An endothermic process is any process which requires or absorbs thermal energy from its surroundings, usually in the form of heat. It may be a chemical process, such as dissolving ammonium nitrate in water, or a physical process, such as the melting of ice cubes. The term was coined by Marcellin Berthelot from the Greek rootsendo-, derived from the word "endon" meaning "within", and the root "therm", meaning "hot" or "warm" in the sense that a reaction 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 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. For example:N2+3H2=2NH3+92kj.
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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. 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. The concept is frequently applied in physical sciences to, for example, chemical reactions, where thermal energy is converted to chemical bond energy. Endothermic analysis only accounts for the enthalpy change of a reaction. The full energy analysis of a reaction is the Gibbs free energy, which includes an entropy and temperature term in addition to the enthalpy. A reaction will be a spontaneous process at a certain temperature if the products have a lower Gibbs free energy even if the enthalpy of the products is higher. Entropy and enthalpy are different terms, so the change in entropic energy can overcome an opposite change in enthalpic energy and make an endothermic reaction favorable. Eg: Burning of fuel Melting of Ice
High-energy neutrons can produce tritium from lithium-7 in an endothermic reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravonuclear test produced an unexpectedly high yield.
