Redox chemistry of vanadium and chromiumEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student reduces yellow acidified VO₂⁺(aq) with different reducing agents. Standard electrode potentials: VO₂⁺ + 2H⁺ + e⁻ ⇌ VO²⁺ + H₂O, E° = +1.00 V; VO²⁺ + 2H⁺ + e⁻ ⇌ V³⁺ + H₂O, E° = +0.34 V; V³⁺ + e⁻ ⇌ V²⁺, E° = −0.26 V; Zn²⁺ + 2e⁻ ⇌ Zn, E° = −0.76 V; Sn⁴⁺ + 2e⁻ ⇌ Sn²⁺, E° = +0.15 V.

   (a)

   Which row gives the colours of vanadium in the +5, +4, +3 and +2 oxidation states, in that order?

   [1 mark]

   • Ayellow, blue, green, violet
   • Bblue, yellow, green, violet
   • Cyellow, green, blue, violet
   • Dviolet, green, blue, yellow

   (b)

   Which reducing agent can reduce VO²⁺ to V³⁺ but cannot reduce V³⁺ to V²⁺?

   [1 mark]

   • AZn(s)
   • BFe²⁺(aq)
   • CSn²⁺(aq)
   • DSn⁴⁺(aq)

   (c)

   Explain, using E° values, why an excess of zinc in acid reduces VO₂⁺ all the way to V²⁺.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student adds an excess of acidified iron(II) sulfate solution to a solution containing VO₂⁺(aq). Standard electrode potentials: VO₂⁺ + 2H⁺ + e⁻ ⇌ VO²⁺ + H₂O, E° = +1.00 V; VO²⁺ + 2H⁺ + e⁻ ⇌ V³⁺ + H₂O, E° = +0.34 V; Fe³⁺ + e⁻ ⇌ Fe²⁺, E° = +0.77 V.

   (a)

   What is E°cell for the reaction between VO₂⁺ and Fe²⁺ to form VO²⁺ and Fe³⁺ under standard conditions?

   [1 mark]

   • A+1.77 V
   • B+0.23 V
   • C−0.23 V
   • D−0.43 V

   (b)

   Which is the main vanadium species present once the reaction with excess Fe²⁺ has finished?

   [1 mark]

   • AVO₂⁺
   • BV³⁺
   • CV²⁺
   • DVO²⁺

   (c)

   Explain, using E° values, why iron(II) ions cannot reduce VO²⁺ to V³⁺.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Orange acidified potassium dichromate(VI) solution is warmed with an excess of zinc granules. The mixture turns green and then blue. Standard electrode potentials: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ ⇌ 2Cr³⁺ + 7H₂O, E° = +1.33 V; Cr³⁺ + e⁻ ⇌ Cr²⁺, E° = −0.41 V; Zn²⁺ + 2e⁻ ⇌ Zn, E° = −0.76 V.

   (a)

   Describe the colour changes seen, and state the oxidation number of chromium in each coloured species, as the dichromate(VI) is reduced by zinc.

   [3 marks]

   (b)

   (i) Write the ionic equation for the reduction of Cr₂O₇²⁻ to Cr³⁺ by zinc in acid. (ii) Calculate E°cell for the reduction of Cr³⁺ to Cr²⁺ by zinc and state what the value shows.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist working on a chrome-plating process prepares chromium compounds in different oxidation states. Chromium(III) hydroxide is oxidised in alkaline solution by hydrogen peroxide, and the product is later acidified. Standard electrode potentials in alkaline solution: HO₂⁻ + H₂O + 2e⁻ ⇌ 3OH⁻, E° = +0.87 V; CrO₄²⁻ + 4H₂O + 3e⁻ ⇌ Cr(OH)₃ + 5OH⁻, E° = −0.13 V.

   (a)

   Describe how a solution of chromium(III) ions can be converted into a solution of dichromate(VI) ions. Include reagents, conditions, colour changes and oxidation numbers, a balanced equation for the oxidation of Cr³⁺, and a calculation showing that the oxidation is feasible.

   [6 marks]

   (b)

   Yellow chromate(VI) ions and orange dichromate(VI) ions are in equilibrium. Write the equation for the equilibrium and explain what is seen when acid is added to a yellow solution and then alkali is added. State whether the interconversion is a redox reaction.

   [6 marks]

   Total for question 4: 12 marks

End of questions