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

The change in color of aqueous CuSO₄ from sky blue to deep blue upon the addition of NH₃ can be explained by the formation of a coordination complex. Initially, in aqueous solution, copper sulfate exists as the complex \([Cu(H_2O)_4]^{2+}\), which is responsible for the sky blue color.

When NH₃ is added, it acts as a stronger ligand than water (H₂O). The ligands in the copper complex can be replaced due to the lability of \([Cu(H_2O)_4]^{2+}\). As a result, water molecules are displaced by ammonia molecules, forming a new complex: \([Cu(NH_3)_4]^{2+}\). This new complex exhibits a deep blue color, significantly different from the initial sky blue.

Thus, the correct explanation is that \([Cu(H_2O)_4]^{2+}\) is a labile complex and changes to \([Cu(NH_3)_4]^{2+}\) because NH₃ is a stronger ligand than H₂O.

Therefore, the answer is: \([Cu(H_2O)_4]^{2+}\) is labile complex and changes to \([Cu(NH_3)_4]^{2+}\) as NH₃ is stronger ligand than H₂O.

Answer 2

The color change of aqueous CuSO₄ from sky blue to deep blue upon the addition of NH₃ can be explained through coordination chemistry. Initially, in the aqueous solution, Cu²⁺ ions are present as the complex \([Cu(H_2O)_4]^{2+}\) which exhibits a sky blue color. Water molecules act as ligands in this complex, forming a coordination sphere around the copper ion.

When NH₃ is added to the solution, it acts as a ligand and replaces the water molecules in the complex. NH₃ is a stronger ligand than H₂O. This is due to the greater electron-donating ability of NH₃, which stabilizes the complex more effectively. As a result, the aqua complex \([Cu(H_2O)_4]^{2+}\) is transformed into the tetraamminecopper(II) complex \([Cu(NH_3)_4]^{2+}\), which has a deep blue color.

The reaction can be represented as follows:

\([Cu(H_2O)_4]^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+} + 4H_2O\)

This reaction highlights the coordination change in copper(II) ion when exposed to different ligands, leading to a noticeable change in color.

Therefore, the correct explanation for the color change is: \([Cu(H_2O)_4]^{2+}\) is a labile complex and changes to \([Cu(NH_3)_4]^{2+}\) as NH₃ is a stronger ligand than H₂O.