Question

When hydrochloric acid is added to cobalt nitrate soulution at room temperature, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink. On the basis of this information mark the correct answer. $\left[Co\right(H_{2}O{)}_6{]}^{3+}\left(aq\right)+4Cl{)}^{-}\left(aq\right)\rightleftharpoons \left[CoC{l}_4{]}^{2-}\right(aq)+6H_{2}O(l)$
(Pink) (Blue)

• A. $\Delta H>0$ for the reaction.
• B. $\Delta H<0$ for the reaction.
• C. $\Delta H=0$ for the reaction.
• D. The sign of $\Delta H$ cannot be predicted ont he basis of this information.

Answer 1

The correct option is A $\Delta H>0$ for the reaction.

Positive value of enthalpy change indicates that the reaction is endothermic in the forward direction. When temperature is lowered, the equilibrium shifts to backward direction. At room temperature, forward reaction forms blue colored $[CoC{l}_4{]}^{2-}$. When the reaction mixture is cooled, the equilibrium shifts to backward direction and pink colored $\left[Co\right(H_{2}O{)}_6{]}^{3+}$ predominates.