Half-equations and ionic equationsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student places a piece of zinc in blue copper(II) sulfate solution. After some time the blue colour fades and a brown-red solid forms on the zinc.

   (a)

   Which equation is the half-equation for the oxidation of zinc?

   [1 mark]

   • AZn²⁺ + 2e⁻ → Zn
   • BZn + 2e⁻ → Zn²⁺
   • CZn → Zn²⁺ + 2e⁻
   • DZn → Zn²⁺ + e⁻

   (b)

   Which half-equation represents the formation of the brown-red solid?

   [1 mark]

   • ACu → Cu²⁺ + 2e⁻
   • BCu²⁺ → Cu + 2e⁻
   • CCu²⁺ + e⁻ → Cu
   • DCu²⁺ + 2e⁻ → Cu

   (c)

   Write the ionic equation for the reaction, including state symbols.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Acidified potassium manganate(VII) solution is used to titrate solutions containing iron(II) ions. The purple MnO₄⁻ ion is reduced to the almost colourless Mn²⁺ ion, and Fe²⁺ is oxidised to Fe³⁺.

   (a)

   Which is the correct half-equation for the reduction of MnO₄⁻ in acidic solution?

   [1 mark]

   • AMnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
   • BMnO₄⁻ + 4H⁺ + 5e⁻ → Mn²⁺ + 2H₂O
   • CMnO₄⁻ + 8H⁺ + 3e⁻ → Mn²⁺ + 4H₂O
   • DMnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O + 5e⁻

   (b)

   How many moles of Fe²⁺ ions are oxidised by one mole of MnO₄⁻ ions?

   [1 mark]

   • A1
   • B5
   • C4
   • D8

   (c)

   Write the overall ionic equation for the reaction between acidified MnO₄⁻ ions and Fe²⁺ ions.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student studies two redox reactions of iron ions in aqueous solution. In the first, iron(III) ions oxidise iodide ions. In the second, acidified dichromate(VI) ions oxidise iron(II) ions.

   (a)

   Write the half-equation for the reduction of Fe³⁺, the half-equation for the oxidation of I⁻ to I₂, and the overall ionic equation.

   [3 marks]

   (b)

   Dichromate(VI) ions, Cr₂O₇²⁻, are reduced to Cr³⁺ ions in acid. Construct the half-equation for this change, and combine it with the half-equation Fe²⁺ → Fe³⁺ + e⁻ to give the overall ionic equation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is practising constructing ionic equations for redox reactions in acidic solution by balancing atoms, charge and electrons.

   (a)

   Acidified manganate(VII) ions oxidise hydrogen peroxide to oxygen. The half-equation for manganate(VII) is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. Construct the half-equation for the oxidation of H₂O₂ to O₂, combine the two to give the overall ionic equation, and explain which species is the reducing agent.

   [6 marks]

   (b)

   Zinc in acid reduces VO₂⁺ ions first to VO²⁺ and then to V³⁺. The half-equations are VO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O and VO²⁺ + 2H⁺ + e⁻ → V³⁺ + H₂O. Write the half-equation for zinc, construct the overall ionic equation for each of the two steps, and use oxidation numbers to show that vanadium is reduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions