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

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Answer & Solution

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 \(\mathrm{H}_2 \mathrm{O}\)

Step-by-step Solution

Detailed explanation

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…

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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