An Investigation of the Various Oxidation States of Vanadium
Introduction
Vanadium has a range of oxidation states from 0 to 5. The following experiments are designed to
identify the colours of vanadium solutions for the various oxidation states, the ease with which the
oxidation states can be formed and the stability of the compound once made. The practical work is
divided into 2 sections.
Table 1
Standard reduction potential, Eo
*
-1.0
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
Zn2+(aq) + 2e == Zn(s)
-0.8
-0.6
-0.4
-0.2
<---------->
Fe 2+(aq) + 2e == Fe(s)
<---------->
V3+(aq) + e == V2+(aq)
<---->
Sn 2+(aq) + 2e ==== Sn
<--------->
0.0
0.2
0.4
0.6
0.8
1.0
SO42-(aq)+4H+(aq)+2e==2H2O(l)+SO2(aq)
<----------->
VO2+ +2H++2e==2H2O+V3+
Cu2+(aq)+2e==Cu(s)
<---->
<---------->
2+
S2O6 +4H +2e==2H2O+2SO2
I2(aq)+2e===2I-(aq)
<---->
<---->
3+
2+
Fe (aq)+e===Fe (aq)
<---->
+
+
2+
VO2 (aq)+2H (aq)+e==VO (aq)+H2O(l)
<---->
Br2(aq)+2e=2Br-(aq)
<---->
1.2
*
1.4 MnO4-(aq)+8H+(aq)+5e===Mn2+(aq)+H2O(l)
*
<----------------------------->
*
1.6
* * * * * * * * * * * +-----+-----+-----+-----+-----+-----+-----+-----+-----+
7
6
5
4
3
2
1
0
-1
-2
Oxidation number
Apparatus: Test tubes and rack, bunsen burner, filter funnel and filter paper.
Chemicals: Solid ammonium vanadate, sodium sulphate(III), Na2SO3, crystals (for preparing SO2(aq),
tin (powder), zinc(powder), copper.
Bench solution: sodium thiosulphate, iron(II) sulphate potassium iodide, potassium bromide, iodine in
potassium iodide solution, sulphuric acid, iron(III) nitrate.
Procedure
Begin the experiment with a solution of ammonium metavanadate; this is best made by dissolving
little of the solid in bench sulphuric acid. Mix solutions of substances as indicated in the following
table and carefully record your observations. In each case add the other solution to the vanadium
solution, or boil the solid with the vanadium solution and centrifuge or filter quickly.
Table 2
Substances to be mixed
1. Ammonium metavanadate + iron(II) sulphate
2. Ammonium metavanadate + potassium iodide
3. Ammonium metavanadate + sulphuric acid + Na2SO3(s)
4. Ammonium metavanadate + tin
5. Ammonium metavanadate + zinc
Observations
Use the table of Eo values given in Table 1, and your observations made and recorded in Table 2, to
deduce the colours of the various oxidation states of vanadium. Record your deductions in Table 3.
(You must be careful to deduce colours bearing in mind the colours of other solutions likely to be
present. For instance, mixing blue and yellow will produce a green solution even if no chemical
reaction occurs.) If iodine is present; the colour of the other products can only be seen if the iodine is
converted to colourless iodide ions or removed in a solution of tetrachloromethane.
Table 3
Oxidation state
V(V)
V(IV)
V(III)
V(II)
Colour in solution
Again using value Eo in Table 1 and your deductions in Table 3, predict the likely experimental result
when substances are mixed as in Table 4. Your predictions should cover the expected oxidation state of
vanadium in the final solution and the likely observations.
Questions for discussion
1. Explain how you derived your prediction in table 4 (question 1,3, 5 only)
2. How did you make your solutions of V(II), V(III) and V(IV) for the experiment in table 4?
3. Comment on the stability of various oxidation state of vanadium.
4. After completing your predictions, test your predictions by carrying out the actual experiments.
After completing your prediction, show them to your teacher. After marking, test your prediction by
carrying out the actual experiments.
Table 4
Substances
to be mixed
1. V(IV) + V(II)
2. V(V) + V(III)
3. V(V) + V(II)
4. V(IV) + Fe(III)
5. V(III) + Fe(III)
6. V(IV) + I(-1)
7. V(V) + Br(-1)
8. V(III) + I(0)
9. V(IV) + I(0)
10. V(V) + Cu(0)
1.
Prediction
Experimental result
Explain the term 'electrochemical force (e.m.f.) of a chemical cell'.
2.
What is the oxidation state of vanadium in ammonium metavanadate?
3.
4.
5.
What is the ion present (in table 2) when vanadium has an oxidation number of 4?
Why is standard electrode important in electrochemistry?
If the standard reduction potential of an electrode A is negative while the standard reduction
potential of an electrode B is positive, what physical significance should be observed? What
experimental data should you search for the confirmation of the physical significance?
If you predict that the position of a redox equilibrium favours the product side but no product is
found, what explanation can you give?
6.
End