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Question: When concentrated \({\text{HCl}}\) is added to an aqueous solution of \({\text{CoC}}{{\text{l}}_{\te...

When concentrated HCl{\text{HCl}} is added to an aqueous solution of CoCl2{\text{CoC}}{{\text{l}}_{\text{2}}}, its colour changes from reddish pink to deep blue. Which complex ion gives colour in this reaction?
A) [CoCl6]4−{\left[ {{\text{CoC}}{{\text{l}}_{\text{6}}}} \right]^{4 - }}
B) [CoCl6]3−{\left[ {{\text{CoC}}{{\text{l}}_{\text{6}}}} \right]^{3 - }}
C) [Co(H2O)6]2+{\left[ {{\text{Co}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}} \right]^{2 + }}
D) [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }}

Explanation

Solution

To solve this we must know the reaction between the aqueous solution of cobalt chloride i.e. CoCl2{\text{CoC}}{{\text{l}}_{\text{2}}} and concentrated hydrochloric acid i.e. HCl{\text{HCl}}. Initially the aqueous solution of cobalt chloride is reddish pink in colour which after the reaction becomes deep blue.

Complete step-by-step answer:
Cobalt chloride i.e. CoCl2{\text{CoC}}{{\text{l}}_{\text{2}}} is a reddish pink coloured powder which has a slightly sharp odour.When concentrated hydrochloric acid is added to an aqueous solution of cobalt chloride, its colour changes from reddish pink to deep blue.

We have aqueous solution of cobalt chloride. In aqueous solution, the cobalt ion gets hydrated. This hydrated cobalt ion then reacts with the chloride ions from the concentrated hydrochloric acid.

The reaction is as follows:
[Co(H2O)6]2++4Cl−→Heat[CoCl4]2−+6H2O{\left[ {{\text{Co}}{{\left( {{{\text{H}}_{\text{2}}}{\text{O}}} \right)}_{\text{6}}}} \right]^{2 + }} + 4{\text{C}}{{\text{l}}^ - }\xrightarrow{{{\text{Heat}}}}{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }} + 6{{\text{H}}_2}{\text{O}}
Thus, when concentrated hydrochloric acid is added to an aqueous solution of cobalt chloride, [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }} complex ion is produced. The complex ion [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }} is deep blue in colour.
Cobalt chloride i.e. CoCl2{\text{CoC}}{{\text{l}}_{\text{2}}} is a weak Lewis acid. It reacts with chloride ions form hydrochloric acid i.e. HCl{\text{HCl}} and produces a salt containing tetrahedral [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }} ion.

Thus, the complex ion that gives colour in the reaction when concentrated HCl{\text{HCl}} is added to an aqueous solution of CoCl2{\text{CoC}}{{\text{l}}_{\text{2}}} is [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }}.

Thus, the correct option is (D) [CoCl4]2−{\left[ {{\text{CoC}}{{\text{l}}_{\text{4}}}} \right]^{2 - }}.

Note: Cobalt chloride is blue in colour when it is anhydrous and becomes deep magenta coloured when hydrated. Thus, cobalt chloride is used to detect the presence of water. The elements or ions exhibit colours due to the presence of unpaired electrons in the d-orbital. Cobalt ion has unpaired electrons in the d-orbital and thus it shows colour.