5.3.1Unit 5. Transition Metals and Organic Nitrogen Chemistry
Vanadium Chemistry — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 6 parts Lesson 21 of 43 Unit quiz
Part 1 of 6Oxidation States and Colours
Vanadium shows four common oxidation states, each with a distinct colour.
+5
- VO₂⁺ is yellow and is the starting point when ammonium vanadate(V) is dissolved in acid.
+4
- VO²⁺ is blue, formed by the first stage of reduction.
+3
- V³⁺ is green in solution as the aqua ion.
+2
- V²⁺ is violet and is a strong reducing agent that is oxidised by air.
Part 2 of 6Electrode Potentials
Electrode Potentials
VO₂⁺/VO²⁺
- VO₂⁺ + 2H⁺ + e⁻ → VO²⁺ + H₂O, E⦵ = +1.00 V.
VO²⁺/V³⁺
- VO²⁺ + 2H⁺ + e⁻ → V³⁺ + H₂O, E⦵ = +0.34 V.
V³⁺/V²⁺
- V³⁺ + e⁻ → V²⁺, E⦵ = -0.26 V.
Comparison
- Zn²⁺/Zn at -0.76 V is lower than all three, so zinc can reduce vanadium all the way to +2.
Part 3 of 6Stepwise Reduction With Zinc
1
Zinc and acid are added to the yellow vanadium(V) solution, and the solution turns green as blue VO²⁺ mixes with yellow VO₂⁺.
2
As reduction continues, the solution becomes blue when all the vanadium is in the +4 state.
3
Further reduction gives the green V³⁺ ion once the VO²⁺ is converted.
4
Finally, the solution turns violet because of V²⁺ forming when the reduction is complete.
5
The reactions can be stopped at an intermediate state by using a weaker reducing agent with a less negative E⦵.
Part 4 of 6Choosing a Reducing Agent
Choosing a Reducing Agent
Fe²⁺
- The Fe³⁺/Fe²⁺ potential of +0.77 V reduces VO₂⁺ (+1.00 V) to VO²⁺ but cannot reduce VO²⁺ (+0.34 V) any further.
Rule
- A species reduces another only if its own E⦵ is lower than that of the half-cell being reduced, giving a positive E⦵cell.
Tin(II)
- Sn⁴⁺/Sn²⁺ has E⦵ = +0.15 V, which is lower than +0.34 V, so Sn²⁺ can reduce VO²⁺ to V³⁺ but not further.
Part 5 of 6Worked Example
Question Calculate E⦵cell for zinc reducing V³⁺ to V²⁺ and say whether the reaction is feasible.
- Zinc is the reducing agentZn → Zn²⁺ + 2e⁻, with E⦵ = -0.76 V for Zn²⁺/Zn.
- E⦵cell = E⦵(V³⁺/V²⁺) - E⦵(Zn²⁺/Zn) = -0.26 - (-0.76) = +0.50 V.
- The value is positive.
Answer E⦵cell = +0.50 V, so the reaction is feasible
Part 6 of 6Exam Checklist & Tips
5.3.1 – Key Points for the Exam
- Oxidation state The charge on vanadium in a given species, from +5 to +2.
- Stepwise reduction Successive reduction from one oxidation state to the next.
- VO₂⁺ The yellow vanadium(V) ion.
- Reducing agent A species that gives electrons to another.
Common Mistakes
- Writing V²⁺ as green instead of violet.
- Forgetting that VO₂⁺ and VO²⁺ contain oxygen.
- Assuming Fe²⁺ can reduce vanadium(IV) to vanadium(III).
- Mixing up the order of colours down the reduction sequence.
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