Vanadium and chromium redox chemistryEdexcel International A Level Chemistry: Mind map

Exam questions on Vanadium and chromium redox chemistry

1. 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
2. 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
3. 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