Vanadium and chromium redox chemistryEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student adds excess zinc to a yellow solution of acidified vanadate(V) ions, VO₂⁺, and sees the colour change in stages. Standard electrode potentials: VO₂⁺/VO²⁺ +1.00 V; VO²⁺/V³⁺ +0.34 V; V³⁺/V²⁺ −0.26 V; Zn²⁺/Zn −0.76 V.

   (a)

   What is the colour of the V³⁺(aq) ion?

   [1 mark]

   • AYellow
   • BBlue
   • CViolet
   • DGreen

   (b)

   What is the final oxidation number of vanadium when the zinc is in excess?

   [1 mark]

   • A+2
   • B+3
   • C+4
   • D+5

   (c)

   Calculate the standard cell potential for the reduction of V³⁺ to V²⁺ by zinc and state whether the reaction is feasible.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A chemist has a blue solution containing VO²⁺ ions and several possible reagents. Standard electrode potentials: VO₂⁺/VO²⁺ +1.00 V; VO²⁺/V³⁺ +0.34 V; V³⁺/V²⁺ −0.26 V; Cr₂O₇²⁻/Cr³⁺ (acidic) +1.33 V; Fe³⁺/Fe²⁺ +0.77 V; Sn⁴⁺/Sn²⁺ +0.15 V; Zn²⁺/Zn −0.76 V.

   (a)

   Which reagent would reduce VO²⁺ to V³⁺ but not reduce V³⁺ any further?

   [1 mark]

   • AFe²⁺
   • BSn²⁺
   • CZn
   • DCr₂O₇²⁻

   (b)

   Which species could oxidise VO²⁺ to VO₂⁺?

   [1 mark]

   • AFe³⁺
   • BSn⁴⁺
   • CCr₂O₇²⁻ in acidic solution
   • DZn²⁺

   (c)

   Use E° values to explain why Fe²⁺ ions can reduce VO₂⁺ to VO²⁺ but cannot reduce VO²⁺ to V³⁺.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Excess zinc is added to an acidified solution of potassium dichromate(VI), which is orange. Standard electrode potentials: Cr₂O₇²⁻/Cr³⁺ (acidic) +1.33 V; Cr³⁺/Cr²⁺ −0.41 V; Zn²⁺/Zn −0.76 V.

   (a)

   Describe and explain the colour changes seen as the excess zinc reduces the dichromate(VI).

   [3 marks]

   (b)

   Write the half-equation for zinc, and an overall ionic equation for the reduction of Cr₂O₇²⁻ to Cr³⁺ by zinc in acid, and calculate E°cell for this reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Chromium(VI) exists in aqueous solution as orange dichromate(VI) ions, Cr₂O₇²⁻, and yellow chromate(VI) ions, CrO₄²⁻, which are interconverted according to the equilibrium Cr₂O₇²⁻ + H₂O ⇌ 2CrO₄²⁻ + 2H⁺. A technician also wants to prepare orange dichromate(VI) from a green solution of chromium(III) ions.

   (a)

   Describe how the technician can convert the green chromium(III) solution into orange dichromate(VI) ions, giving the reagents, observations and an equation.

   [6 marks]

   (b)

   A sample of orange potassium dichromate(VI) solution is treated first with sodium hydroxide solution and then with excess dilute sulfuric acid. Describe and explain what is seen, and deduce whether either change is a redox reaction.

   [6 marks]

   Total for question 4: 12 marks

End of questions