How is combustion energy released?

[avicenna]

Is it possible that in combustion, the chemical energy of reaction is released wholly through photon emission. Say as in simple burning of wood.

 

[osilmag]

Photon are usually emitted from electrons changing energy levels. The energy from combustion is released from the breaking of hydrocarbon bonds and forming carbon dioxide.

 

[Baluncore]

Is it possible that in combustion, the chemical energy of reaction is released wholly through photon emission.

Not when you burn iron slowly, at a low temperature. Then it forms a rust-red oxide that has higher volume, that forces things apart. The colour in reflected light changes, but it does not emit photons.

Maybe you need to define combustion.

 

[russ_watters]

The question sounds a bit unclear to me, but I was under the impression that most of the energy of combustion is usually in molecular motion. The combustion products are hot.

 

[hutchphd]

Say as in simple burning of wood

Think of a candle. There are at least two distinct regions of emission: blue and yellow. Thie blue light is direct emission from from electronic transitions as the vaporized hydrocarbons convert to water and carbon dioxide and particulate carbon. This hot mix of gasses and particulates is buoyant and flows upward. The particulate carbon is hot enough (and large enough) to glow as a black body and at >2000K producing a very bright yellow emission. Your eye is sensitive to the yellow.
Flame dynamics happen at many scales.

 

[avicenna]

The question sounds a bit unclear to me, but I was under the impression that most of the energy of combustion is usually in molecular motion. The combustion products are hot.

OK. I will restrict it to simply burning carbon in oxygen giving CO₂. Let's assume there is no initial kinetic energy in the carbon and oxygen molecule. Then, somehow they combine to form CO₂. In what energy form is the combustion reaction energy released if not wholly as radiation photon.

If your reply is molecular motion, can you elaborate with some details of chemical energy to molecular motion.

 

[avicenna]

Think of a candle. There are at least two distinct regions of emission: blue and yellow. Thie blue light is direct emission from from electronic transitions as the vaporized hydrocarbons convert to water and carbon dioxide and particulate carbon. This hot mix of gasses and particulates is buoyant and flows upward. The particulate carbon is hot enough (and large enough) to glow as a black body and at >2000K producing a very bright yellow emission. Your eye is sensitive to the yellow.
Flame dynamics happen at many scales.

Thanks. This is the closest I get; I will examine further. Can I say the chemical energy is converted first wholly to radiation photon.

[edit] If yes to photon, can it be general for all combustion (oxidation process ) reactions.

 

[DrClaude]

Let's assume there is no initial kinetic energy in the carbon and oxygen molecule.

Then you don't have combustion. You need a spark (or something similar) to initiate the process. What you get after is basically a domino effect of hot, combusting material initiating the combustion of adjacent material.

Can I say the chemical energy is converted first wholly to radiation photon.

Absolutely not! It is the other way around. The chemical reaction of combustion will result in hot, excited molecules that then d'excité by emission of radiation.

 

[tech99]

The atoms in a compound attract one another. If you pull them apart you are putting energy in; if you let them combine you obtain energy out. Combustion of carbon is a chemical reaction involving the making of bonds, so that we have energy out - it is exothermic. On the other hand the combustion of a hydrocarbon will involve breaking and making bonds but we have a net surplus of energy out. Photon emission is a result of high temperatures following the reaction.

 

[avicenna]

Then you don't have combustion. You need a spark (or something similar) to initiate the process. What you get after is basically a domino effect of hot, combusting material initiating the combustion of adjacent material.

Absolutely not! It is the other way around. The chemical reaction of combustion will result in hot, excited molecules that then d'excité by emission of radiation.

Can you explain this. Burning of firewood/coal will released chemical energy which is converted to heat which is basically kinetic energy of the molecules. The only mechanism which transfers kinetic energy between two molecules is collision. So which of the molecules first get the increased in kinetic energy and how.

 

[tech99]

This is called the initiation energy. We have to break a few bonds to initiate the reaction, as when lighting a fire.

 

[avicenna]

This is called the initiation energy. We have to break a few bonds to initiate the reaction, as when lighting a fire.

Can you be more specific. There is a continuous conversion of chemical energy to heat.

 

[russ_watters]

Can you be more specific. There is a continuous conversion of chemical energy to heat.

Note that the definition of "heat" is thermal energy transfer. Heat is not itself a type of energy. The conversion here is chemical energy to kinetic energy (i think for most fires...) and different chemical energy (excited electron state or ionized).

 

[tech99]

Once we start the reaction, by providing the initiation energy, using a match, for instance, the reaction is self sustaining. This is because, for a fire, the energy released by bonds being made is greater than the energy absorbed by bonds breaking.

 

[avicenna]

Please! Can someone tell other than the obvious. When you google for combustion, etc, you get hundreds of hits telling how in a wood fire or combustion, chemical energy is converted to thermal energy - whatever! But how? No one is telling the secret!

I have already said the only mechanism of kinetic energy exchange between two molecules is collision - no others. So please give some details how the chemical energy get magically transformed to thermal energy - or whatever energy you call it.

By just saying chemical energy gets converted to thermal energy explains nothing of the actual mechanism.

 

[Baluncore]

By just saying chemical energy gets converted to thermal energy explains nothing of the actual mechanism.

