Q10 / 180 · COMEDK 2023 (Morning Shift) · Chemistry

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

  • Correct answer
    A
    $\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$ is labile complex and changes to $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$ as $\mathrm{NH}_3$ is stronger ligand than $\mathrm{H}_2 \mathrm{O}$
  • B
    $\mathrm{Cu}^{2+}$ changes to $\mathrm{Cu}^{+}$
  • C
    $\mathrm{Cu}^{2+}$ forms hydrate
  • D
    $\mathrm{Cu}^{+}$ changes to $\mathrm{Cu}^{2+}$

Solution

Aqueous copper(II) sulfate exists as the complex ion $\left[\mathrm{Cu}(\mathrm{H}_2\mathrm{O})_4\right]^{2+}$ (or more accurately $\left[\mathrm{Cu}(\mathrm{H}_2\mathrm{O})_6\right]^{2+}$), which is sky blue in color.

Upon the addition of ammonia ($\mathrm{NH}_3$), the ammonia molecules act as stronger ligands than water molecules and displace them from the coordination sphere of the copper(II) ion.

The reaction is represented as $\left[\mathrm{Cu}(\mathrm{H}_2\mathrm{O})_4\right]^{2+} + 4\mathrm{NH}_3 \rightarrow \left[\mathrm{Cu}(\mathrm{NH}_3)_4\right]^{2+} + 4\mathrm{H}_2\mathrm{O}$.

The resulting complex $\left[\mathrm{Cu}(\mathrm{NH}_3)_4\right]^{2+}$ (tetraamminecopper(II) ion) exhibits a deep blue color due to the change in the crystal field splitting energy caused by the stronger $\mathrm{NH}_3$ ligands.

Answer: $\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}$ is labile complex and changes to $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$ as $\mathrm{NH}_3$ is stronger ligand than $\mathrm{H}_2 \mathrm{O}$

Hence, the correct answer is option: 'A'.

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