Question

Aqueous copper sulphate solution (blue in colour) gives:
a green precipitate with aqueous potassium fluoride.

a bright green solution with aqueous potassium chloride.
Explain these experimental results.

Answer 1

Aqueous copper sulphate (blue) is $\left[Cu\right(H_{2}O{)}_4]S{O}_4$

$\left[Cu\right(H_{2}O{)}_4]S{O}_{4\left(aq\right)}\rightleftharpoons \underset{\left[Blue\right]}{\left[Cu\right(H_{2}O{)}_4{]}^{2+}}+S{O}_4^{2-}$
$\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ is a labile complex in which ligands $H_{2}O$ get easily replaced by $F^{-}$ ion of KF (as in (i) part) and by $C{l}^{-}$ ion of KCl (as in (ii) part).
(i). $\left[Cu\right(H_{2}O{)}_4{]}_{\left(aq\right)}^{2+}+4F_{\left(aq\right)}^{-}\to \underset{Greenppt.}{[Cu{F}_4{]}^{2-}}+4H_{2}O$

(ii).$\left[Cu\right(H_{2}O{)}_4{]}_{\left(aq\right)}^{2+}+4C{l}_{\left(aq\right)}^{-}\to \underset{Brightgreensolution}{[CuC{l}_4{]}_{\left(aq\right)}^{2-}}+4H_{2}O$

In both these cases, the weak field ligand water is replaced by the $F^{-}$ and $C{l}^{-}$ions.