Feasibility of redox reactionsEdexcel International A Level Chemistry: Subtopic test

1. 1
   Standard electrode potentials: Cl₂(g)|Cl⁻(aq) +1.36 V; Br₂(aq)|Br⁻(aq) +1.09 V; I₂(aq)|I⁻(aq) +0.54 V.

   (a)

   Which reaction is thermodynamically feasible under standard conditions?

   [1 mark]

   • ABr₂ + 2Cl⁻ → 2Br⁻ + Cl₂
   • BI₂ + 2Br⁻ → 2I⁻ + Br₂
   • CI₂ + 2Cl⁻ → 2I⁻ + Cl₂
   • DBr₂ + 2I⁻ → 2Br⁻ + I₂

   (b)

   A reaction has E°cell = +0.55 V at 298 K. Which statement is correct?

   [1 mark]

   • AThe reaction must occur rapidly
   • BThe total entropy change is positive and K is greater than 1
   • CThe total entropy change is negative
   • DThe equilibrium constant K is less than 1

   (c)

   Chlorine water is added to aqueous potassium bromide. Calculate E°cell for the reaction and state whether it is feasible.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Copper(I) ions in aqueous solution can undergo disproportionation. Standard electrode potentials: Cu²⁺(aq) + e⁻ ⇌ Cu⁺(aq) +0.15 V; Cu⁺(aq) + e⁻ ⇌ Cu(s) +0.52 V.

   (a)

   In the disproportionation 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s), what are the oxidation numbers of copper in the two products?

   [1 mark]

   • A+2 and 0
   • B+2 and +2
   • C0 and 0
   • D+3 and 0

   (b)

   Which statement about aqueous Cu⁺ ions is correct, based on these data?

   [1 mark]

   • ACu⁺ is stable because it has an intermediate oxidation state
   • BCu²⁺ oxidises Cu to form Cu⁺ spontaneously
   • CCu⁺ is unstable and forms Cu²⁺ and Cu
   • DCu⁺ is reduced to Cu²⁺

   (c)

   Use the data to show that copper(I) ions disproportionate in aqueous solution under standard conditions.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   The reaction 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq) is studied at 298 K. Standard electrode potentials: Fe³⁺(aq)|Fe²⁺(aq) +0.77 V; I₂(aq)|I⁻(aq) +0.54 V. Use F = 96 500 C mol⁻¹ and R = 8.31 J K⁻¹ mol⁻¹. For a cell reaction in which n electrons are transferred: ΔS_total = nFE°cell / T and ln K = nFE°cell / (RT).

   (a)

   Calculate E°cell and the total entropy change for this reaction at 298 K.

   [3 marks]

   (b)

   Calculate ln K and K for the reaction at 298 K, and state what the value of K shows about the position of equilibrium.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A teacher points out two cases where predictions from standard electrode potentials seem to disagree with experiment. Standard electrode potentials: MnO₄⁻(aq), H⁺(aq)|Mn²⁺(aq) +1.51 V; O₂(g), H⁺(aq)|H₂O(l) +1.23 V; Cl₂(g)|Cl⁻(aq) +1.36 V; MnO₂(s), H⁺(aq)|Mn²⁺(aq) +1.23 V.

   (a)

   Acidified potassium manganate(VII) solution can be stored for weeks in a stoppered bottle, even though the data predict that it should oxidise water to oxygen. Explain this.

   [6 marks]

   (b)

   Chlorine is made in the laboratory by warming manganese(IV) oxide with concentrated hydrochloric acid, even though the data predict that the reaction is not feasible. Explain why the reaction happens.

   [6 marks]

   Total for question 4: 12 marks

End of questions