Vanadium and chromium redox chemistryEdexcel International A Level Chemistry: Flashcards
What these 12 flashcards ask
- Colour of VO₂⁺ and its oxidation number?
- Colour of VO²⁺ and its oxidation number?
- Colour of V³⁺ and V²⁺?
- What reduces vanadium from +5 to +2?
- Rule for a reducing agent using E° values?
- How is E°cell calculated?
- Colour change on reducing dichromate(VI) with zinc?
- Overall equation for zinc reducing dichromate(VI) to Cr³⁺?
- How can Cr³⁺ be oxidised to chromate(VI)?
- Equation for oxidation of Cr³⁺ by alkaline hydrogen peroxide?
- Equilibrium between dichromate(VI) and chromate(VI)?
- What is seen on adding alkali to dichromate(VI)?
Exam questions on Vanadium and chromium redox chemistry
- 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.Calculate the standard cell potential for the reduction of V³⁺ to V²⁺ by zinc and state whether the reaction is feasible.2 marks
- 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.Use E° values to explain why Fe²⁺ ions can reduce VO₂⁺ to VO²⁺ but cannot reduce VO²⁺ to V³⁺.2 marks
- 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.Describe and explain the colour changes seen as the excess zinc reduces the dichromate(VI).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).