Exothermic (heat out) reactions are a form of exergonic (energy out) reaction that occurs when the bond energy of the reactants is greater than the bond energy of the products and the surplus energy is released as heat.

The excess energy is conserved do to the law of conservation of energy and matter which states that mater and energy cant be created of destroyed but can change form. For example, the energy holding wood together is released in the form of light and heat when it is combined to ##O_2## to produce ##CO_2## and ##H_2O## .