Why do exothermic reactions release energy?
Exos= outside , thermos=temperature or heat So within the definition itself it can be known that in exothermic reactions heat is released.
In chemical reactions two processses take place, breaking of old bonds and formation of new bonds. The energy supplied is use in breaking of bonds, but energy is release by making bonds. In exothermic reactions, the energy supplied is sufficient for chemical reaction to occur (to break existing bonds) and the excess energy is given in the form of heat (from making new bonds).
If the reaction is exothermic, it means that the total energy it took to break the bonds is less than the total energy released when forming new bonds -- so net energy is released in the form of heat.
for example 2