Variable oxidation states and redox titrationsAQA A-Level Chemistry: Subtopic test

1. 1
   Ammonium vanadate(V) dissolves in dilute sulfuric acid to give a yellow solution containing VO₂⁺ ions. When zinc is added, the solution changes colour from yellow to blue, then green, then violet.

   (a)

   What is the oxidation state of vanadium in the blue species VO²⁺?

   [1 mark]

   • A+4
   • B+2
   • C+3
   • D+5

   (b)

   Which ion is responsible for the final violet colour?

   [1 mark]

   • AVO₂⁺
   • BVO²⁺
   • CV³⁺
   • DV²⁺

   (c)

   The standard electrode potentials are E° (Zn²⁺/Zn) = −0.76 V, E° (V³⁺/V²⁺) = −0.26 V and E° (V²⁺/V) = −1.18 V. Explain why zinc reduces V³⁺ to V²⁺ but does not reduce V²⁺ to vanadium metal.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   The electrode potential of a transition metal ion changing from a higher to a lower oxidation state depends on its environment. For Co³⁺ + e⁻ → Co²⁺ the standard electrode potential is +1.82 V when the ions are hexaaqua complexes and +0.11 V when they are hexaammine complexes. The half-equation for the manganate(VII) ion in acid is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.

   (a)

   Which statement about the cobalt(II) complexes is correct?

   [1 mark]

   • A[Co(H₂O)₆]²⁺ is oxidised more readily than [Co(NH₃)₆]²⁺
   • B[Co(NH₃)₆]²⁺ is oxidised more readily than [Co(H₂O)₆]²⁺
   • CBoth complexes are oxidised equally readily
   • DAmmonia ligands make cobalt(III) less stable than water ligands do

   (b)

   How does the electrode potential for MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O change when the pH of the solution is increased?

   [1 mark]

   • AIt becomes more positive
   • BIt stays the same
   • CIt becomes less positive
   • DIt changes sign immediately

   (c)

   Air oxidises [Co(NH₃)₆]²⁺ in ammonia solution but does not oxidise [Co(H₂O)₆]²⁺. Use the data, and take the electrode potential for the reduction of oxygen under these conditions to be +0.40 V, to explain why.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A teacher has two unlabelled liquids, propanal and propanone. She prepares Tollens' reagent, which contains the complex ion [Ag(NH₃)₂]⁺, and warms a sample of each liquid with it in a clean test tube.

   (a)

   State what is observed with each liquid and identify the organic product formed from propanal.

   [3 marks]

   (b)

   Write half-equations for the reduction of [Ag(NH₃)₂]⁺ and the oxidation of propanal, and combine them to give the overall equation.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Potassium manganate(VII), KMnO₄, is used in redox titrations in acidic solution, where the manganate(VII) ion is reduced to Mn²⁺. A student uses it to analyse an iron supplement tablet and then a solution of sodium ethanedioate.

   (a)

   A 1.50 g iron supplement tablet is dissolved in dilute sulfuric acid and the solution made up to 250 cm³. A 25.0 cm³ portion requires 19.0 cm³ of 0.0100 mol dm⁻³ KMnO₄ for complete reaction. Calculate the percentage by mass of iron in the tablet, assuming all the iron is Fe²⁺. (Ar of Fe = 55.8)

   [6 marks]

   (b)

   A 25.0 cm³ sample of sodium ethanedioate solution is acidified, warmed to about 60 °C and titrated with 0.0200 mol dm⁻³ KMnO₄, requiring 18.50 cm³. Calculate the concentration of the ethanedioate solution and state how the end-point is recognised and why the mixture is warmed.

   [6 marks]

   Total for question 4: 12 marks

End of questions