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From what I understand (please correct me if I'm wrong), combustion is an exothermic oxidation reaction where, for example, a fuel like methane is combined with oxygen, i.e. CH4 + 2O2 → CO2 + 2H2O. My question is in this reaction, in what forms is the energy released? I know it releases light and heat, but how is the heat created? Is all the energy released as photons, which then increase the kinetic energy of the surrounding molecules? Does the reaction intrinsically cause the kinetic energy of the molecules to increase somehow?

The reason I'm interested is I'm curious about what would be the most efficient way of capturing all of the energy released from this reaction. E.g. with some combination of heat exchange and radiation capture.

matt_black
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JBaczuk
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    https://chemistry.stackexchange.com/questions/55914/how-is-heat-physically-released-in-an-exothermic-process https://chemistry.stackexchange.com/questions/119093/how-does-the-energy-released-during-a-bond-formation-typically-manifest-itself-o – Mithoron May 05 '22 at 16:06
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    But if you really want practical side then https://en.wikipedia.org/wiki/Fuel_cell – Mithoron May 05 '22 at 16:09
  • If the real question here is the one in the last paragraph, I suspect editing the title would make this a much better question. The title is generic; the last paragraph is interesting. – matt_black May 06 '22 at 08:25
  • My back-of-the envelope calculation says ~10^4 times more thermal energy than light energy is released by burning methane. The thermal output of a stove-top burner on high is ~2 kW. I'd estimate the light energy is ~< 0.1 W. – theorist May 06 '22 at 08:29
  • @theorist do you consider "light energy" to be all of the EM radiation or just the visible portion of the spectrum? – JBaczuk May 09 '22 at 16:30
  • @JBaczuk Ah, good point. I was considering only visible light, not other portions of the EM spectrum. And IR emissions could be substantial. Though now I'm curious how much of the thermal energy released by the combustion of methane is in the form of IR. I suppose that would depend on the combustion conditions. – theorist May 09 '22 at 19:19

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There are alternatives to combustion as a way of releasing energy but only in specific reactions

In a typical combustion reaction (or any exothermic reaction) it is hard to control how the energy gets released. The reacting molecules bump into each other, react with each other and the resulting excess energy is distributed rapidly in a variety of ways. Some goes into vibrational energy, some to kinetic energy, some into electronic transitions (hence light). But however the very specific reaction starts off, the energy is rapidly redistributed across many other molecules (other molecular interactions happen frequently even in the gas phase and result in vibrational or kinetic energy being spread around other, neighbouring, molecules in ways that end up as "heat").

This makes it hard to modify how the excess energy is extracted from the system (but, having said that, many boilers or engines are very efficient at extracting heat energy).

Occasionally, though, it is possible to drive a reaction down a path where how the energy is extracted is not primarily heat. Trivially, this is what glow-sticks do. But the amount of energy there is small and it isn't even obvious that the majority appears as light. But it is also what Fuel Cells do.

Fuel cells use moderately complex systems where a catalyst on a surface can encourage the energy to emerge as electricity. The simplest are based on the reaction between hydrogen and oxygen. We could burn those gases to get mostly heat. But a fuel cell encourages the gases to combine to give water but instead of producing most of the excess energy as heat, capture it as electricity which can be directly used to drive machinery without needing a turbine or generator to transform heat into electricity. And some, more complex, fuel cells can use methane.

Most fuel cells rely on complex setups and precious metal catalysts to encourage the reaction to happen on a surface where instead of heat, (and simplifying a lot) electrons can be captured and used to drive an electrical circuit.

This can be more "efficient", but the cost of the devices and their inflexibility compared to heat-driven generators means they are not usually a cheaper way to generate electricity. Yet.

matt_black
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    The king of this sort of process would seem to be cellular respiration, which has an efficiency of 80%-90% even after taking the elecrochemical potential it generates and converting it to chemical energy. For comparison: Steam-electric power plant: ~30% - 50%. Combined cycle gas turbine electric power plant: ~50% -60%. Fuel cell: ~50%-60%. https://www.nature.com/articles/s42003-020-01192-w – theorist May 06 '22 at 08:23
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All the chemical energy comes from the relative positions of electrons being attracted to nuclei. Efficiency depends on the path that the energy is transferred. Efficiency is an engineering concern; Making everything work right electrically and mechanically. Thermodynamics just does its best to determine what is possible. Thermodynamics tells us that to get the maximum work out of a heat engine the temperature difference must be a large as possible and the final T as low as possible. [The lowest practical T is ambient.] [By the Way that is one reason Global warming is giving us problems; We are raising the ambient T.] The maximum work possible from a process is given by the Free Energy Function: G = H-TS. This means that energy must be lowered and entropy increased to do work. If one looks at it closely the only way we can survive as a functioning entity is to maximize the increase of entropy of the Universe that we cause. We must use LOW Entropy energy as efficiently as possible and Expel to the Universe High entropy Energy maintaining a reasonable temperature to function properly.

Back to the first sentence! An electrochemical potential difference is the maximum work that can be gleaned from a chemical process. Delta G = EnF. So a chemical cell gives the most hope of using chemical energy efficiently. The battery people are working on this. The only other method of high efficiency is a simple photochemical process the emission or capturing or of a photon or the transfer of an electron between energy levels by emission or capture of a photon. Even that interacts with the motion of molecules. That gives us a clue [I must admit this was in the back of my mind, another Commenter stated this recently and I am giving my incomplete thoughts here If I can find the comment I will quote it]. Transfer of energy relies on conservation of momentum and total energy in every collision. Collision between molecules involves distortion of the electron orbitals. The idea that the distortion resulted in differential infrared emission and absorption between the different bonds makes sense both as a means of energy transfer by conduction and as the cause of black body radiation. Energy is transferred by transfer of electrons to lower lower energy orbitals because the nuclear attractions in some chemicals are greater than others. To do work this energy must be converted to electricity thru an electrical cell or to heat by intermolecular collisions that lower the quantum state energy into thermal energy, heat and the difference in heat content exploited [less efficient], any energy loss by direct emission of photons is sending possible low entropy energy to the universe and should be contained.

jimchmst
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