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Atraveller
2010-Jul-07, 06:14 AM
What happens when you pass a 2.45GHz signal through a concentration of Hydrogen? Will it become plasma?

Ivan Viehoff
2010-Jul-07, 11:16 AM
2.45GHz is the frequency of a microwave oven. The food I eat has a lot of hydrogen in it, in the form of water, fats, carbohydrates and proteins especially, which does not normally seem to become plasma when I heat my meal up in the microwave. I suppose if you heated my dinner up sufficiently some of it would become plasma, as for example occurs in a flame. Food can catch alight in a microwave oven, a phenomenom which occurs especially with foods high in stuff like raisins. You have to be especially careful heating up British Christmas pudding.

If it was specifically molecular hydrogen you had in mind, then that is an apolar molecule, so that would tend to be unaffected by the microwaves, whose main effect is dielectric heating. Microwave ovens are most effective with OH bonds, though because it works on rotation solid water is affected much less than liquid water. Poly-alcohols such as glycerol and sugars (in solution) are very strongly affected.

korjik
2010-Jul-07, 01:53 PM
No

AriAstronomer
2010-Jul-07, 02:16 PM
very nice explanation ivan, it's been a while since I reviews my high school science text book haha. It's often difficult to put a 'name to face' when it comes to frequencies. Didnt know the microwave oven is 2.45GHz.

dgh64
2010-Jul-07, 02:36 PM
Hmm... interesting.

So, what would be the ideal frequency for turning molecular H2 into plasma?

Atraveller
2010-Jul-07, 11:59 PM
If it was specifically molecular hydrogen you had in mind, then that is an apolar molecule, so that would tend to be unaffected by the microwaves, whose main effect is dielectric heating. Microwave ovens are most effective with OH bonds, though because it works on rotation solid water is affected much less than liquid water. Poly-alcohols such as glycerol and sugars (in solution) are very strongly affected.

Thanks Ivan, Great explaination - and was what I thought - It was molecular hydrogen I was wondering about.

Dgh64 does ask an interesting question - What frequency would strongly affect molecular hydrogen? Would it be true to call that a resonant frequency?

Geo Kaplan
2010-Jul-08, 02:29 AM
Thanks Ivan, Great explaination - and was what I thought - It was molecular hydrogen I was wondering about.

Dgh64 does ask an interesting question - What frequency would strongly affect molecular hydrogen? Would it be true to call that a resonant frequency?

I don't recall the H-H bond strength (and I'm too lazy to look it up), but let's guess that it's within a factor of 2 of the ionization energy of a single hydrogen atom. In round numbers, let's posit that it's around 25-30eV. That corresponds to a frequency of about 7PHz, or somewhere in the UV range (~10x the frequency of blue light).

It would be ok to call it a resonant frequency, but be careful. There are various vibrational modes (rotational, linear, etc), each with its eigenfrequency. But the bond strength gives you a valuable calibration on what neighborhood you need to be in for strong interactions to take place.

korjik
2010-Jul-08, 03:19 AM
I don't recall the H-H bond strength (and I'm too lazy to look it up), but let's guess that it's within a factor of 2 of the ionization energy of a single hydrogen atom. In round numbers, let's posit that it's around 25-30eV. That corresponds to a frequency of about 7PHz, or somewhere in the UV range (~10x the frequency of blue light).

It would be ok to call it a resonant frequency, but be careful. There are various vibrational modes (rotational, linear, etc), each with its eigenfrequency. But the bond strength gives you a valuable calibration on what neighborhood you need to be in for strong interactions to take place.

Bond strengths are around 1 eV.

Geo Kaplan
2010-Jul-08, 03:25 AM
Bond strengths are around 1 eV.

I don't think that can possibly be right. If it really were around 1eV, it would be opaque to near-IR, which hydrogen gas most certainly is not. I also expect the covalent bond to be stronger, not weaker, than the single-atom ionization energy of 13.6eV.

But if you can cite an authoritative source to the contrary, I am happy to be educated.

Nick Theodorakis
2010-Jul-08, 04:12 AM
H-H bond energy is 436 kJ/mol or 104 kcal/mol:

http://en.wikipedia.org/wiki/Bond_energy

(yeah, I know, wikipedia, but those sound about right, and other sources agree)

This page:

http://en.wikipedia.org/wiki/Joule_per_mole

claims a kJ/mol is about .0104 eV per particle, so that would make it around 4 and half eV for an H-H bond.

Nick

Geo Kaplan
2010-Jul-08, 04:24 AM
H-H bond energy is 436 kJ/mol or 104 kcal/mol:

http://en.wikipedia.org/wiki/Bond_energy

(yeah, I know, wikipedia, but those sound about right, and other sources agree)

This page:

http://en.wikipedia.org/wiki/Joule_per_mole

claims a kJ/mol is about .0104 eV per particle, so that would make it around 4 and half eV for an H-H bond.

Nick

Thanks, Nick. I'm glad to see confirmation that the numbers do correspond to UV, although I see that my intuition about the covalent bond strength is wrong. Rather than ~double the ionization potential, it's more like one-half. I'll figure it out.

Thanks again!

Nick Theodorakis
2010-Jul-08, 02:28 PM
Thanks, Nick. I'm glad to see confirmation that the numbers do correspond to UV, although I see that my intuition about the covalent bond strength is wrong. Rather than ~double the ionization potential, it's more like one-half. I'll figure it out.

Thanks again!

My guess is that it's the difference between removing an electron altogether vs. moving them from a sigma bonding orbital to their s atomic orbitals. As for turning molecular hydrogen to plasma (as opposed to just breaking their bonds) I suspect the numbers would come out different.

Nick