COMEDK · Chemistry · 24. Coordination Compounds

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

A \(\mathrm{Cu}^{2+}\) forms hydrate
B \(\mathrm{Cu}^{2+}\) changes to \(\mathrm{Cu}^{+}\)
C \(\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}\) 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 \(\mathrm{H}_2 \mathrm{O}\)
D \(\mathrm{Cu}^{+}\)changes to \(\mathrm{Cu}^{2+}\)

Verified Solution

Answer & Solution

Correct Answer

(C) \(\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}\) 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 \(\mathrm{H}_2 \mathrm{O}\)

Step-by-step Solution

Detailed explanation

As, \(\mathrm{H}_2 \mathrm{O}\) is a weak field ligand and thus \(\left[\mathrm{Cu}\left(\mathrm{H}_2 \mathrm{O}\right)_4\right]^{2+}\) absorbs red light in visible spectrum. But the colour appears blue because…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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