Aqueous copper sulphate solution (blue in colour) gives: (i) a green precipitate with aqueous potassium fluoride and (ii) a bright green solution with aqueous potassium chloride. Explain these experimental results.
When aqueous potassium fluoride is added to aqueous copper sulphate solution, the F⁻ ions act as ligands and replace H₂O molecules from the coordination sphere of [Cu(H₂O)₆]²⁺ complex. This results in the formation of a green precipitate of copper fluoride complex.
When aqueous potassium chloride is added to aqueous copper sulphate solution, the Cl⁻ ions act as ligands and replace H₂O molecules from the coordination sphere. This forms a bright green solution of copper chloride complex. The difference in colour is due to different crystal field splitting caused by different ligands.
Explanation
The textbook context establishes that ligands influence the colour of complexes through crystal field splitting. The blue colour of aqueous CuSO₄ is due to [Cu(H₂O)₆]²⁺ complex. When different ligands (F⁻ or Cl⁻) replace H₂O, they cause different crystal field splitting, resulting in different colours. F⁻ forms an insoluble green precipitate, while Cl⁻ forms a soluble bright green complex. This is analogous to the Ni-en example in the context where different ligand combinations produce different colours (green, blue/purple, violet).