Half-equations and balancing redox equationsAQA A-Level Chemistry: Subtopic test

1. 1
   Bromine is extracted from seawater by bubbling chlorine gas through it. The overall reaction is Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq).

   (a)

   Which is the half-equation for the oxidation process?

   [1 mark]

   • ACl₂ + 2e⁻ → 2Cl⁻
   • B2Br⁻ → Br₂ + 2e⁻
   • CBr₂ + 2e⁻ → 2Br⁻
   • D2Cl⁻ → Cl₂ + 2e⁻

   (b)

   Which row correctly describes the chlorine in this reaction?

   [1 mark]

   • AIt is oxidised and is a reducing agent
   • BIt is reduced and is a reducing agent
   • CIt is oxidised and is an oxidising agent
   • DIt is reduced and is an oxidising agent

   (c)

   Write the half-equation for the reduction of chlorine and explain why electrons do not appear in the overall equation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In acidic solution, orange dichromate(VI) ions, Cr₂O₇²⁻, are reduced to green chromium(III) ions, Cr³⁺, and water is formed. A student reacts acidified dichromate(VI) ions with a solution of iron(II) ions, which are oxidised to iron(III) ions, Fe³⁺.

   (a)

   The half-equation for the reduction is Cr₂O₇²⁻ + 14H⁺ + ?e⁻ → 2Cr³⁺ + 7H₂O. How many electrons are needed to balance it?

   [1 mark]

   • A6
   • B3
   • C7
   • D12

   (b)

   What is the total charge on the left-hand side of the fully balanced half-equation Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O?

   [1 mark]

   • A0
   • B+2
   • C+6
   • D+12

   (c)

   The iron(II) half-equation is Fe²⁺ → Fe³⁺ + e⁻. Combine it with the dichromate(VI) half-equation to give the overall ionic equation.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Iron(II) ions in a solution are titrated against 0.0200 mol dm⁻³ acidified potassium manganate(VII). In acidic solution the MnO₄⁻ ion is reduced to Mn²⁺ and the Fe²⁺ ion is oxidised to Fe³⁺. A 25.0 cm³ sample of the iron(II) solution required 22.40 cm³ of the manganate(VII) solution for complete reaction.

   (a)

   Write the half-equation for the reduction of the MnO₄⁻ ion in acid and the half-equation for the oxidation of Fe²⁺, and combine them to give the overall ionic equation.

   [3 marks]

   (b)

   Calculate the concentration, in mol dm⁻³, of the iron(II) ions in the 25.0 cm³ sample.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The concentration of hydrogen peroxide in a bleaching solution is found by titration with acidified potassium manganate(VII). In acidic solution the MnO₄⁻ ion is reduced to Mn²⁺ and hydrogen peroxide, H₂O₂, is oxidised to oxygen. A 10.0 cm³ sample of the bleaching solution was diluted to 250 cm³ in a volumetric flask. Portions of 25.0 cm³ of the diluted solution required 18.40 cm³ of 0.0200 mol dm⁻³ manganate(VII) solution for complete reaction.

   (a)

   Construct the half-equation for each of the reduction of MnO₄⁻ and the oxidation of H₂O₂ in acid, and combine them to give the overall ionic equation. Explain why the half-equations must be multiplied by different numbers.

   [6 marks]

   (b)

   Calculate the concentration, in g dm⁻³, of hydrogen peroxide in the original bleaching solution. (Mr of H₂O₂ = 34.0)

   [6 marks]

   Total for question 4: 12 marks

End of questions