Where does the energy from combustion come from?

Question

I am aware that combustion reactions are exothermic. The energy stored in the chemical bonds of the products is less than the energy stored in the chemical bonds of the reactants. The difference in energies manifests itself as energy released into the environment. However, this sounds somewhat mystical. How does breaking bonds result in thermal energy, or greater kinetic energy of surrounding molecules? The most satisfying answer that I have seen so far is: the repulsion between like-charge molecules held in a covalent bond is constrained by the covalent bond; once something breaks that covalent bond, the molecules move away from each other, increasing their kinetic energy. Does anyone have further insights on this?

Answer 1

Better is to say "Energy released by forming bonds of combustion products is bigger than the energy needed to break bonds of combustion reactants.".

Particularly breaking $\ce{C-C}$, $\ce{C-H}$ and $\ce{O=O}$ bonds needs less energy than is released by forming $\ce{O-H}$ and $\ce{C=O}$ bonds. The nature of the released energy of chemical bonds is electrostatic potential energy being lower than for separated atoms. In other words, breaking the bond does not release energy, it consumes energy, like if you detach two magnets or two macro objects with the opposite charge. It is the forming of bonds that releases energy, via interaction with other molecules while the bond is forming. (*)

When a bond is being created, the bond gets very high vibration energy(**). It has to pass at least part of this energy somewhere else, otherwise, the bond breaks again. This is frequently done in a mechanical way, when moving atoms in this vibrating bond pushing other atoms or molecules, passing a part of their energy.

Such a bystanding molecule transforms this energy to its electron excitation or some form (translation, vibration, rotation) of molecular kinetic energy.

The increased kinetic energy of molecules after the reaction contributes to the increase of the average kinetic energy of molecules of the mixture being combusted. And, as consequence, increasing temperature as the statistical measure of the average kinetic energy (per degree of freedom) for very large samples of interacting molecules.

E.g. $$\ce{R-CH2-H(slow) + O(slow) -> R-CH2(fast) + O-H(fast)}$$

or

$$\ce{H(slow) + O(slow) + N2(slow) -> O-H(fast) + N2(fast)}$$

Note that

$$\ce{H(slow) + O(slow) -> O-H(fast) }$$

is not possible due to the momentum conservation law.

Some energy can be released as photons too. Some are emitted by excited molecular bonds ($\ce{C-C}$, $\ce{C-H}$), but most of the radiation in the context of combustion and flame is caused by black body thermal radiation of condensed carbon atom chains, soot or solid particles.

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(*) Explanation of quantum chemistry is the topic for thick textbooks, but illustration based on classical analogies would serve well. The proton and the electron in hydrogen atoms, even if in motion, have lower energy than if they were at rest but free. You need to provide energy to separate them, as they attract each other. Similarly as for planets in the Solar system. When two hydrogen atoms are bound in a hydrogen molecule, each electron is around other electrons to repulse each other, but also around another proton to attract each other. The mean squared distance electron-proton is significantly smaller than the mean squared distance electron-electron or proton-proton, so the total energy is even smaller than for separate atoms. Reaching states with a lower total energy of electrostatically bound systems is the source of chemical energy.

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(**) The energy of a hydrogen molecule - as an example - depends on the distance of hydrogen atoms. It decreases with shortening distance, reaches the minimum, and starts to increase again when atoms keep getting closer. As the result, there is pushing together if they are too distant and pushing away if they are too close. As the result, the hydrogen molecule is able to vibrate along the bond, as if both atoms were connected by a spring, trying to keep them at the particular distance = the length of $\ce{H-H}$ bond. It is just a coarse approximation as the behavior is non-linear. If too close, repulsion raises really steeply. If too far, attraction stops raising and starts decreasing toward zero.