Question

Recall the changing oxidation states of vanadium during a chemical reaction (You can use Internet for finding a good demonstration). Using that, answer the following questions:

1. Describe your observations.

2. What evidence did you observe to suggest chemical reactions were occurring?

3. How many oxidation states of vanadium were formed?

4. List the oxidation states and their colours.

Accepted Answer

Vanadium has oxidation states in its compounds of $+ 5, + 4, + 3$ and $+ 2$ . Here, we can use the reaction involving the reduction of vanadium $(V)$ in stages to vanadium $(II)$ .

The usual source of vanadium in the $+ 5$ oxidation state is ammonium metavanadate, ${NH}_{4} {VO}_{3}$ . This is not very soluble in water and is usually first dissolved in sodium hydroxide solution. The solution can be reduced using zinc and an acid (either hydrochloric acid or sulphuric acid), usually using moderately concentrated acid.

[The reaction is done in a small flask, it is normally stoppered with some cotton wool. This allows hydrogen (produced from a side reaction between the zinc and acid) to escape. At the same time it stops much air from entering. This prevents re-oxidation of the lower oxidation states of vanadium (particularly the $+ 2$ state) by oxygen in the air].

1. The observations can be given as,

Continuous colour changes are observed at different oxidation states.

2. We know that, colour change is one of the characteristics of a chemical reaction. Here, colour changes are observed. This gives us evidence that, chemical reactions were occurring.

3. Three oxidation states of vanadium were formed from this reduction reaction. That is, $V^{4 +}, V^{3 +}$ and $V^{2 +}$ .

4. During the reaction, when the reaction mixture is warmed, some individual important colours are shown. That is,

Reduction from $V^{5 +}$ (yellow colour) to $V^{4 +}$ (blue colour).

Reduction from $V^{4 +}$ (blue colour) to $V^{3 +}$ (green colour).

Reduction from $V^{3 +}$ (green colour) to $V^{2 +}$ (purple colour).

Gary Horner Solutions for Chapter: Evidence, Exercise 9: Changing oxidation states of vanadium

Gary Horner Chemistry Solutions for Exercise - Gary Horner Solutions for Chapter: Evidence, Exercise 9: Changing oxidation states of vanadium

Questions from Gary Horner Solutions for Chapter: Evidence, Exercise 9: Changing oxidation states of vanadium with Hints & Solutions

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