What is the difference between an exothermic and an endothermic reaction?

Step-by-step solution

The energy level difference between S and P. If ‘P’ is at a lower level than ‘S’, the reaction is an exothermic reaction . Whether it is an exothermic or spontaneous reaction or an endothermic or energy requiring reaction, the ‘S’ has to go through a much higher energy state or transition state. The difference in average energy content of ‘S’ from that of this transition state is called ‘activation energy’.