Ncert Exercises & Solutions

When hydrochloric acid is added to cobalt nitrate solution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink
\([Co(H_2O)_6]^{3+} (aq) + 4Cl^-(aq) \rightleftharpoons [CoCl_4]^{2-} (aq) + 6H_2O(l)\\ ~~~\small{(Pink})~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\small{ (Blue)}\)

On the basis of the information given above, mark the correct answer:

1.	∆ H > 0 for the reaction
2.	∆ H < 0 for the reaction
3.	∆ H = 0 for the reaction
4.	The sign of ∆ H cannot be predicted on the basis of the given information.

Hint: For endothermic reaction, equilibrium shifts in forward direction on heating.

Explanation:

In this reaction,
\([Co(H_2O)_6]^{3+} (aq) + 4Cl^-(aq) \rightleftharpoons [CoCl_4]^{2-} (aq) + 6H_2O(l)\\ ~~~\small{(Pink})~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\small{ (Blue)}\)

On cooling, the equilibriums shifts backward direction or on heating, the equilibrium shifts toward forward direction. Hence,

reaction is endothermic, i.e.,

∆ H > 0

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