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Question: The formation of \( CoC{l_4}^{2 - } \) from \( C{o^{2 + }} \) and \( C{l^ - } \) is endothermic. Are...

The formation of CoCl42−CoC{l_4}^{2 - } from Co2+C{o^{2 + }} and Cl−C{l^ - } is endothermic. Are the color changes that accompany heating and cooling of equilibrium mixture in accord with Le Chatelier’s principle?

Explanation

Solution

To answer the question we need to understand Le-Chatelier’s principle. Le-Chatelier’s principle explains the effect of change in concentration, pressure, temperature, or addition of inert gas to a reversible reaction equilibrium. Equilibrium will shift in such direction i.e. either forward or backward direction to “undo” all the changes made.

Complete answer:
To answer the question let us first understand the effect of change in temperature on reaction equilibrium as per Le-Chatelier’s principle. If temperature increases, equilibrium shifts to that direction which proceeds through absorption of heat.
(A)If the reaction is endothermic (heat is absorbed) and we increase the temperature the reaction will shift in the forward direction. By this method, the excess heat is shifted towards the product side.
(B) If the reaction is exothermic (heat is released) and we increase the temperature the reaction will shift in the backward direction. By this method, the excess heat is shifted towards the reactant side.
In the question, we are asked about are the color changes that accompany the heating and cooling of equilibrium mixture resulting in formation of CoCl42−CoC{l_4}^{2 - } from Co2+C{o^{2 + }} and Cl−C{l^ - } in accord with Le Chatelier’s principle, so the answer is yes, these color changes are in accord with the principle.
Both Co2+C{o^{2 + }} and Cl−C{l^ - } in the reactants exist in the form of a hexahydrate complex [Co(H2O)6]2+{\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }} . In the aqueous medium, the following equilibrium exists between the complexes-
[Co(H2O)6]2++4Cl−+heat⇌CoCl42−+6H2O{\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }} + 4C{l^ - } + heat \rightleftharpoons CoC{l_4}^{2 - } + 6{H_2}O
Let us draw some conclusions from the change-
(A)The reaction is endothermic i.e. it absorbs heat during the change.
(B)Here the complex [Co(H2O)6]2+{\left[ {Co{{\left( {{H_2}O} \right)}_6}} \right]^{2 + }} is pink in color while the complex CoCl42−CoC{l_4}^{2 - } is blue.
(C)Here the chloride ion enters the coordination sphere and converts the octahedral complex into a tetrahedral complex.
(D) Following Le-Chatelier’s principle, if we increase the temperature (provide heat) the reaction shifts in the forward direction forming more of the products turning the solution into blue, and if we decrease the temperature (freeze the mixture or on cooling) it will shift the reaction in the backward direction producing pink solution.

Note:
Apart from temperature changes, several other changes can affect the color change. If we increase the concentration of chloride ion suppose by adding of HClHCl the reaction shifts to the forward direction i.e. produces blue color however if we add AgNO3AgN{O_3} then there is a removal of chloride ion from the reactants and the equilibrium will shift in the backward direction i.e. produces a pink solution.