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.
Complete the table below as you progress through the experiement.
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.
1, Write half equations for the reduction of:
(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+?