In reactions (nuclear, atomic, mechanical) in which some quantity is changed into some other quantity (e.g., two cars into a mass of tangled metal), energy is usually emitted (exothermic reaction) or absorbed (endothermic reaction). This energy, according to Einstein's Special Theory of Relativity comes from a change in the rest mass of the reactants. This change in mass AM is related to the reaction energy AE" by the famous relation
= AMc2. (4.1)
Here AM = Mjnitjai — Mfinaj, i.e., the loss of rest mass between the initial mass of the reactants and the mass of the final products. If mass is lost in the reaction ^initial > -^finab then AE > 0 and the reaction is exothermic. If mass is gained in the reaction Mflnaj > Mjnitja], then AE < 0 and the reaction is endothermic.