<P> The term endothermic process describes the process or reaction in which the system absorbs energy from its surroundings, usually in the form of heat . The term was coined by Marcellin Berthelot from the Greek roots endo -, derived from the word "endon" (ἔνδον) meaning "within" and the root "therm" (θερμ -) meaning "hot" or "warm" intended sense is that of a reaction that depends on absorbing heat if it is to proceed . The opposite of an endothermic process is an exothermic process, one that releases, "gives out" energy in the form of heat . Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs, though in reality it only refers to where the energy goes, without necessarily being in the form of heat . All chemical reactions involve both the breaking of existing and the making of new chemical bonds . A reaction to break a bond always requires the input of energy and so such a process is always endothermic . When atoms come together to form new chemical bonds, the electrostatic forces bringing them together leave the bond with a large excess of energy (usually in the form of vibrations and rotations). If that energy is not dissipated, the new bond would quickly break apart again . Instead, the new bond can shed its excess energy - by radiation, by transfer to other motions in the molecule, or to other molecules through collisions - and then become a stable new bond . Shedding this excess energy is the exothermicity that leaves the molecular system . Whether a given overall reaction is exothermic or endothermic is determined by the relative contribution of these bond breaking endothermic steps and new bond stabilizing exothermic steps . </P> <P> The concept is frequently applied in physical sciences to, for example, chemical reactions, where thermal energy (heat) is converted to chemical bond energy . </P>

Where does the energy in endothermic reactions come from