The thermal effect of water formation reaction is 286 kJ. what will be the thermal effect of its decomposition. Why?

286 kJ

Explanation: The heat change value in the forward reaction is equal to the heat change value in the reverse reaction. The only thing that changes is the sign on the heat value, such that if the forward reaction is endothermic then the reverse reaction is exothermic.

In this case;

The enthalpy change of formation of water is - 286 kJ/mol, this means that the reaction of formation of water is an exothermic reaction. The decomposition of water which is the reverse of formation has an enthalpy change of + 286 kJ, the reaction is an endothermic reaction. Therefore, the thermal effect of decomposition of water has the same value as the the thermal effect of formation which is 286 kJ