Question

Aqueous $CuS{O}_4$ changes its colour from sky blue to deep blue on the addition of $N{H}_3$ because:

• A. $C{u}^{2+}$ forms a hydrate
• B. $\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ is a labile complex and changes to $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ as $N{H}_3$ is a stronger ligand than $H_{2}O$
• C. $C{u}^{2+}$ changes to $C{u}^{+}$
• D. All of the above

Answer 1

The correct option is B $\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ is a labile complex and changes to $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ as $N{H}_3$ is a stronger ligand than $H_{2}O$

$\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ has a weak ligand $H_{2}O$ and absorbs red light of the visible spectrum. The colour that appears is blue. $\left[Cu\right(H_{2}O{)}_4{]}^{2+}$ is a labile complex and changes to $\left[Cu\right(N{H}_3{)}_4{]}^{2+}$ by the addition of strong ligand $N{H}_3$.

It has a strong ligand $N{H}_3$ and absorbs yellow light of the visible spectrum. The colour that appears is deep blue (indigo).