D-And F-Block Elements Ques 493

Question: When concentrated $ HCl $ is added to an aqueous solution of $ CoCl_2 $ its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction-

Options:

A) $ {{[CoCl_4]}^{2-}} $

B) $ {{[CoCl_6]}^{3-}} $

C) $ {{[CoCl_6]}^{4-}} $

D) $ {{[Co{{(H_2O)}_6}]}^{2+}} $

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Answer:

Correct Answer: A

Solution:

Aqueous of solution of $ CoCl_2 $ contains $ {{[Co{{(H_2O)}_6}]}^{2+}} $ which is pinkish in colour so option [d] is incorrect.

Reduction potential of $ C{{o}^{3+}}\xrightarrow{{}}C{{o}^{2+}} $ is high so option[b] is incorrect.

$ C{{o}^{2+}} $ does not oxidises easily to $ C{{o}^{3+}} $ .

It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes.

So $ {{[CoCl_4]}^{2-}} $ is deep blue in colour.



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