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Author: Subject: Autoignition temperature of mixed gases
International Hazard

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[*] posted on 10-11-2019 at 16:45
Autoignition temperature of mixed gases

Vaguely like sulfur lowering the ignition temperature of gunpowder or two metals that when mixed the melting point becomes lower, are there any examples where two flammable gases with roughly equal ignition temperatures, work synergistically to become a mixture that has a much lower autoignition temperature in air?
Not like hypergolic reactions but stable mixtures such as hydrogen and say methanol vapor just for example, 500 C and 470 C respectively.

As an aside but not related ...

"The presence of “inert species” for example nitrogen or solid surfaces pose further
complications. They are generally not recognized to participate in the reaction, but may have a catalyzing role in some reactions and may affect the process as heat sinks or alter diffusivity. The replacement of nitrogen with helium in a methane-air mixture will for instance triples the burning velocity." (Glassman 1987).

[Edited on 11-11-2019 by Morgan]
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[*] posted on 10-11-2019 at 17:10

For what it's worth, He has about 2.6 the thermal velocity of N2, at the same T.

I've wondered what is known about the ignition process. Suppose you have a mixture of H2 and O2, for example, in some proportion that is capable of ignition. Now if some volume of the gas is heated above the ignition temperature, say by a match, then you get an explosion, that is, the reaction initiated by the hot spot propagates.

But what is the minimum size of a hot spot (in such a situation) that will propagate? A microgram? A nanogram? A single atom? Surely not the latter, otherwise cosmic rays would ignite the mixture.

In some cases UV light will ignite a mixture (H2 + Cl2, I am told). But again the questions, how many photons? When the final ignition takes place, is it due to a single photon? Or do they have to gang up somehow?

Obviously the answer depends on the details, but I'd be curious to know what are typical ranges.

Any other SF Bay chemists?
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[*] posted on 13-12-2019 at 14:31

My recollection is that it is red light from an LED that explodes a pure mixture of H2 and Cl2 with the propagation of a chain reaction. See my prior comments at .

Unfortunately, the chain reaction can be halted in the presence of a small amount of oxygen.

Caution: I have posted on SM somewhere a link to a video where it is quite evident that the H2/Cl2/Light chain reaction is quite powerful!

[Edited on 13-12-2019 by AJKOER]
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[*] posted on 13-12-2019 at 19:44

Again AJOKER is incorrect. Blue light is required to break the Cl-Cl bond and explode H2/Cl2.

Source: I have done this. A transparent balloon of H2/Cl2 is stable under light from an incandescent bulb but explodes when illuminated with a blue laser pointer.

As below, so above.
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[*] posted on 13-12-2019 at 20:49

It even explodes when flashed by a flash from a camera!
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