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

8.2.5 Vanadium Oxidation States

Aim:

To observe and identify the colour changes associated with the reduction of vanadium from its highest oxidation state (V) to lower oxidation states (IV, III, II) by using zinc metal in acidic conditions.

Vanadium as a Transition Metal:

Vanadium (V) is a transition metal known for having variable oxidation states. This makes vanadium an ideal element to study oxidation-reduction (redox) reactions, as each oxidation state exhibits distinct colours in solution:

• V(V) (vanadate ion, $VO₃⁻$) is yellow
• V(IV) ($VO²⁺$) is blue
• V(III) ($V³⁺$) is green
• V(II) ($V²⁺$) is violet By reducing vanadium ions in acidic conditions using zinc, you can observe the stepwise reduction of vanadium from V(V) to V(II) and the corresponding colour changes.

Materials and Equipment:

• Ammonium vanadate(V) ($NH₄VO₃$)
• 1.0 mol dm⁻³ hydrochloric acid ($HCl$)
• Zinc metal (small pieces)
• Test tube and test tube rack
• Spatula
• Dropping pipette
• Stopwatch or timer
• Stopper (optional, for $V²⁺$observation)

Method:

1. Prepare the Vanadium(V) Solution:

• Using a spatula, add approximately ¼ of a spatula of ammonium vanadate(V) ($NH₄VO₃$) to a test tube.
• Use a dropping pipette to fill the test tube halfway with 1.0 mol dm$⁻³$ hydrochloric acid ($HCl$).
• Gently shake the test tube to dissolve the solid and form the yellow vanadate(V) solution.

2. Initial Observation:

• Record the colour of the vanadate(V) ion in solution.
• This should be yellow, indicating the presence of V(V).

3. Add Zinc Metal:

• Carefully add one small piece of zinc to the test tube.
• Gently shake the test tube to initiate the reduction of vanadium.

4. Observe Colour Changes:

• Observe the colour changes over the course of 15 minutes as the zinc reduces vanadium ions step by step from V(V) to V(II).
• The following changes should be noted:
• V(V) → V(IV): Yellow to blue
• V(IV) → V(III): Blue to green
• V(III) → V(II): Green to violet

5. Handling $V²⁺$(Optional Step):

• Once the solution turns violet, indicating the formation of $V²⁺$, you may place a stopper on the test tube to prevent oxidation. In the presence of oxygen (air), $V²⁺$can be oxidised back to $V³⁺$ causing the solution to turn green again.
• Stopping the air contact will allow you to observe the violet colour of $V²⁺$more clearly.

Results Table:

Oxidation State	Ion Formula	Observed Colour
V(V)	$VO₃⁻$	Yellow
V(IV)	$VO²⁺$	Blue
V(III)	$V³⁺$	Green
V(II)	$V²⁺$	Violet

Explanation

Reduction Process:

• Zinc acts as a reducing agent, providing electrons that reduce vanadium from its higher oxidation states (V) to lower ones.
• The reduction happens step by step, and each oxidation state has a distinctive colour, which can be used to track the progress of the reaction. Colour Changes Explained:

The different colours observed during the reduction correspond to the changing oxidation states of vanadium:

• V(V) (yellow) is reduced to V(IV) (blue), indicating that vanadium has gained one electron.

• A further reduction to V(III) (green) occurs as vanadium gains another electron.

• Finally, V(II) (violet) forms as the vanadium ion gains one more electron. Oxidation in Air:

• The $V²⁺$(violet) ion is highly susceptible to oxidation when exposed to air, which converts it back to $V³⁺$(green).

• To prevent this, use a stopper to minimise contact with oxygen, allowing the violet colour of $V²⁺$to be observed longer.

Conclusion:

This practical demonstrates the ability of vanadium, as a transition metal, to exist in multiple oxidation states. The stepwise reduction of vanadate (V) ions in acidic conditions using zinc shows distinct colour changes that correspond to different oxidation states of vanadium.