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2.5.5Inorganic Chemistry

Variable Oxidation States

5 min read 5 parts Lesson 22 of 27

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Part 1 of 5Why Oxidation States Vary

2.5.5 Variable Oxidation States
Why Oxidation States Vary
Similar energies
  • The 3d and 4s sub-shells are close in energy, so successive electrons can be removed without a huge jump in ionisation energy.
Loss of 4s
  • Ions form by losing 4s electrons first, which is why +2 is common.
More states
  • Further loss of 3d electrons gives higher states, such as +3, +4 and above.
Stability
  • Compounds in a given oxidation state are stable if energy released in forming bonds compensates the ionisation energy.

Part 2 of 5Common Oxidation States

Ti
  • Titanium commonly shows +2, +3 and +4, with +4 the most stable state in compounds such as TiO₂.
V
  • +2, +3, +4, +5; the aqueous ions are V²⁺ violet, V³⁺ green, VO²⁺ blue and VO₂⁺ yellow.
Cr
  • +2, +3, +6; Cr³⁺(aq) is green, CrO₄²⁻ is yellow and Cr₂O₇²⁻ is orange.
Mn
  • +2, +4, +6, +7; Mn²⁺(aq) is pale pink and MnO₄⁻ is purple.
Fe, Cu
  • Iron shows +2 and +3, whereas copper shows +1 and +2.

Part 3 of 5Redox Changes

1
Reducing agents such as zinc in acid reduce vanadate(V), VO₂⁺, stepwise from yellow to blue, green and finally violet.
2
Manganate(VII) is a strong oxidising agent: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O, so purple turns almost colourless.
3
Dichromate(VI) is reduced in acid: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, orange changing to green.
4
Chromate(VI) and dichromate(VI) are interconverted without redox: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O.
5
Fe²⁺ is oxidised to Fe³⁺ by oxidising agents, and Fe³⁺ is reduced back to Fe²⁺ by reducing agents.

Part 4 of 5Copper(I)

Copper(I)
Disproportionation
  • Copper(I) is unstable in aqueous solution and disproportionates: 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s).
Why
  • The hydration of Cu²⁺ releases enough energy to make this change favourable in water.
Solid compounds
  • Some solid copper(I) compounds, such as red Cu₂O, are stable.

Part 5 of 5Exam Checklist & Tips

2.5.5 – Key Points for the Exam

  • Oxidation state The charge an atom would have if its bonds were completely ionic.
  • Disproportionation A reaction in which one species is both oxidised and reduced.
  • Oxidising agent A species that takes electrons from another.
  • Reducing agent A species that donates electrons.

Common Mistakes

  • Saying 3d electrons are removed before 4s electrons.
  • Describing the chromate–dichromate change as a redox reaction.
  • Confusing the purple MnO₄⁻ with the pale pink Mn²⁺ ion.
  • Thinking all transition metals show +3 as the most stable state.
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