ABSTRACT
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 ∆M is related to the reaction energy ∆E by the famous relation
∆E = ∆M c2. (4.1)
Here ∆M ≡ Minitial −Mfinal, 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 Minitial > Mfinal, then ∆E > 0 and the reaction is exothermic. If mass is gained in the reaction Mfinal > Minitial, then ∆E < 0 and the reaction is endothermic.
