When concentrated HCl is added to an aqueous solution of \(\mathrm{CoCl_2}\), its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction ?
Show Hint
- \(\mathrm{[CoCl_6]^{4-}}\)
- \(\mathrm{[CoCl_6]^{3-}}\)
- \(\mathrm{[CoCl_4]^{2-}}\)
- \(\mathrm{[Co(H_2O)_6]^{2+}}\)
The Correct Option is C
Solution and Explanation
On adding concentrated HCl (high \(\mathrm{Cl^-}\) concentration), water ligands are replaced:
\(\mathrm{[Co(H_2O)_6]^{2+} + 4Cl^- \rightleftharpoons [CoCl_4]^{2-} + 6H_2O}\).
The product is a tetrahedral complex of cobalt(II) which is deep blue. Complexes (1) and (2) are not formed under these conditions.
Answer: (3) \(\mathrm{[CoCl_4]^{2-}}\)
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