Variable Oxidation States (AQA A-Level Chemistry): Revision Notes

6.2.5 Variable Oxidation States

Variable Oxidation States of Transition Elements

Transition elements are unique in that they can exist in multiple oxidation states, which allows them to form a variety of compounds with distinct chemical properties. The ability of transition metals to switch between oxidation states is essential in many redox reactions, including industrial processes, biological systems, and analytical techniques.

Oxidation States of Vanadium

Vanadium is a classic example of a transition element with multiple oxidation states. It exhibits four primary oxidation states (V, IV, III, and II), each associated with a unique colour in solution:

• Vanadium(V): Yellow solution, $\text{VO}_2^+$
• Vanadium(IV): Blue solution, $\text{VO}^{2+}$
• Vanadium(III): Green solution, $\text{V}^{3+}$
• Vanadium(II): Violet solution, $\text{V}^{2+}$

These oxidation states can be interconverted through a series of reduction reactions, typically involving zinc as a reducing agent in an acidic solution.

Stepwise Reduction of Vanadium(V)

Reduction of $\text{VO}_2^+$ (Vanadium(V)) to $\text{VO}^{2+}$ (Vanadium(IV))

Colour Change: Yellow to blue

Half-Equations:

Reduction:

$$\text{VO}_2^+ + 2\text{H}^+ + e^- \rightarrow \text{VO}^{2+} + \text{H}_2\text{O}$$

Oxidation (Zinc):

$$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$

Overall Reaction:

$$2\text{VO}_2^+ + 4\text{H}^+ + \text{Zn} \rightarrow 2\text{VO}^{2+} + 2\text{H}_2\text{O} + \text{Zn}^{2+}$$

Reduction of $\text{VO}^{2+}$ (Vanadium(IV)) to $\text{V}^{3+}$ (Vanadium(III))

Colour Change: Blue to green

Half-Equations:

Reduction:

$$\text{VO}^{2+} + 2\text{H}^+ + e^- \rightarrow \text{V}^{3+} + \text{H}_2\text{O}$$

Oxidation (Zinc):

$$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$

Overall Reaction:

$$2\text{VO}^{2+} + 4\text{H}^+ + \text{Zn} \rightarrow 2\text{V}^{3+} + 2\text{H}_2\text{O} + \text{Zn}^{2+}$$

Reduction of $\text{V}^{3+}$ (Vanadium(III)) to $\text{V}^{2+}$ (Vanadium(II))

Colour Change: Green to violet

Half-Equations:

Reduction:

$$\text{V}^{3+} + e^- \rightarrow \text{V}^{2+}$$

Oxidation (Zinc):

$$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$

Overall Reaction:

$$2\text{V}^{3+} + \text{Zn} \rightarrow 2\text{V}^{2+} + \text{Zn}^{2+}$$

These colour changes can be used to visually track the reduction process of vanadium ions in solution.