Redox chemistry of vanadium and chromiumEdexcel A-Level Chemistry: Mind map

Exam questions on Redox chemistry of vanadium and chromium

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.
   Explain, using E° values, why an excess of zinc in acid reduces VO₂⁺ all the way to V²⁺.2 marks
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.
   Explain, using E° values, why iron(II) ions cannot reduce VO²⁺ to V³⁺.2 marks
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.
   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