When a reaction is exothermic it releases energy often denoted in kJ/mol. This is due to the total enthalpy of the reactant(s) being higher than the enthalpy of the product(s) and thus needs to release the excess energy to something, somewhere. This can be done via multiple forms such as heat & light.

But how do these forms of energy occur from the excess energy?

For example, if exothermic reaction takes place in an closed system and the environment is an arbitrary amount of degrees warm and this reaction is indefinite. Does the temperature keep rising because it keeps adding energy to entropy of the system or does it warm itself up to like 300K with the excess energy and due to how warmth distributes itself is the max temperature of the system 300K.

  • $\begingroup$ By "how", do you mean what happens on an atomic level? Otherwise I don't get the question... $\endgroup$ Jun 26 at 18:35
  • $\begingroup$ @CharlesTucker3 if possible, rather on an intuitive level. It can get rid of the excess energy via e.g. heat & light, but what happens that causes that energy to turn into heat or light $\endgroup$
    – Noah
    Jun 26 at 18:38
  • 2
    $\begingroup$ In a closed system, a reaction can not go on indefinitely: after a while at least one of the reactants will be used up. $\endgroup$ Jun 27 at 7:57
  • $\begingroup$ Related: physics.stackexchange.com/q/712402/247642 $\endgroup$ Jun 27 at 9:09

3 Answers 3


For example, one (or more) of the reaction partners is after the chemical reaction not in the ground state, but in an excited state. The electron will sooner or later fall back into the ground state and a photon is released, meaning that light is produced. The photon can be absorbed by other molecules in the surrounding, exciting them, and so on. Alternatively, collisions with the excited atom can occur increasing the kinetic energy of other molecules, resulting in more heat.


Here is more information related to the basic question "How does an exothermic reaction release energy?"

Classical thermodynamics accounts for the release of energy using such concepts as "enthalpy of formation, or energy of formation". For an exothermic reaction, the energy of formation results in the internal energy of the products being less than the internal energy of the reactants and this difference is the energy released. We now know, using $E = mc^2$, that the decrease in internal energy is due to a decrease in the rest mass of the products compared to the reactants; that is the "energy of formation" is a change in rest mass. For an endothermic formation there is an increase in the rest mass of the products compared to the reactants.

This applies to nuclear as well as chemical reactions, but the energy released in very much greater for the nuclear reaction. The equivalent of energy of formation for a nuclear reaction is called the Q value of the reaction which is the total rest masses of the reactants minus the total rest masses of the products. Q is positive for an exothermic nuclear reaction, and Q is negative for an endothermic nuclear reaction.

The answer by @Charles Tucker 3 addresses the other issues in your question.


The energy release din chemical reaction comes from the difference in binding energies of the reactants and the reaction products. Thus, if we consider two atoms $A$ and $B$ that can join in a molecule, forming abound state $AB$ with energy $E_b$, then the energy conservation gives us: $$ \frac{mv_A^2}{2} + \frac{mv_B^2}{2}=\frac{mv_{AB}^2}{2}-E_b $$ or, if the energy is release as electromagnetic radiation (photon): $$ \frac{mv_A^2}{2} + \frac{mv_B^2}{2}=\frac{mv_{AB}^2}{2}+h\nu-E_b $$

That is the collision of the two atoms may result in them forming abound state, with the product having higher kinetic energy than the reagents: $ A +B = AB + E_b. $ (The energy of the bound state $AB$ is negative: $E_{AB}=-E_b$.)

The molecule may also dissociate into atoms (for stable molecules it rarely that it happens spontaneously, but it can be hit by another atom, providing the energy): $ AB+E_b= A + B $

As a reversible chemical reaction can go both ways,$A + B \leftrightarrow AB$, eventually a balance is established, where the number of new molecules becomes equal to the number of molecules that dissociate, and the reaction stops. Thus, in a closed system, although on the microscopic level the reaction would continue going infinitely in both directions, it stops on the microscopic level. If a reaction is maintained at constant temperature (isothermic), the exotermic reaction will result in excess energy that would be lost to the environment, and it will go till it cannot generate any excess heat anymore (in which case it reaches a kind of equilibrium described above).


  • @JohnDarby correcly points out that the excess heat can be described as a mass defect. This description works well for nuclear reactions, but is less convenient for chemical ones, since the binding energy is much smaller than the rest energy of the reactants and products. Still, $E=mc^2$ applies here as well.
  • As a crash course on chemical reactions for physicists I recommend Introduction to Physics and Chemistry of Combustion: Explosion, Flame, Detonation by Michael Liberman.

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