Predicting feasibility and limitationsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Acidified potassium manganate(VII) solution is added to a solution of iron(II) sulfate at 298 K. Standard electrode potentials: MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l), E° = +1.51 V; Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V.

   (a)

   What is E°cell for the reaction MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O?

   [1 mark]

   • A+2.28 V
   • B−0.74 V
   • C+3.70 V
   • D+0.74 V

   (b)

   Which statement about this reaction is correct?

   [1 mark]

   • AE°cell is positive, so ΔS(total) is positive and the reaction is thermodynamically feasible
   • BE°cell is positive, so the reaction must be fast
   • CE°cell is positive, so ΔS(total) is negative
   • DE°cell is positive, so K is less than 1

   (c)

   State the relationship between E°cell and ΔS(total), and explain why a positive E°cell shows that a reaction is feasible.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Zinc powder is added to aqueous copper(II) sulfate at 298 K. Standard electrode potentials: Zn²⁺(aq) + 2e⁻ ⇌ Zn(s), E° = −0.76 V; Cu²⁺(aq) + 2e⁻ ⇌ Cu(s), E° = +0.34 V. Use ΔS(total) = nFE°cell / T and ln K = nFE°cell / RT, with F = 96 500 C mol⁻¹ and R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   What is the value of ΔS(total) for the reaction Zn + Cu²⁺ → Zn²⁺ + Cu?

   [1 mark]

   • A+356 J K⁻¹ mol⁻¹
   • B+712 J K⁻¹ mol⁻¹
   • C+212 300 J K⁻¹ mol⁻¹
   • D−712 J K⁻¹ mol⁻¹

   (b)

   What is the value of the equilibrium constant, K, for this reaction at 298 K?

   [1 mark]

   • A85.7
   • B4.1 × 10¹⁸
   • C1.7 × 10³⁷
   • D5.9 × 10⁻³⁸

   (c)

   Explain what the value of K shows about the extent of this reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician stores acidified potassium manganate(VII) solution for use as a titrant, and finds that it stays purple for weeks. Standard electrode potentials: MnO₄⁻(aq) + 8H⁺(aq) + 5e⁻ ⇌ Mn²⁺(aq) + 4H₂O(l), E° = +1.51 V; O₂(g) + 4H⁺(aq) + 4e⁻ ⇌ 2H₂O(l), E° = +1.23 V.

   (a)

   Use the data to predict whether acidified manganate(VII) ions should oxidise water to oxygen.

   [3 marks]

   (b)

   The solution is in fact stable for weeks. Explain why the prediction in (a) does not match what is observed, and give one further reason why a prediction from E° values may be wrong.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist studies the stability of ions of copper and iron in aqueous solution at 298 K. Standard electrode potentials: Cu⁺(aq) + e⁻ ⇌ Cu(s), E° = +0.52 V; Cu²⁺(aq) + e⁻ ⇌ Cu⁺(aq), E° = +0.15 V; Fe²⁺(aq) + 2e⁻ ⇌ Fe(s), E° = −0.44 V; Fe³⁺(aq) + e⁻ ⇌ Fe²⁺(aq), E° = +0.77 V.

   (a)

   Explain, using the data, why copper(I) ions disproportionate in aqueous solution.

   [6 marks]

   (b)

   Predict whether iron(II) ions disproportionate under standard conditions, and explain how the data for iron differ from those for copper(I).

   [6 marks]

   Total for question 4: 12 marks

End of questions