Question

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.

Answer 1

Aqueous copper sulphate solution is blue in colour due to presence of $\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ ions.
(i) It reacts with KF to form green coloured $[Cu{F}_4{]}^{2-}$.
$\left[Cu\right(H_{2}O{)}_4{]}^{2+}+4F^{-}\to [Cu{F}_4{]}^{2-}$

It reacts with KCl to form bright green coloured $[CuC{l}_4{]}^{2-}$.
$\left[Cu\right(H_{2}O{)}_4{]}^{2+}+4C{l}^{-}\to [CuC{l}_4{]}^{2-}$
In both examples, the weak field ligand water is replaced by fluoride and chloride ions.