Oxides, oxoanions and important compounds

The essentials

Transition metals form oxides and oxoanions in which the metal can be in a high oxidation state. As the oxidation state rises, oxides become more acidic: Mn₂O₇ and CrO₃ are acidic, while MnO is basic. Two important compounds are potassium dichromate and potassium permanganate, both powerful oxidising agents.

Key points

• Potassium dichromate (K₂Cr₂O₇) is made from chromite ore (FeCr₂O₄). The ore is fused with sodium carbonate in air to give sodium chromate, which is acidified to sodium dichromate; treatment with KCl gives orange crystals of K₂Cr₂O₇.
• Chromate (yellow) and dichromate (orange) interconvert with pH: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O.
• In acid: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (orange to green). It oxidises I⁻ to I₂, Fe²⁺ to Fe³⁺, H₂S to S and Sn²⁺ to Sn⁴⁺.
• Potassium permanganate (KMnO₄) is made by fusing MnO₂ with KOH and an oxidant to give green K₂MnO₄, which disproportionates in neutral or acidic solution to purple permanganate. Commercially, manganate is oxidised electrolytically.
• In acid: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O. In neutral or faintly alkaline solution: MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻.
• KMnO₄ oxidises Fe²⁺, oxalate (C₂O₄²⁻ to CO₂), I⁻ and H₂S in acid. Acid used is H₂SO₄, not HCl, because HCl would be oxidised to Cl₂.
• Both oxoanions are tetrahedral around the metal; permanganate's intense colour is due to charge transfer, not d–d transitions, since Mn(VII) is d⁰.

Exam tips

• KMnO₄ acts as its own indicator in titrations: the first excess drop gives a permanent pink colour.

Common mistakes

• Using 3 electrons for permanganate in acid; it is 5 in acid and 3 in neutral or weakly alkaline medium.