Question

\([Co(H_{2}O)_{6}]^{2+}\) (Pink) \(+4Cl^{-}\rightleftharpoons[CoCl_{4}]^{2-}\) (Blue) \(+6H_{2}O\) (Endothermic reaction). If the mixture is transferred from room temperature to a freezing ice bath, what is expected to happen?

• A. Colour will remain the same
• B. Colour will become deeper blue
• C. It will become colourless
• D. Colour will become pink

Hint:

Remember this important equilibrium: \[ \boxed{ [Co(H_2O)_6]^{2+} \;\rightleftharpoons\; [CoCl_4]^{2-} } \] \[ \boxed{ \begin{aligned} \text{Pink} &\rightarrow [Co(H_2O)_6]^{2+}\\ \text{Blue} &\rightarrow [CoCl_4]^{2-} \end{aligned} } \] Heating favours the blue complex, whereas cooling favours the pink complex.

Answer 1

The Correct Option is D

In the equilibrium $[Co(H_2O)_6]^{2+} + 4Cl^- \rightleftharpoons [CoCl_4]^{2-} + 6H_2O$, water is a product. By Le Chatelier's principle, adding more water increases the concentration of a product and shifts the equilibrium to the left, converting the blue $[CoCl_4]^{2-}$ back to the pink $[Co(H_2O)_6]^{2+}$.