As I understand it, exothermic reactions release energy. This energy can be released through the generation of heat, sound, light, or electricity.


  1. Is it true that exothermic reactions can release energy in the form of heat, sound, light or electricity?

  2. Is it possible to alter the form of energy released by an exothermic reaction?

  • $\begingroup$ Separately, you might be interested in comparing/contrasting exothermic with exergonic. $\endgroup$
    – Nat
    Commented Jul 25, 2019 at 13:08
  • $\begingroup$ The semantics here are kinda interesting. Literally, an exothermic reaction is a reaction that releases heat. "Exothermic reaction", Wikipedia, says it can release light or heat. "Exothermic process", Wikipedia, says that it can release heat, light, electricity, or sound, as you've listed in the question statement. $\endgroup$
    – Nat
    Commented Jul 25, 2019 at 13:15
  • $\begingroup$ @Nat Interesting what is said in Wikipedia. The IUPAC definition is however rather clear. goldbook.iupac.org/terms/view/E02269 $\endgroup$ Commented Jul 25, 2019 at 22:51
  • $\begingroup$ @JeffreyJWeimer Yeah, it can vary depending on the theoretical framework. For example, IUPAC also defines a "molecule" as being uncharged, though this requirement tends to be more useful in the context of simplistic chemical species, while it's pretty senseless to think of molecules that way in Biochemistry. Likewise, exothermal processes put off "heat", but "heat" itself is a loose concept -- for example, if we zoom out, microscopic sound becomes macroscopic heat. $\endgroup$
    – Nat
    Commented Jul 28, 2019 at 10:33
  • $\begingroup$ @Nat Sound is not heat at any level, nor is the inverse true. Sound is more akin to transport of mechanical work as fluctuations of pressure and density. Heat is transported by variations in internal energy of molecules. $\endgroup$ Commented Jul 28, 2019 at 13:29

1 Answer 1


A fundamental mistake in your treatise is how you (and Wikipedia) use the terms exothermic and endothermic. These terms define the direction of heat flow between the system and its surroundings, not the direction of overall energy flow. Indeed, the IUPAC definition is explicit and defines the terms exothermic (and endothermic) solely based on the enthalpy change of the reaction, which is heat flow at constant pressure (see the IUPAC definition here). Work is combined with heat as the two major forms of energy flow. Other forms of energy flow between a system and its surroundings are as you mention: sound, light, and electricity.

So, by specific example: We do not label a reaction that emits light "exothermic", we label it fluorescent.

To the main point of your question: The mode of energy release (or uptake) by a chemical reaction is inherent to the reaction itself; one cannot change the external conditions of a chemical reaction to make that reaction release one type of energy in a completely different type of energy. So as a general example, a reaction that is solely exothermic and not electrochemical or fluorescent cannot be made to be electrochemical or fluorescent just by changing the external pressure or temperature or concentrations.

For reactions that emit more than one type of energy, one may be able to change the external conditions to change the ratio of energy. By example, an electrochemical reaction that is also exothermic may have a reaction enthalpy that depends on temperature at the same time that its Nernst energy depends on temperature (and species concentrations). In this case, you might be able to use temperature to change the ratio of electrochemical potential (voltage) of the reaction relative to the amount of heat released during the reaction.


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