Redox chemistry of vanadium and chromiumEdexcel A-Level Chemistry: Flashcards
What these 13 flashcards ask
- Give the colours of vanadium in the +5, +4, +3 and +2 states.
- What does excess zinc in acid do to VO₂⁺(aq)?
- How do you decide if a reducing agent can reduce a species using E° values?
- Why can Fe²⁺ reduce VO₂⁺ but not VO²⁺?
- What is the colour of Cr₂O₇²⁻ and what is chromium's oxidation number?
- What colours form when dichromate(VI) is reduced by zinc in acid?
- Write the equation for Cr₂O₇²⁻ reduced to Cr³⁺ by zinc.
- Why can zinc reduce Cr³⁺ to Cr²⁺?
- How is Cr³⁺ oxidised to chromate(VI)?
- Write the equation for oxidation of Cr³⁺ by H₂O₂ in alkali.
- Write the chromate(VI)/dichromate(VI) equilibrium.
- Is chromate(VI) to dichromate(VI) a redox reaction?
- What happens to the colour on adding alkali to orange dichromate(VI)?
Exam questions on Redox chemistry of vanadium and chromium
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).