A chemical reaction is an event involving electrochemical interaction between elements which result in the formation of ionic, covalent or hydrogen bonds. In some cases the reaction results in a release of heat energy while in others the absorption of it. In Chemistry these two opposing statuses of heat transfer are referred to as endothermic or exothermic reactions. All chemical reactions can be classified as one or the other.
The word “therm” stems from the Greek lexicon and refers to heat. The prefix “endo” is also Greek and translates in English to “into” while the prefix “exo”correlates in English to “out from.” An endothermic reaction is therefore one which absorbs heat, and exothermic, one that releases heat.
The actual heat in the case of both reactions is carried into and out from the atoms involved in the chemical reaction by photons in the infrared rage of the electromagnetic spectrum. Again, an exothermic reaction emits infrared photons while an endothermic reaction absorbs photons from the space around the atoms involved in the reaction.
In physics, photons are thought of as transient forms of energy. Standing still, a photon would have zero mass, but because photons travel through space at an extreme velocity of 186,282 miles per second, they exhibit mass like existence (as prescribed by Einstein’s theory of special relativity). This mass like existence is represented theoretically as pure energy. The term photons was coined by Albert Einstein, and the emission of them by electrons was part of his theory called the photoelectric effect.
According to the Bohr model (developed by Danish Physicist Neils Bohr) of atomic structure, electrons orbit the nucleus of the atom in shells or energy levels designated K,L,M, N, O and P. The outermost shell can never have more than 8 electrons in it. Any time that an electron jumps from one orbit or energy level to another (referred to as a “quantum leap”) a photon of energy is either released or absorbed by the electron. When the quantum jump is to a lower energy level, a photon is absorbed and when the electron jumps to a higher energy shell an electron is emitted. While Bohr’s classical model provides a good prototype to visualize the transient properties of electrons, its not really accurate. In 1925, the German Physicist Werner Heisenberg proposed a new theory of “Quantum mechanics” which better expressed the energy/matter relationships of subatomic particles. Incidentally, Bohr, who died in 1962, never believed in the existence of photons.
While its easier to visualize chemical reactions in terms of individual atoms reacting with other individual atoms, in reality most reactions involved millions and billions of atoms reacting simultaneously. Furthermore, unless the elemental components of the reaction are two hydrogen atoms, there are usually more than one or two electrons which become transient during the reaction. Predicting exactly what’s happening on an atom by atom basis serves no practical purpose for a chemist, although it is of interest to the physicists. For the chemist, it suffices to know that a specific reaction will be endothermic or exothermic, and to what degree.
Most reactions actually involve both exothermic as well as endothermic events at the quantum level, but the chemist makes an assessment only of the net effect. If there is a net absorption of photons the reaction is rated endothermic and if the net positive emission of photons is greater, then the reaction is exothermic. For a given quantity of reactants the actual endothermic or exothermic effect can be accurately predicted and would be expressed in Joules, Calories or BTUs (Brithish Thermal Units).