Question

What is the formula of brown ppt?
(A) $ C{u_2}{I_2} $
(B) $ C{u_2}{I_2} + {I_3}^ - $
(C) $ Cu{I_2} $
(D) $ CuS{O_4} $

Answer 1

Hint: Copper is a transition metal with the ability to show variable oxidation states and also d-d transitions that are responsible for imparting colour to its compounds and precipitates. But all oxidation states of copper cannot show d-d transitions.

Complete answer:
The electronic configuration of copper in its $ + 1 $ oxidation state is $ 3{d^{10}} $ and that of copper in its $ + 2 $ oxidation state is $ 3{d^9} $ . Thus, copper in its $ + 2 $ oxidation state has one vacant space in the d orbital for an electron but copper in its $ + 1 $ oxidation state has no vacant spaces and a completely filled d-orbital.
Transition metals ions that have a fully filled d-orbital are incapable of showing d-d transitions that are mainly responsible for imparting colour to these ions. Each ion in its particular oxidation state shows a characteristic colour depending upon the wavelength of the d-d transitions that take place.
Salts formed by copper $ C{u^{2 + }} $ ions have a characteristic blue colour due to the presence of d-d transitions associated with the partially filled d-orbital. Hence, $ Cu{I_2} $ and $ CuS{O_4} $ are blue coloured salts.
The copper ions in $ + 1 $ oxidation state are mainly colorless or white due to the absence of d-d transitions. Therefore a copper salt alone cannot form a brown precipitate. But iodine gas in the presence of potassium iodide forms triiodide ions which are initially yellow but turn dark brown in colour upon standing.
 $ {I_2} + KI \to {I_3}^ - $
These triiodide ions get adsorbed on the surface of colourless precipitate of copper resulting in a coloured precipitate
 $ (C{u_2}{I_2} + {I_3}^ - ) $ this combination is brown in colour.
Hence, the correct option is (b) and the brown precipitate is due to the formation of $ (C{u_2}{I_2} + {I_3}^ - ) $ .

Note:
Not all transition metal compounds and complexes are coloured solely due to the d-d transitions. Many complexes get their colour due to the presence of ligands that are conjugated in nature or due to charge transfer phenomenon.