1 mark): In the UK, water companies typically treat drinking water with chlorine gas at a concentration of 0.500 mg dm–3 or less. Which statement about UK drinking water
- June 2024, AS Paper 1, Q9 (1 mark): Which reaction does not show disproportionation of chlorine? A MnO2 + 4HCl MnCl 2 + Cl 2 + 2H2O B Cl 2 + H2O HCl + HCl O C 2Cl O2 + 2NaOH NaCl O2 + NaCl O3 + H2
- June 2023, AS Paper 2, Q3(b) (3 marks): Chlorine reacts with calcium hydroxide to form Ca(OCl )2, which is the active ingredient in bleaching powder. 2Cl 2 + 2Ca(OH)2 CaCl 2 + Ca(OCl )2 + 2H2O This is
- June 2022, Paper 3, Q2(a)(ii) (3 marks): Calcium hypochlorite, Ca(Cl O)2, is heated. The Ca(Cl O)2 decomposes to form CaCl 2 and Ca(Cl O3)2. This is a disproportionation reaction. Write an equation for
- June 2022, AS Paper 1, Q24(a)(i) (1 mark): What is meant by the term disproportionation? [1]
- June 2022, AS Paper 1, Q24(a)(ii) (3 marks): Construct the equation for the reaction of chlorine with dilute sodium hydroxide. Use your equation to explain that disproportionation has taken place. Equation
- November 2021, AS Paper 1, Q23(a) (1 mark): Chlorine is used to kill bacteria in drinking water. State one risk in using chlorine in drinking water. [1]
- November 2020, Paper 1, Q21(b) (1 mark): Chlorine is used in water treatment. State one benefit and one risk of using chlorine in water treatment. Benefit Risk [1]
- November 2020, AS Paper 2, Q3(d)(ii) (3 marks): This reaction is an example of disproportionation. Use oxidation numbers to explain why. Include the meaning of the term disproportionation. [3]
- June 2018, Paper 3, Q1(a)(v) (1 mark): An element which reacts with water to form an acidic solution. [1]
- June 2018, AS Paper 2, Q3(b)(i) (3 marks): Iodine reacts with water as shown below. I2 + H2O HI + HIO Using oxidation numbers, explain why this reaction is a disproportionation. [3]
- June 2018, AS Paper 2, Q3(b)(ii) (1 mark): Chlorine is used to purify water on a large scale. State one disadvantage of using chlorine for the purification of drinking water. [1]
- June 2017, Paper 1, Q17(c)(i) (3 marks): State what is meant by disproportionation and use oxidation numbers to show that disproportionation has taken place. [3]
- June 2016, AS Paper 1, Q23(b) (1 mark): Chlorine is used in water treatment. State one benefit and one risk of chlorine in water treatment. Benefit Risk [1]
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