[Nick23]

Hello, I couldn't find the sandbox section, so I'm posting here. Are there any tables or formulas helping to calculate how many photons are emitted during the chemical reaction ? For example if I set alight a chip of Magnesium, is there a constant of how many photons per unit per time will be released during oxidation ? Thank you.

[Corribus]

You can treat emitted photons like a product in a chemical reaction, in which the usual rules governing reaction rates apply. So, then, to answer your question, you need to think about how you determine the amount of products formed during any chemical reaction.

[Nick23]

You can treat emitted photons like a product in a chemical reaction, in which the usual rules governing reaction rates apply. So, then, to answer your question, you need to think about how you determine the amount of products formed during any chemical reaction.

Thank you for the answer. It's been years since I studied chemistry in school, but from what I gather you can calculate the energy released in heat in J/mol. But is the light released coherent with these values ? Is there a proportion of how much heat and how much light gets released ? If I have a thin film of Magnesium over the layer of optic fiber, is it possible to calculate how many photons can be harvested when the reaction starts ? Sorry if the questions sound stupid.

[Corribus]

I apologize, I should have read your question more carefully. The light produced by burning mangesium is not a direct product of the reaction itself, but rather a biproduct of the high temperature at which magnesium burns. Therefore it's not so simple a problem as I previously described.

It's going to be a hard problem to solve in the way you want, for a number of reasons. The biggest of these might be that the photons produced take on a distribution of energies. You can specify that probability distribution pretty exactly, but accounting for all that energy and then transcribing it to determine the total number of photons is going to be a difficult problem.

I'd probably start with the heat of combustion of magnesium. From this you can determine the total amount of heat produced by combusting a certain mass of the metal. Then you can determine the spectral distribution of photons. Quantifying the # of photons is the difficult part. Aside from the energy distibution and the quantity of magnesium burning, you also have the kinetic aspect to worry about. I'd have to think about the best way to do it.  There might be some ways to simplify the problem and give you an estimate - how precise of an answer do you need?

[Nick23]

how precise of an answer do you need?

It's more of a general interest, so it doesn't have to be precise at all.

You can specify that probability distribution pretty exactly

Could you suggest any readings on how this is done ? I would leave the spectral distribution for latter times, when I understand the basics ). Also, what is kinetic aspect ? Is it how the movement of reacting materials influences the way photons travel ?

[Nick23]

"The heat of formation of magnesium oxide is -601.83 kJ/mol.  When magnesium burns, approximately 10% of the energy of combustion occurs as light, a value unapproached among known transformations of energy used in the production of light."
 What would I need to transform 10% to photons ? Are there any generally known chemical reactions that produce even more light over a short time ?

[Borek]

What would I need to transform 10% to photons ?

For a rough estimate it is enough to do the energy balance - 10% of the energy produced, average energy of the visible light photon and you are done.

That won't be accurate as magnesium burns so hot it emits in UV, so there are many more photons emitted (and if you include IR, even more).

[Corribus]

If you know the total amount of energy, you should just be able to perform and integration over the energy probability distribution to get the total number of photons of each energy produced. This of course assumes the 10% statistic is correct, and that it (and the temperature) is a constant value over the course of the reaction.  Experimentally it's easy to determine the % of energy converted as light - you should be able to determine total energy released by basic thermodynamic considerations, and measure the amount of energy released as heat by calorimetry. The difference will be predominantly light. (A sort of reverse procedure is able to be used to measure the absolute fluorescence yield of a chromophore, but it's a very difficult measurement as I understand it, at least compared to more measures that measure the yield relative to a standard.)

Are there any generally known chemical reactions that produce even more light over a short time ?

By total light energy output, and total photon output, it will be difficult to beat burning magnesium. But it's a hard question to answer generally, since it's not known what you are after. For example, burning magnesium is a continuum spectrum. Other reactions may produce less total photons, but have better monochromicity, say. (All photons having same energy.)