Fundamentally, a quantum step generates photons with a quantised energy, plus or minus a variable thermal broadening of the individual photon energy.
When that broadened photon provides energy to initiate a different quantum step, (also thermally broadened), there will be some thermal energy left over that is not needed.
The chemical reaction energy is quantised, but for spontaneous chemical reactions, excess chemical energy will be released as more heat.

https://en.wikipedia.org/wiki/Doppler_broadening

 

[hutchphd]

The mechanism that allows the molecules and atoms to maintain a stable configuration is called Quantum Mechanics. The system (say a candle in this case) is moving from higher energy states to lower energy states in a complicated way. The candle (wax +wick) had more energy than the final candle nib. The energy has been carried away by EM field and various constituent gas atoms and particulates which have left the area.

Please! Can someone tell other than the obvious. When you google for combustion, etc, you get hundreds of hits telling how in a wood fire or combustion, chemical energy is converted to thermal energy - whatever! But how? No one is telling the secret!

I have already said the only mechanism of kinetic energy exchange between two molecules is collision - no others. So please give some details how the chemical energy get magically transformed to thermal energy - or whatever energy you call it.

By just saying chemical energy gets converted to thermal energy explains nothing of the actual mechanism.

Quantum Mechanics for the system describes it all, with the complete detail immensely complicated. That is the magic and even the simplest system will be beyond your understanding
QED.

 

[avicenna]

The mechanism that allows the molecules and atoms to maintain a stable configuration is called Quantum Mechanics. The system (say a candle in this case) is moving from higher energy states to lower energy states in a complicated way. The candle (wax +wick) had more energy than the final candle nib. The energy has been carried away by EM field and various constituent gas atoms and particulates which have left the area.Quantum Mechanics for the system describes it all, with the complete detail immensely complicated. That is the magic and even the simplest system will be beyond your understanding
QED.

At least you say `the complete detail immensely complicated'. So I would not enquire further.

 

[Baluncore]

It takes heat to increase the rate of chemical reactions.
At absolute zero, if a photon were emitted, it could only go back to the quantised state it came from.
Without thermal broadening, the variety of chemical reactions we see, would not be possible.

 

[hutchphd]

So I would not enquire further.

I did not say that to dissuade your inquiry but rather to indicate there is no general complete answer to your question. As usual everyone lives with an approximation sufficient to his or her requirements and your requirements are not clear !
So the questions need specificity.

 

[avicenna]

I did not say that to dissuade your inquiry but rather to indicate there is no general complete answer to your question. As usual everyone lives with an approximation sufficient to his or her requirements and your requirements are not clear !
So the questions need specificity.

I have no complaints at all about your replies. Your comments are fair, that the one word "combustion" involves many levels of difficulties. I don't ask further as I have no knowledge of QM.

 

[DrClaude]

Quantum mechanics is definitely not necessary to understand combustion. The basic thing to know is that combustible molecules have an energy profile of the type

Starting at A, a molecule is stable, until it gains enough energy to reach point B, after which it can continue on to C, with the energy difference between A and C ending up as electronic, ro-vibrational, or kinetic energy. That extra energy, mostly through collisions, will push another molecule from A to C, and so on.

Looking deeper into it, combustion is complex because that simple picture must be replicated over many chemical species. Take for example the combustion of methane, CH₄. In the simple case, A in the diagram would be CH₄ + O₂, with C being CO₂ + H₂O, with the balanced reaction being

CH₄ + 2 O₂ → CO₂ + 2 H₂O

In reality, the reaction has the intermediaries

All hot intermediaries can relax by emission of radiation, mostly in the visible and the infra-red.

 

[TeethWhitener]

Quantum mechanics is definitely not necessary to understand combustion.

Seconded.

Let's assume there is no initial kinetic energy in the carbon and oxygen molecule. Then, somehow they combine to form CO₂.

No, they don’t. Look at @DrClaude ’s diagram in post 22. Just as a ball sitting still in one valley won’t roll into the second valley (ignoring QM), a chemical species in one valley needs energy input to get to the second valley. This is called activation energy. For combustion reactions, the activation energy is smaller than the potential energy difference between reactants and products (IOW, the hill between the valleys is less tall than the altitudes of the valleys). This means that, once the reagents get over the energy hill (activation barrier), they go down to the second valley and have enough energy (be it translational/kinetic, rotational, vibrational, etc.) that they can give some of it to other reagents to help them over that hill. This is what makes combustion self-sustaining.

 

[avicenna]

Thanks for the reply. I think my initial assumption of all "chemical energy" to photon is simplistic.

Someone at stack exchange brought up equipartition of energy. So the chemical energy would be distributed to: translational KE, rotational KE, vibrational KE and some radiations.

So simple burning of wood in air is not simple!

 

[DrClaude]

Someone at stack exchange brought up equipartition of energy. So the chemical energy would be distributed to: translational KE, rotational KE, vibrational KE and some radiations.

The equipartition theorem only applies to quadratic degrees of freedom, so it doesn't work for electronic energy. It also only works at equilibrium, which doesn't always apply (even locally) during combustion.

Also, "chemical energy" is the term used to describe the energy related to bond forming and bond breaking. In combustion, chemical energy is converted into the other forms of energy.