plumiRedox and electrode potentials: OCR A‑Level Chemistry A questions
Every OCR A-Level Chemistry A exam question on 5.2.3 Redox and electrode potentials, 2016–2025, with its mark scheme and examiner’s report.
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Q20(a)(i)
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Q20(a)(ii)
The whole paper ↗![June 2025 Paper 1, question 20(a)(ii): Calculate the standard cell potential of this cell. standard cell potential = V [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43201-jun25-20-aii-qp.webp?v=3)
Q20(b)
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Q21(d)(i)
The whole paper ↗![June 2025 Paper 1, question 21(d)(i): Calculate the percentage by mass of CuSO4•5H2O in Mixture A. Give your answer to 3 significant figures. Percentage of CuSO4•5H2O in Mixture A = % [5]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43201-jun25-21-di-qp.webp?v=3)
Q21(d)(ii)
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Q4(a)(ii)
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![June 2025 Paper 3, question 4(a)(iii): Construct an ionic equation for the reaction of SO3 [2]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun25-04-aiii-qp.webp?v=3)
Q5(c)(i)
The whole paper ↗![June 2025 Paper 3, question 5(c)(i): What is the standard electrode potential of the negative electrode in this cell? Standard electrode potential = V [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun25-05-ci-qp.webp?v=3)
Q5(c)(ii)
The whole paper ↗![June 2025 Paper 3, question 5(c)(ii): Construct the equation for the reaction at the negative electrode of this cell. [2]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun25-05-cii-qp.webp?v=3)
Q21(b)
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Q21(c)
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Q21(d)(i)
The whole paper ↗![June 2024 Paper 1, question 21(d)(i): A lithium-ion cell involves redox systems 1 and 5. Construct the overall cell equation for a lithium-ion cell. [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43201-jun24-21-di-qp.webp?v=3)
Q21(d)(ii)
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Q3(b)(i)
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Q3(b)(ii)
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Q3(b)(iii)
The whole paper ↗![June 2024 Paper 3, question 3(b)(iii): The uncertainty in each burette reading is ±0.05 cm3. Calculate the percentage uncertainty for the titre in Titration 1. percentage uncertainty = % [1]](https://pub-e8a9dbab8e2a4d089f52b2a45e0d0b50.r2.dev/bank/h43203-jun24-03-biii-qp.webp?v=3)
Q3(b)(iv)
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All 77 questions on Redox and electrode potentials, as a list
- June 2025, Paper 1, Q10 (1 mark): The overall equation for a hydrogen–oxygen fuel cell is: H2(g) + 1 2O2(g) H2O(l) The half-equation at the negative electrode is: H2(g) + 2OH–(aq) 2H2O(l) + 2e–
- June 2025, Paper 1, Q11 (1 mark): Two redox systems are shown below. Mg2+(aq) + 2e– E ɵ= – 2.37 V Mg(s) Pb2+(aq) + 2e– E ɵ = – 0.13 V Pb(s) Which species is the strongest reducing agent? Mg2+(aq
- June 2025, Paper 1, Q20(a)(i) (4 marks): A student sets up a standard cell based on redox systems 2 and 4. Draw a labelled diagram to show how this cell could be set up to measure its standard cell pot
- June 2025, Paper 1, Q20(a)(ii) (1 mark): Calculate the standard cell potential of this cell. standard cell potential = V [1]
- June 2025, Paper 1, Q20(b) (4 marks): A student predicts that cobalt metal will reduce acidified VO2+ ions to form V2+ ions in two stages. Explain why this two-stage reduction should happen in terms
- June 2025, Paper 1, Q21(d)(i) (5 marks): Calculate the percentage by mass of CuSO4•5H2O in Mixture A. Give your answer to 3 significant figures. Percentage of CuSO4•5H2O in Mixture A = % [5]
- June 2025, Paper 1, Q21(d)(ii) (1 mark): The student repeats their method, but after carrying out Step 1 they leave the solution exposed to the air for a few days. Some of the iron(II) sulfate is oxidi
- June 2025, Paper 3, Q4(a)(ii) (7 marks): Complete the flowchart by adding formulae in the boxes. white precipitate white precipitate Ag+(aq) Ba2+(aq) acidic solution containing three ions Cl2(aq)/H2O(I
