Exothermic and endothermic reactions

Exothermic reactions transfer energy as heat to their surroundings. Endothermic reactions absorb heat from their surroundings. Breaking bonds requires energy; forming bonds releases energy. The overall balance determines the heat change. If bond breaking requires 100 energy units and bond formation releases 150 units, the net result is a release of 50 units. The reaction is exothermic, even though some energy was needed to break the original bonds.