What is the difference between an exothermic and an endothermic...

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’.