Oxidation, reduction and redox equationsAQA A-Level Chemistry: Topic test

1. 1
   Magnesium ribbon continues to burn when it is lowered into a gas jar of carbon dioxide, forming white magnesium oxide and black specks of carbon: 2Mg(s) + CO₂(g) → 2MgO(s) + C(s).

   (a)

   Which species is the reducing agent in this reaction?

   [1 mark]

   • AMg
   • BCO₂
   • CC
   • DMgO

   (b)

   What is the change in the oxidation state of carbon?

   [1 mark]

   • A−4 to 0
   • B+2 to 0
   • C+4 to 0
   • D+4 to −4

   (c)

   Write half-equations for the oxidation process and for the reduction process in this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chlorine gas is bubbled into cold, dilute aqueous sodium hydroxide to make household bleach: Cl₂(g) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l).

   (a)

   What is the oxidation state of chlorine in sodium chlorate(I), NaClO?

   [1 mark]

   • A−1
   • B+1
   • C+3
   • D+5

   (b)

   Which statement about the chlorine in this reaction is correct?

   [1 mark]

   • AIt is oxidised only
   • BIt is reduced only
   • CIts oxidation state does not change
   • DIt is both oxidised and reduced

   (c)

   Write the half-equation for the reduction of chlorine in this reaction, and state the change in oxidation state of the chlorine that is oxidised.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Copper reacts with dilute nitric acid to form blue copper(II) nitrate solution and nitrogen monoxide gas, which turns brown in air. The overall equation is 3Cu(s) + 8H⁺(aq) + 2NO₃⁻(aq) → 3Cu²⁺(aq) + 2NO(g) + 4H₂O(l).

   (a)

   Write half-equations for the oxidation of copper and for the reduction of nitrate ions to nitrogen monoxide in acidic solution, and name the oxidising agent.

   [3 marks]

   (b)

   Use oxidation states to show that the overall equation is balanced for electron transfer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Sulfur dioxide takes part in two redox reactions. In the first, it decolourises acidified potassium manganate(VII) solution, forming sulfate ions, SO₄²⁻, and manganese(II) ions, Mn²⁺. In the second, it reacts with hydrogen sulfide gas to form solid sulfur and water: SO₂(g) + 2H₂S(g) → 3S(s) + 2H₂O(l).

   (a)

   For the reaction with acidified manganate(VII) ions, give the oxidation states that change, write both half-equations, and combine them to give the overall ionic equation.

   [6 marks]

   (b)

   In the second reaction, sulfur dioxide acts as an oxidising agent, whereas in the first reaction it acts as a reducing agent. Use oxidation states and half-equations to explain this, and show how the half-equations combine to give the equation for the second reaction.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Acidified potassium manganate(VII) oxidises ethanedioate ions, C₂O₄²⁻, to carbon dioxide when the mixture is warmed. The manganate(VII) ions are reduced to manganese(II) ions.

   (a)

   What is the oxidation state of carbon in the ethanedioate ion, C₂O₄²⁻?

   [1 mark]

   • A+2
   • B+4
   • C+3
   • D−3

   (b)

   Which is the correct half-equation for the oxidation of ethanedioate ions to carbon dioxide?

   [1 mark]

   • AC₂O₄²⁻ → 2CO₂ + 2e⁻
   • BC₂O₄²⁻ + 2e⁻ → 2CO₂
   • CC₂O₄²⁻ → 2CO₂ + e⁻
   • DC₂O₄²⁻ → CO₂ + 2e⁻

   (c)

   The half-equation for the reduction is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Combine this with the oxidation half-equation to give the overall ionic equation.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   In the thermite reaction, powdered aluminium reacts with iron(III) oxide at a very high temperature, producing molten iron: Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(l). The reaction is used to weld railway lines.

   (a)

   Which statement identifies the reducing agent and gives the reason?

   [1 mark]

   • AFe₂O₃, because the oxidation state of iron falls
   • BAl₂O₃, because it is the oxide that is formed
   • CFe, because it is the product of the reaction
   • DAl, because its oxidation state rises

   (b)

   How many moles of electrons are transferred when one mole of Fe₂O₃ reacts?

   [1 mark]

   • A3
   • B6
   • C2
   • D12

   (c)

   Write the half-equation for the oxidation of aluminium and the half-equation for the reduction of iron(III) ions to iron in this reaction.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Iodate(V) ions oxidise iodide ions in acidic solution, forming iodine: IO₃⁻(aq) + 5I⁻(aq) + 6H⁺(aq) → 3I₂(aq) + 3H₂O(l).

   (a)

   State the oxidation state of iodine in IO₃⁻, in I⁻ and in I₂, and identify the oxidising agent and the reducing agent.

   [3 marks]

   (b)

   Write half-equations for the formation of iodine from iodate(V) ions and from iodide ions, and show how they combine to give the equation in the question.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Two redox reactions are studied. In the first, copper(I) oxide dissolves in dilute sulfuric acid, giving copper metal and a blue solution of copper(II) ions: Cu₂O(s) + 2H⁺(aq) → Cu(s) + Cu²⁺(aq) + H₂O(l). In the second, ammonia is burned in oxygen over a platinum catalyst in the manufacture of nitric acid: 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g).

   (a)

   Use oxidation states to explain why the first reaction is a redox reaction of a particular type, and say what happens to the oxidation states of oxygen and hydrogen. Show how the copper half-equations combine.

   [6 marks]

   (b)

   For the second reaction, use oxidation states to identify the oxidising agent and reducing agent, and use half-equations in acidic form to show how the overall equation arises.

   [6 marks]

   Total for question 8: 12 marks

End of questions