Question

Aqueous CuSO$_4$ changes its colour from sky blue to deep blue on addition of NH$_3$ because

• A. Cu\(^{2+}\) forms hydrate
• B. Cu\(^{2+}\) changes to Cu\(^{+}\)
• C. \([Cu(H_2O)_4]^{2+}\) is labile complex and changes to \([Cu(NH_3)_4]^{2+}\) as NH\(_3\) is stronger ligand than H\(_2\)O
• D. Cu\(^{+}\) changes to Cu\(^{2+}\)

Hint:

In coordination chemistry, ligands like NH\(_3\) can replace weaker ligands like H\(_2\)O, leading to changes in the color of the solution due to the formation of new complexes.

Answer 1

The Correct Option is C

When ammonia (NH\(_3\)) is added to an aqueous solution of CuSO\(_4\), it reacts with the copper ion (Cu\(^{2+}\)) to form a complex. Initially, Cu\(^{2+}\) exists as a hydrated ion \([Cu(H_2O)_4]^{2+}\), which gives the sky-blue color. 
However, NH\(_3\), being a stronger ligand, displaces water molecules from the coordination sphere of Cu\(^{2+}\) and forms a new complex, \([Cu(NH_3)_4]^{2+}\), which has a deep blue color. 
The transition to this new complex leads to the color change.