Question

Assertion: On cooling in a freezing mixture, the colour of the mixture turns to pink from deep blue for a reaction.
$Co({H_2}O)_6^{2 + }(aq) + 4C{l^ - }(aq) \rightleftharpoons CoCl_4^{2 - }(aq) + 6{H_2}O(l)$
Reason: The reaction is endothermic in forward reaction, so on cooling the reaction, deep blue colour appears
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
(C) Assertion is correct but Reason is incorrect
(D) Assertion is incorrect but Reason is correct
(D) Both Assertion and Reason are incorrect

Answer 1

Hint: One of the most unique properties of transition metal complexes is the display of a wide range of colors. This is due to the different types of complexes that are formed when the ligands get binded at the central atom site. The splitting of the d-orbital is the main reason for the impartation of color.

Complete answer:
In the reaction:
$Co({H_2}O)_6^{2 + }(aq) + 4C{l^ - }(aq) \rightleftharpoons CoCl_4^{2 - }(aq) + 6{H_2}O(l)$
This is an endothermic reaction.
First, the cobalt is in an octahedral state because it is bound to $6$ water molecules. After the reaction, cobalt occurs in tetrahedral form and gets bonded to $4$ chlorine ions. Chloride ions are bigger than water molecules and there isn’t enough space to fit all the atoms. So an octahedral spin complex becomes a tetrahedral spin complex. This phenomenon has occurred due to chlorine ions who got entry into the coordination sphere. The colour of the reaction changes due to this shift.
Now when we apply Le Chatelier’s principle it states that when the temperature has raised the equilibrium will shift to forward reaction and more blue colour ions get formed from pink ones and on freezing equilibrium shifts backward. The structure changes due to a change in temperature so due to the endothermic reaction more blue colour appears.
Hence, we can say that Both Assertion and Reason are correct and Reason is the correct explanation for Assertion. Therefore, option (A) is correct.

Note:
The colour in coordination compounds can be explained in terms of CFT. For example, (\[Ti\left( {{H_2}O} \right)_6^{3 + }\] is violet in colour. This is an octahedral complex with a single electron in ${t_{2g}}$, level in the ground state of the complex. The eg level is the next higher state for the electron.