Illustrating the oxidation states of vanadium

ILLUSTRATING THE OXIDATION STATES OF VANADIUM
The purpose of this experiment is to illustrate the presence of several different oxidation states for vanadium, and to show how it is possible to change form one oxidation state to another.
Introduction
Starting with a solution containing vanadium (V) in acid conditions. The solution is ammonium metavanadate NH₄ VO₃,
The VO₃^- ion in the ammonium salt is converted to VO₂⁺ by acid:

VO₃^-(aq) + 2H⁺ (aq) <=====> VO₂⁺ (aq) + H₂O(l)

You then use powdered zinc as a reducing agent. Colour changes indicate the formation of other oxidation states.
Procedure

Complete the table below as you progress through the experiement.

Ion	VO2^+	VO^2+	V^3+	V^2+
Colour	yellow	blue	green	violet
Oxidation state
Name

1. Pour carefully about 25 - 40 cm³ of the supplied ammonium vanadate in acid solution (0.1M ammonium meta vanadate) (TOXIC) into a conical flask.

2. To the conical flask add 1 - 2 g (one spatula measure) of zinc dust, a little at a time. Swirl the flask at intervals and record any observed colour changes in a copy of the Results Table.
Sometimes, the solution turns green before it turns blue. Can you explain why?
3. When the solution has become violet, filter about 2 cm³ into a test-tube. This is vanadium (II) solution.
4. To this tube of Vanadium (II) add, a little at a time, acidified 0.01M potassium manganate (VII) solution, gently shaking gently after each addition, until no further change is observed.

Note down the relevant colours and observations.

Questions:
1, Write half equations for the reduction of:

• 
  VO₂⁺  VO²⁺
  
• 
  VO²⁺  V³⁺
  
• 
  V³⁺  V²⁺
  

(Hint: the solution is acidified)
2, What species is getting oxidized? Can you write a half equation for this reaction?
3, Can you write complete equations for all three reactions? (You will need to combine each reduction half equation with the oxidation half equation)
4, Write a half equation for the reduction of MnO₄^-
5, Can you deduce the half equation for the relevant vanadium ion with regards to Q4?
6, Can you combine the half equations in Q4 and Q5 to come up with an overall equation for the reaction?
7, What can you deduce about the relative oxidizing ability of MnO₄^- compared to V²⁺?