When hydrochloric acid is added to the cobalt nitrate solution at room temperature, the following reaction takes place :

The solution is blue at room temperature. However, it turns pink when cooled in a freezing mixture. Based on this information, which of the following expression is correct for the forward reaction?

Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

Since the reaction shifts to backward direction on cooling, this means that the backward reaction is an exothermic reaction. Therefore, the forward reaction is an endothermic reaction and ∆H > 0. On the other hand, enthalpy cannot be zero(C option) because this above-given process is not isothermal(constant temperature).