A level Chemistry · Edexcel 9CH0 · Topic 15C
Reactions of transition metal ions
Key points
- Ligand exchange: e.g. [Cu(H₂O)₆]²⁺ (blue) + 4Cl⁻ → [CuCl₄]²⁻ (yellow/green) + 6H₂O, with a change of coordination number and shape.
- NaOH(aq) removes protons from aqua ions giving precipitates: Cu(OH)₂ blue, Fe(OH)₂ green, Fe(OH)₃ orange-brown, Co(OH)₂ blue/pink, Cr(OH)₃ grey-green, Mn(OH)₂ off-white.
- Amphoteric hydroxides (Cr(OH)₃, Zn(OH)₂) dissolve in excess NaOH, e.g. forming [Cr(OH)₆]³⁻ (dark green).
- Ammonia gives the same precipitates in small amounts; in excess, some form ammine complexes: [Cu(NH₃)₄(H₂O)₂]²⁺ deep blue, [Co(NH₃)₆]²⁺ straw, [Cr(NH₃)₆]³⁺ purple.
- Some precipitates oxidise in air: Fe(OH)₂ turns orange-brown, Mn(OH)₂ darkens.
- Redox: Cr³⁺ is oxidised to chromate(VI) by hydrogen peroxide in alkali; Co²⁺ ammine complexes oxidise in air.
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