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I don't understand how energy is released in combustion. We have a reaction of a combustible material with oxygen and this releases energy. In what form is this energy released? It should be some waves or a medium in which the energy is released! And I suppose that released energy causes the products of the combustion to have extremely high kinetic energy (ie. temperature), right? Anyway, can you answer me please.

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    $\begingroup$ I Googled combustion energy and was sent to the Wikipedia entry Heat of combustion. What searches have you done that failed to bring results? $\endgroup$
    – Kyle Kanos
    Commented Nov 25, 2015 at 11:24
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    $\begingroup$ I 've been to that webpage, it doesn't answer my question. Have you read my question in full, or you just spotted two words, combustion and heat? $\endgroup$
    – ergon
    Commented Nov 25, 2015 at 12:20
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    $\begingroup$ I saw the question In what form is this energy released? & then did the Google search. That page clearly states that energy is released as heat, so I made my comment. $\endgroup$
    – Kyle Kanos
    Commented Nov 25, 2015 at 12:27
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    $\begingroup$ en.wikipedia.org/wiki/Heat $\endgroup$
    – Kyle Kanos
    Commented Nov 25, 2015 at 19:06
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    $\begingroup$ Does this answer your question? How to thermodynamically understand process of burning a piece of coal? $\endgroup$
    – Roger V.
    Commented Feb 4, 2023 at 16:43

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Notice, the combustion is an exothermic chemical reaction with the oxygen that liberates the energy mainly in the form of heat, comparatively less amount of light, sound etc. Reactant molecules react with the oxygen that results in breaking of old bonds & formation of new bonds with comparatively less bonding energy. Thus, releasing the rest amount of energy in form of heat during combustion.

In the combustion process, the heat energy comes from the chemical energy stored in the combustible material being burnt.

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Note: Existing answers and comments are already quite satisfactory. However, I think the Feynman video linked below has great pedagogical value, and this is why I post this complementary answer. For the sake of completeness, here are some very closely related questions: Fire: Why is Carbon combustible and Why is Oxygen needed?, Why does combustion release energy?, How much energy is needed to make fire?, Relation of particle velocity, temperature, and reaction activation energy, Understand why combustion releases energy?. Finally, Is combustion a phase transition? is also relevant and its answers point to some good references.

Quoting Richard Feynman (video interview here), energy is released in combustion because you are releasing "solar energy" (if you are burning wood). I suggest spending 4 minutes to see the whole Feynman video because it starts from carbon and oxygen and goes down to the fact that, ultimately, we live in a Universe that is still very far from nuclear chemical equilibrium.

From this beautiful description of Feynman, you can easily extrapolate and understand the combustion of something that is not necessarily wood, or exothermic reactions in general... but also make the connection with deep questions like "the Sun as a reservoir of low-entropy": with the combustion of wood, this low-entropy energy is transformed in high-entropy energy.

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I think that what you are looking for is the energy at the molecular level. There are reactions where there is energy release as electromagneic energy, like in chemoluminescence. But the heat we get from combustion is due to the increase in the kinetic energy of the molecules of the products of combustion. If the reaction is exotermic, the potential energy of the reactant molecules is higher than that of the product. The difference is converted in kinetic energy, a little like a ball going down hill from higher potential to lower potential. The increase kinetic energy of the molecules means increased internal energy (which is unproperly called heat). The same mechanism applies to a nuclear reactor. The energy of the reaction goes primarily to the neutrons emitted during the reaction and the other products. Sometimes the energy released by the reaction may go into internal energy of atoms and these get excited on above the ground state. When they get back to the ground state they emit the extra energy as photons.

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The internal energy of the reactants is greater than the internal energy of the products in an exothermic reaction (chemical or nuclear), such as combustion. The decrease in internal energy causes an increase in kinetic energy of the products relative to the reactants.

For combustion, this increase in kinetic energy causes an increase in temperature of the air (heating) and emission of photons (light).

Due to Einstein, we know this decrease in internal energy is a decrease in the mass of the reactants through $E = mc^2$. See I don't understand why combustion releases energy? on this exchange.

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Yes heat is waves in the EM field (mainly IR) .... when you stand close to a campfire its mainly the IR that you feel. There are other wavelengths as well .... for sure visible orange and blue flame is also heat energy. There is UV but this is closer into the flame .... exciting electrons and causing a lot of molecular vibration which is kinetic energy heat.

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