- June 2025, Paper 3, Q4(a)(iii) (2 marks): Construct an ionic equation for the reaction of SO3 [2]
- June 2025, Paper 3, Q5(c)(i) (1 mark): What is the standard electrode potential of the negative electrode in this cell? Standard electrode potential = V [1]
- June 2025, Paper 3, Q5(c)(ii) (2 marks): Construct the equation for the reaction at the negative electrode of this cell. [2]
- June 2024, Paper 1, Q21(b) (4 marks): Potassium iodate tablets prevent the uptake of radioactive iodine in the human body following a nuclear accident. The mass of potassium iodate(V), KIO3, in a ta
- June 2024, Paper 1, Q21(c) (4 marks): Standard electrode potentials are measured by comparison with a reference half-cell. Draw a labelled diagram to show how the standard electrode potential could
- June 2024, Paper 1, Q21(d)(i) (1 mark): A lithium-ion cell involves redox systems 1 and 5. Construct the overall cell equation for a lithium-ion cell. [1]
- June 2024, Paper 1, Q21(d)(ii) (3 marks): Hydrogen-oxygen fuel cells can operate in acidic or in alkaline conditions. Show that for acidic and alkaline hydrogen–oxygen fuel cells, the standard cell pote
- June 2024, Paper 3, Q3(b)(i) (2 marks): In the titration, both iron(II) ions and ethanedioate, C2O4 Construct half-equations for the oxidation of iron(II) and ethanedioate ions. Oxidation of iron(II)
- June 2024, Paper 3, Q3(b)(ii) (3 marks): The student prepares a 250.0 cm3 solution of iron(II) ethanedioate by dissolving 1.295 g of FeC2O4•xH2O, in dilute sulfuric acid. The student titrates 25.0 cm3
- June 2024, Paper 3, Q3(b)(iii) (1 mark): The uncertainty in each burette reading is ±0.05 cm3. Calculate the percentage uncertainty for the titre in Titration 1. percentage uncertainty = % [1]
- June 2024, Paper 3, Q3(b)(iv) (6 marks): This question is about compounds and ions of iron(II) and iron(III) that contain ethanedioate ions, 2–. C2O4 (iv)* In the titration, 5 mol of iron(II) ethanedio
- June 2023, Paper 1, Q12 (1 mark): The equations show the electrode potentials of the half-cells used in a lithium-ion cell. E ө / V Li+ + e– Li –3.04 Li+ + CoO2 + e– LiCoO2 +1.16 Which statement
- June 2023, Paper 1, Q21(a)(i) (1 mark): State the colour change seen at the end point of the titration. from to [1]
- June 2023, Paper 1, Q21(a)(ii) (2 marks): The student’s titration results are shown in the table below. The trial titre has been omitted. 1 2 3 Final volume / cm3 12.65 25.60 38.35 Initial volume / cm3
- June 2023, Paper 1, Q21(a)(iii) (3 marks): Determine the concentration, in mol dm–3, of iron(II) ions in the river water. concentration = mol dm–3 [3]
- June 2023, Paper 1, Q21(b) (4 marks): The student modifies the experiment in (a) to determine the combined concentration of iron(II) and iron(III) ions in the river water. The student’s method is sh
- June 2023, Paper 1, Q22(d) (2 marks): An acidified solution containing Cr2O7 reaction to form a solution containing Cr3+ ions and VO2 + ions. Construct the overall equation for this reaction. [2]
- June 2023, Paper 3, Q6(c)(i) (1 mark): Balance the half-equation for the oxidation process in this reaction. H+ + AuCl4 – + e– Au + HCl [1]
- June 2023, Paper 3, Q6(c)(ii) (3 marks): In the reduction process in this reaction, HNO3 and H+ react together to form 2 oxides: X (Mr = 30) and Z (Mr = 18). Determine the formulae of X and Z and write
- June 2022, Paper 1, Q4 (1 mark): Hydrogen peroxide, H2O2, can be oxidised by manganate(VII) ions under acid conditions as shown below. –(aq) + 5H2O2(aq) + 6H+(aq) 2Mn2+(aq) + 5O2(g) + 8H2O(l) 2
- June 2022, Paper 1, Q13 (1 mark): Standard electrode potentials for two redox systems are shown below. Cr3+(aq) + 3e– Cr(s) –0.74 V Cr3+(aq) + e– Cr2+(aq) –0.42 V What is the standard electrode
- June 2022, Paper 1, Q22(b)(ii) (4 marks): Use the information below to explain why the pale green precipitate turns brown when left to stand in air and construct an equation for the reaction which occur
