Redox processes: electron transfer and changes in oxidation number (oxidation state): past paper questions

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• Question 10May/June 2026, paper 11Multiple choice1 mark

  Which statement about the dichromate ion, ${\text{Cr}}_2{\text{O}}_7^{2-}$, is correct?

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• Question 9Oct/Nov 2025, paper 11Multiple choice1 markHas a diagram

  Sulfite ions, ${\text{SO}}_3^{2-}$, react separately with zinc and with manganese dioxide. $a$, $b$, $c$, $d$, $w$, $x$, $y$ and $z$ are all whole numbers.

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• Question 11Oct/Nov 2024, paper 11Multiple choice1 markHas a diagram

  In alkaline solution, ${\text{MnO}}_4^{-}$ ions oxidise ${\text{SO}}_3^{2-}$ ions to ${\text{SO}}_4^{2-}$ ions. The ${\text{MnO}}_4^{-}$ ions are reduced to ${\text{MnO}}_2$. What is the ratio of the two ions in the balanced chemical equation for this reaction?

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• Question 11Oct/Nov 2023, paper 11Multiple choice1 mark

  $\mathrm{H}\mathrm{O}\mathrm{C}\mathrm{l}(\mathrm{a}\mathrm{q})$ is the molecule that kills bacteria when chlorine is added to water. The following reaction produces this molecule.

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• Question 11May/June 2026, paper 11Multiple choice1 markHas a diagram

  The equation for the reaction between nitric acid and arsenous acid, ${\text{H}}_3{\text{AsO}}_3$, is shown. $$x{\text{HNO}}_3+y{\text{H}}_3{\text{AsO}}_3\to x\text{NO}+y{\text{H}}_3{\text{AsO}}_4+z{\text{H}}_2\text{O}$$ What are the numerical values of $x$ and $y$?

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• Question 10Oct/Nov 2025, paper 11Multiple choice1 mark

  Which equation shows hydrogen acting as an oxidising agent?

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• Question 11Oct/Nov 2024, paper 12Multiple choice1 mark

  Equations for some reactions of hydrogen peroxide are given.

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• Question 3Oct/Nov 2023, paper 12Multiple choice1 mark

  Zinc reacts with concentrated nitric acid giving three products only: zinc nitrate, an oxide of nitrogen and water. $3.0\text{ moles}$ of zinc react with $8.0\text{ moles}$ of nitric acid. Zinc nitrate contains ${\text{Zn}}^{2+}$ ions.

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• Question 11May/June 2026, paper 12Multiple choice1 mark

  ${\text{TiO}}_2$ is obtained from iron(II) titanate, ${\text{FeTiO}}_3$. What is the change, if any, in the oxidation number of titanium?

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• Question 14Oct/Nov 2025, paper 11Multiple choice1 mark

  Photochromic glass, used for sunglasses, darkens when exposed to bright light and becomes more transparent again when the light is less bright. The darkness of the glass is due to the presence of silver atoms. The following reactions are involved.

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• Question 12Oct/Nov 2024, paper 12Multiple choice1 markHas a diagram

  The equation for the reaction of aqueous thiosulfate ions, ${\mathrm{S}}_2{\mathrm{O}}_3^{2-}$, and aqueous dioxo-vanadium ions, ${\mathrm{V}\mathrm{O}}_2^{+}$, is shown.

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• Question 12Oct/Nov 2023, paper 12Multiple choice1 mark

  Chlorine reacts with sodium bromide. $$\frac{1}{2}{\text{Cl}}_2+\text{NaBr}\to \text{NaCl}+\frac{1}{2}{\text{Br}}_2$$ Which words correctly describe this reaction?

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• Question 12May/June 2026, paper 12Multiple choice1 mark

  Copper(I) iodide is produced by the reaction shown. $$2{\text{CuSO}}_4\text{(aq)}+4\text{KI(aq)}\to 2\text{CuI(s)}+{\text{I}}_2\text{(aq)}+2{\text{K}}_2{\text{SO}}_4\text{(aq)}$$ Which statement about this reaction is correct?

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• Question 22Oct/Nov 2025, paper 11Multiple choice1 mark

  What happens when iodine solution is added to a solution of sodium bromide?

