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.