- June 2022, Paper 3, Q2(b) (5 marks): A student analyses the redox reactions shown below. State symbols have been omitted. CH3CHO + 2H+ + 2e– E ө = –0.197 V C2H5OH 2– + 14H+ + 6e– 2Cr3+ + 7H2O E ө =
- June 2022, Paper 3, Q2(c) (2 marks): A student bubbles hydrogen sulfide gas, H2S(g), through an acidified solution containing –(aq). manganate(VII) ions, MnO4 A redox reaction takes place, forming
- June 2022, Paper 3, Q6(d) (4 marks): The label on a carton of orange juice lists the mass of vitamin C, in mg, in a typical serving of 150 cm3. The student carries out an investigation to check the
- November 2021, Paper 1, Q3 (1 mark): The unbalanced equation for the reaction of silver with concentrated nitric acid is shown below. –(aq) + ….. H+(aq) → ….. Ag+(aq) + .…. NO(g) + ..… H2O(l) ….. A
- November 2021, Paper 1, Q12 (1 mark): Two redox systems are shown below. Fe2+(aq) + 2e– Eo– = –0.44 V Fe(s) Cu2+(aq) + 2e– Eo– = +0.34 V Cu(s) Which species in the two redox systems is the strongest
- November 2021, Paper 1, Q18(c) (1 mark): Suggest a suitable indicator for this titration and state the colour change at the end point in Step 4. Indicator: Colour from to [1]
- November 2021, Paper 1, Q18(d) (4 marks): Determine the percentage, by mass, of copper in the copper ore. Give your answer to an appropriate number of significant figures. percentage = % [4]
- November 2021, Paper 1, Q18(e)(i) (1 mark): The potassium iodide was not in excess, in Step 3. [1]
- November 2021, Paper 1, Q18(e)(ii) (1 mark): The burette readings were read from the top of the meniscus, in Step 4. [1]
- November 2021, Paper 1, Q18(f) (2 marks): The student then modifies the method in order to obtain a more accurate value for the percentage by mass of copper in the ore. The student decides to use 25.00
- November 2021, Paper 1, Q19(a)(i) (3 marks): Draw a labelled diagram to show how this cell could be set up in the laboratory. [3]
- November 2021, Paper 1, Q19(a)(ii) (1 mark): Construct the equation for the overall cell reaction. [1]
- November 2021, Paper 1, Q19(b)(ii) (2 marks): Explain, in terms of electrode potentials and equilibrium shifts why MnO4 2–(aq) disproportionates in acid conditions. Use the information in Table 19.1. [2]
- November 2021, Paper 1, Q19(c)(i) (1 mark): Write the half-equation at the negative electrode. [1]
- November 2021, Paper 1, Q19(c)(ii) (1 mark): The cell potential is 1.23 V. Calculate the electrode potential of the negative electrode. electrode potential = V [1]
- November 2021, Paper 1, Q19(c)(iii) (1 mark): State one important feature of a fuel cell that is different from a conventional storage cell. [1]
- November 2021, Paper 3, Q5(a)(i) (2 marks): This is a redox reaction. Write half-equations for the reduction and oxidation processes that take place. Reduction Oxidation [2]
- November 2020, Paper 1, Q12 (1 mark): Iodide ions, I−(aq), react with MnO4 −(aq). The unbalanced equation is shown below. I−(aq) + MnO4 −(aq) + H2O(l) IO−(aq) + MnO2(s) + OH−(aq) What is the ratio o
- November 2020, Paper 1, Q19(a) (3 marks): A student sets up a standard cell in the laboratory based on redox systems 3 and 4. Draw a labelled diagram to show how this cell could be set up to measure its
- November 2020, Paper 1, Q19(b) (4 marks): A student warms a mixture of methanal, HCHO, and acidified potassium manganate(VII). The student observes gas bubbles. Explain this observation in terms of elec
- November 2020, Paper 1, Q19(c) (2 marks): Methanoic acid, HCOOH, can be used in a fuel cell. As with all fuel cells, the fuel (HCOOH) is supplied at one electrode and the oxidant (oxygen) at the other e
- November 2020, Paper 1, Q22(c) (5 marks): Sodium sulfite(IV), Na2SO3, is used as a preservative in some foods. Food safety legislation allows a maximum of 850 mg Na2SO3 per kg of burger meat. A chemist
- June 2019, Paper 1, Q15 (1 mark): Four redox systems relevant to hydrogen–oxygen fuel cells are shown below. Eө/V H2O(l) + e− OH−(aq) + ½H2(g) −0.83 H+(aq) + e− ½H2(g) 0.00 ½O2(g) + H2O(l) + 2e−
- June 2019, Paper 1, Q18(b)(ii) (3 marks): The student adds excess iron to a solution containing VO2+(aq) ions, and observes that the colour of the solution changes from blue to green and then to violet.
