Chemical reactions often produce energy changes, because energy is required to break bonds and is released as the bonds form. The total energy of a system must be conserved according to the Law of Conservation of Energy, and therefore, the required or excess energy from a reaction is often taken in or released in the form of thermal energy. This transfer of thermal energy is due to the relative stored chemical energies in the reactants and the products. This stored heat content is known as the enthalpy.
Exothermic reactions are those which release heat to their surroundings, like a thermite reaction: . Exothermic reactions have a negative enthalpy of reaction, as the enthalpy of the products is less than the enthalpy of the reactants. In other words, the energy from the formation of the bonds in the products exceeds the energy released from breaking the bonds in the reactants. Endothermic reactions, on the other hand, absorb heat, and the products feel cold when touched. They have a positive enthalpy as in these reactions, the products have a greater enthalpy than the reactants. A combustion reaction is an example of an exothermic reaction, while decomposition is an example of an endothermic reaction. Although an exothermic reaction releases thermal energy, the reaction still needs to overcome an activation energy barrier to proceed, so this does not imply that the reaction will be spontaneous.
Heat changes from a reaction can be measured experimentally using a calorimeter or can be calculated using Hess's Law .