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Variable Oxidation States (A2 Only)

Chemistry: AQA A Level, topic 4.2.5

Oxidation States of Vanadium

Transition metals have multiple stable ions in different oxidation states. For example, vanadium has four main oxidation states, all of which can be easily prepared. Each vanadium ion is a different colour:

Vanadium (V) - yellow

Vanadium (V) - yellow

Vanadium (IV) - blue

Vanadium (IV) - blue

Vanadium (III) - green

Vanadium (III) - green

Vanadium (II) - violet

Vanadium (II) - violet

Vanadium Ion Synthesis

Here we shall look into preparing various oxidation states of vanadium.

Preparation of vanadium ions

Preparation of vanadium ions

  • Different ions of vanadium can be prepared by successive reduction with zinc in acid solution.
  • Zinc will donate two electrons to vanadium, so to move vanadium down one oxidation state you add half as much zinc as you have vanadium.
  • The equations for each reduction are on the next slides.
Vanadium(V) → vanadium(IV)

Vanadium(V) → vanadium(IV)

  • Vanadium(V) exists as the vanadate ion, VO2+.
  • The reduction equation is:
  • 2VO2+(aq) + Zn(s) + 4H+(aq) → Zn2+(aq) + 2VO2+(aq) + 2H2O(l)
  • There is a colour change from yellow to blue.
Vanadium(IV) → vanadium(III)

Vanadium(IV) → vanadium(III)

  • The reduction equation is:
  • 2VO2+(aq) + Zn(s) + 4H+(aq) → Zn2+(aq) + 2V3+(aq) + 2H2O(l)
  • There is a colour change from blue to green.
Vanadium(III) → vanadium(II)

Vanadium(III) → vanadium(II)

  • The reduction equation is:
  • 2V3+(aq) + Zn(s) → Zn2+(aq) + 2V2+(aq)
  • There is a colour change from green to violet.

Redox Potentials

Redox potentials give information about the stability of a complex. They are dependent on many factors.

Redox potentials

Redox potentials

  • Redox potentials show you how easy a metal can be reduced.
  • A redox potential is just another name for an electrode potential.
  • Higher values mean the ion is less stable and more easily reduced.
  • Standard redox potentials are quoted relative to a standard hydrogen electrode and under standard conditions.
Differences to standard potentials

Differences to standard potentials

  • A redox potential is not always the same as a standard electrode potential.
  • To change a redox potential from the standard electrode potential, you change the conditions.
  • In particular, redox potentials can depend on:
  • Ligands.
  • pH.
Ligands

Ligands

  • Standard redox potentials are measured in aqueous solution.
  • Changing the ligands will alter the redox potential depending on the bond strength to the metal.
pH

pH

  • Certain potentials involve H+.
  • If the concentration of H+ is changed, the redox potential will change.
  • Usually, a higher concentration of H+ means a higher redox potential.

Test yourself on Variable Oxidation States (A2 Only)

  • How many main oxidation states does vanadium have?

  • Which vanadium ion is blue?

  • What is a good metal to use to prepare different oxidation states of vanadium?

  • Which of the following best describes a redox potential?

  • A higher concentration of H+ ions usually means what?

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