- June 2019, Paper 1, Q18(b)(iii) (1 mark): Construct an equation for the first colour change from blue to green. [1]
- June 2019, Paper 1, Q18(c)(i) (1 mark): Explain why the student used 0.00200 mol dm−3 potassium manganate(VII) solution for this titration, rather than the more usual concentration of 0.0200 mol dm−3
- June 2019, Paper 1, Q18(c)(ii) (5 marks): Use the student’s results to determine the mass, in mg, of iron(II) gluconate in one tablet. Give your answer to 3 significant figures. mass of iron(II) glucona
- June 2019, Paper 3, Q6(a)(iv) (2 marks): Construct an equation for the reaction of H2S(g) with H+(aq)/MnO4 −(aq). [2]
- June 2018, Paper 1, Q12 (1 mark): The redox equilibria for a hydrogen–oxygen fuel cell in alkaline solution are shown below. 2H2O(l) + 2e– H2(g) + 2OH–(aq) Eө = –0.83 V 1/2O2(g) + H2O(l) + 2e– 2
- June 2018, Paper 1, Q20(d)(i) (2 marks): Complete Table 20.1 and calculate the mean titre that the student should use for analysing the results. mean titre = cm3 [2]
- June 2018, Paper 1, Q20(d)(ii) (1 mark): The uncertainty in each burette reading is ±0.05 cm3. Calculate the percentage uncertainty in the titre obtained from titration 1. Give your answer to two decim
- June 2018, Paper 1, Q20(d)(iii) (2 marks): Describe and explain how the student should determine the end point of this titration accurately. [2]
- June 2018, Paper 1, Q20(d)(iv) (5 marks): Determine the relative formula mass and formula of the Group 1 iodate(V), B. Show your working. relative formula mass of B = formula of B = [5]
- June 2018, Paper 1, Q21(b)(i) (4 marks): Draw a labelled diagram to show how this cell could be set up to measure its standard cell potential. Include details of apparatus, solutions and the standard c
- June 2018, Paper 1, Q21(b)(ii) (1 mark): Predict the standard cell potential of this cell. standard cell potential = V [1]
- June 2018, Paper 1, Q21(c)(i) (1 mark): Write the overall equation for the oxidation of Fe2+ by acidified H2O2. [1]
- June 2018, Paper 1, Q21(c)(ii) (4 marks): Zinc reacts with acidified Cr2O7 Explain why this happens in terms of electrode potentials and equilibria. Include overall equations for the reactions which occ
- June 2018, Paper 1, Q21(d) (6 marks): This question is about some reactions of d block elements and their ions. Table 21.1 shows standard electrode potentials which will be needed within this questi
- June 2018, Paper 3, Q2(b) (2 marks): Hydrogen peroxide can act as an oxidising agent or as a reducing agent. Some standard electrode potentials are shown below. 2H+(aq) + O2(g) + 2e– E o = +0.68 V
- June 2018, Paper 3, Q3(b)(i) (1 mark): This titration is self-indicating and the student does not need to add an indicator. What colour change would the student observe at the end point? Colour chang
- June 2018, Paper 3, Q3(b)(ii) (6 marks): Calculate the percentage, by mass, of ethanedioic acid in the rhubarb leaves. Give your answer to an appropriate number of significant figures. percentage of et
- June 2017, Paper 1, Q14 (1 mark): A cell is constructed from the two redox systems below. Cu2+(aq) + 2e– E ө = +0.34 V Cu(s) Ag+(aq) + e– E ө = +0.80 V Ag(s) Which statement(s) is/are correct fo
- June 2017, Paper 1, Q22(a)(i) (4 marks): Draw a labelled diagram to show how this cell could be set up to measure its standard cell potential. Include details of the apparatus, solutions and the standa
- June 2017, Paper 1, Q22(a)(ii) (1 mark): Predict the standard cell potential of this cell. standard cell potential = V [1]
- June 2017, Paper 1, Q22(b) (2 marks): In Table 22.1, what is the strongest reducing agent and the strongest oxidising agent? Strongest reducing agent Strongest oxidising agent [2]
- June 2017, Paper 1, Q22(c) (2 marks): Electrode potentials can be used to predict the feasibility of reactions. Construct an overall equation for the predicted reaction between the species in redox
- June 2017, Paper 3, Q3(b) (4 marks): Solutions of hydrogen peroxide decompose slowly into water and oxygen: 2H2O2(aq) 2H2O(l) + O2(g) This reaction is catalysed by manganese dioxide, MnO2(s). Stand
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