Question

When concentrated HCl is added to an aqueous solution of CoCl₂ , its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

(a) [CoCl₄] ^2– 

(b) [CoCl₆] ^3–

(c) [CoCl₆] ^4– 

(d) [Co(H₂O)₆] ²⁺

Answer 1

Correct option: (a) [CoCl₄]^2–

Explanation:

Aqueous solution of CoCl₂ contains [Co(H₂O)₆ ]²⁺ which is pinkish in colour so option (d) is incorrect.

Reduction potential of Co³⁺ → Co²⁺ is high so option(b) is incorrect. Co²⁺ does not oxidises easily to Co³⁺ .

It is general case that symmetrical substituted octahedral complexes are less deeper in colour than tetrahedral complexes. So [CoCl₄ ]^2– is deep blue in colour.