Variable oxidation states and redox titrationsAQA A-Level Chemistry: Flashcards

Exam questions on Variable oxidation states and redox titrations

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.
   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
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.
   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
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.
   State what is observed with each liquid and identify the organic product formed from propanal.3 marks