Most chemical reactions involve the breaking and formation of chemical bonds. It takes energy to break a chemical bond but energy is released when chemical bonds are formed. If more energy is released than consumed, then the chemical reaction evolves heat and is said to be exothermic . On the other hand if the reaction consumes more energy than it releases then the reaction absorbs heat and is said to be endothermic .
Most reactions are exothermic.
The amount of heat generated by a reaction can be quantified. If is positive then the reaction is endothermic and if it is negative the reaction is exothermic.