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• Question 23Oct/Nov 2024, paper 12Multiple choice1 mark

  The name 'chlorate' is used for an anion consisting of chlorine and oxygen only.

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• Question 13Oct/Nov 2023, paper 12Multiple choice1 mark

  The equation for the reaction between aqueous copper ions and aqueous iodide ions is as follows. $$2{\text{Cu}}^{2+}(\text{aq})+4{\text{I}}^{-}(\text{aq})\to 2\text{CuI}(\text{s})+{\text{I}}_2(\text{aq})$$ What is the change in oxidation state of copper?

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• Question 23May/June 2026, paper 12Multiple choice1 mark

  Which is the strongest oxidising agent?

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• Question 3Oct/Nov 2025, paper 12Multiple choice1 mark

  The reaction of hydrogen sulfide with sulfur dioxide gives sulfur as one of the products. The two relevant redox equations are shown.

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• Question 25Oct/Nov 2023, paper 12Multiple choice1 mark

  When concentrated sulfuric acid is added to solid sodium chloride, $\text{HCl}$ is formed but not ${\text{Cl}}_2$. When concentrated sulfuric acid is added to solid sodium iodide, ${\text{I}}_2$ is formed. Which statement explains these observations?

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• Question 34May/June 2026, paper 12Multiple choice1 mark

  Two reactions are described. 1 Ethanoic acid reacts with methanol. 2 Ethanoic acid reacts with sodium carbonate.

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• Question 10Oct/Nov 2025, paper 12Multiple choice1 mark

  One commercially available 'heat pad' contains iron, activated carbon and water. The 'heat pad' is activated by air. This causes the pad to get hotter.

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• Question 11May/June 2024, paper 11Multiple choice1 mark

  Sodium chromate(VI), ${\text{Na}}_2{\text{CrO}}_4$, is manufactured by heating chromite, ${\text{FeCr}}_2{\text{O}}_4$, with sodium carbonate in an oxidising atmosphere. Chromite contains ${\text{Cr}}_2{\text{O}}_4^{2-}$ ions.

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• Question 10May/June 2026, paper 13Multiple choice1 mark

  Aqueous sodium chlorate(I) is added to aqueous iron(II) sulfate and a reaction takes place. Aqueous sodium chlorate(I) behaves as an oxidising agent in this reaction. Which statement about the reaction is correct?

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• Question 11Oct/Nov 2025, paper 12Multiple choice1 mark

  In which substance is the average oxidation number of sulfur the highest?

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• Question 12May/June 2024, paper 11Multiple choice1 mark

  Oxygen can be prepared by the reaction of potassium manganate(VII), ${\text{KMnO}}_4$, hydrogen peroxide, ${\text{H}}_2{\text{O}}_2$, and sulfuric acid, ${\text{H}}_2{\text{SO}}_4$. Each ${\text{H}}_2{\text{O}}_2$ molecule loses two electrons in this reaction.

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• Question 19Oct/Nov 2023, paper 13Multiple choice1 markHas a diagram

  Which row correctly describes the separate reactions of calcium and strontium with water?

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• Question 11May/June 2026, paper 13Multiple choice1 mark

  Which conversion involves the oxidation of an element in the reactant shown?

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• Question 11May/June 2024, paper 12Multiple choice1 mark

  Nitrogen dioxide reacts with water. $$2{\text{NO}}_2+{\text{H}}_2\text{O}\to {\text{HNO}}_2+{\text{HNO}}_3$$ Which statement about this reaction is correct?

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• Question 3Oct/Nov 2023, paper 21Structured12 marksHas a diagram

  Potassium chlorate, $\mathrm{K}\mathrm{C}\mathrm{l}{\mathrm{O}}_3$, is widely used as an oxidising agent and to make ${\mathrm{O}}_2(\mathrm{g})$. Define oxidising agent.

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• Question 21May/June 2026, paper 13Multiple choice1 mark

  Chlorine gas reacts with hot dilute aqueous sodium hydroxide as shown. $$u{\text{Cl}}_2+v{\text{OH}}^{-}\to w{\text{Cl}}^{-}+y{\text{ClO}}_3^{-}+z{\text{H}}_2\text{O}$$ What is the simplest ratio of $w:y$ in the balanced equation?

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