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.
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.
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.
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
Explain how you derived your prediction in table 4 (question 1,3, 5 only)
How did you make your solutions of V(II), V(III) and V(IV) for the experiment in table 4?
Comment on the stability of various oxidation state of vanadium.
Explain the term 'electrochemical force (e.m.f.) of a chemical cell'.
What is the oxidation state of vanadium in ammonium metavanadate?
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?