Chemical reactions always involve a change in energy. Energy is neither created nor destroyed. Energy is absorbed or released in chemical reactions. Chemical reactions can be described as endothermic or exothermic reactions.
Endothermic reactions – chemical reactions in which energy is absorbed are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy.
Exothermic reactions – Chemical reactions in which energy is released are exothermic. The energy is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.