Chemistry of acetylene.

I have always been puzzled by the way that acetylene produces a higher
flame temperature than propane or natural gas, even though it's total
heat output is lower. Also, I have never been able to understand the
nature of the explosive reaction that can occur when acetylene exceeds
15 psi. This reaction is evidently able to occur without oxygen. I
know a little bit about about chemistry, and from what I understand,
it seems to me that the triple carbon bond in the acetylene molecule
would make it more stable, rather than less, as it seems to be. Does
anyone out there have a good explanation of why acetylene behaves the
way it does? Thanks much!
Reply to
matthew
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I can't help a great deal as I am not a chemist but I did pose this sort of question to one of my chemistry teachers and he thought the issue of the instability was due to the 'stress' the triple bonds joining the carbon atoms were under. basically they are not at the angles they would like to be at but rather sort of forced and so likely to break easily.
I had been meaning to ask a similar question, I have always been told that acetylene is likely to spontaneously decompose above about 30 psi so why doesn't it do so when a new bottle registers 250psi on the gauge. I am aware of the dissolving in acetone but I presume at some point within the regulator gaseous acetylene is present at a high pressure to be regulated down.
Reply to
David Billington
Thanks for the reply!
Yes, I have also been curious about that same question.
Reply to
matthew
Maybe it's because there's no oxygen inside there to react with?
Grant Erwin Kirkland, Washington
Reply to
Grant Erwin
"Grant Erwin" wrote: Maybe it's because there's no oxygen inside there to react with? ^^^^^^^^^^^^^^^^ I understand that acetylene becomes unstable and dangerous at pressures above 15PSI, which is why they dissolve it in pulpy liquid mix inside the cylinder. There is no oxygen there, so that can't explain it. Why it doesn't explode in the space at the top of the cylinder, in the regulator neck, etc., is probably due to the very small volume. Possibly it's a heat transfer question--small passages have a larger relative area to get rid of heat, reducing the temperature rise from any reaction that tries to start. If the temperature doesn't go up, the reaction rate doesn't go up.
I believe the explanation for the instability of the triple bond that David B. gave is correct. Chemists draw the carbon atom as a "C", with bonds going "north, south, east and west." This is not a true representation of the atom, but it will do for discussion. In order to form a triple bond, the bonds have to move from their relaxed position to join similar bonds on the other carbon atom, so they are stretched out of shape, and therefore more easily broken.
Reply to
Leo Lichtman
There is cork inside the tank. This is why you are supposed to run the tank in an upright position. Lay it over and it might clog. Some did in Vietnam while I was there. Makes it hard to use.
This keeps it stable. Just like dynamite.
Reply to
Bob Noble
I know that, IIRC kapok and other materials are also used. The question was about when the acetylene gets out of the filler medium and acetone and into the regulator. I presume it is under the pressure indicated on the pressure gauge, 250psi on a new bottle, so why doesn't it break down before it is regulated to a safe low pressure.
Reply to
David Billington
"David Billington" wrote: (clip) why doesn't it break
^^^^^^^^^^^^^^^^^^^^^^ Let's see whether I can clarify what I posted earlier. (This is based on college education, not actual direct knowlege, so don't mistake it for gospel.) The reaction in gaseous acetylene produces heat. The heat causes the reaction to accelerate, which leads to a greater production of heat, which causes more heat, aand so on. If the gas is contained in very small spaces, the heat is conducted away and the system reaches a stable equillibrium. If you increase the volume of gas, its ability to lose heat is decreased, and the equillibrium temperature goes up. At some point it reaches a temperature at which it is not stable--the temperature starts to run away, and you get a rapid reaction--read explosion.
I assume that the bottle with dissolved acetylene, and the spaces filled with undissolved acetylene are kept small enough so the unstable heat transfer condition is not possible.
Reply to
Leo Lichtman
Yes, that makes sense. I am not a chemist, but I know a bit about chemistry and that explanation sounds quite reasonable. Thanks!
Reply to
matthew

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