Ask A Scientist Chemistry Archive

  
name         Dan F.
status       educator
age          30s

Question -   If acetylene has a flammable range of 2.5%-100%, how does
it burn at 100% concentration without an oxidizer?
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My resource CRC Handbook of Chemistry and Physics gives the flammability
limits of acetylene in air as: 2.5 to 80 % on a volume/volume basis. The
reason you might find the number 100% is that acetylene gas is unstable and
will explode without any oxidizer at pressures of 2 atm or more; however,
that is an entirely different reaction than its combustion. Initially,
benzene is formed from 3 molecules of HCCH; however, the reaction becomes
uncontrolled and soot (carbon) and who knows what else is formed. It is not
an easy reaction to study because of its instability.

Vince Calder
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Burning or combustion always means reacting the substance with O2 or some
other oxidant. I believe the data you are quoting assumes ambient air.

Greg (Roberto Gregorius)
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I think somebody may have exaggerated.    Or maybe they have another 
meaning in mind.
The table of inflammability limits in my CRC reference book says 2.5% to 80%.

But, on the conciliatory side, that is for combustion with air.
There is also its self-reaction.

Acetylene is very capable of reacting with itself, making bigger molecules 
of the same average composition, and generating substantial heat doing so.
"Hazardous polymerization" it's called in Materials Safety Sheets.
That triple-bond in the middle is pretty unstable, has much higher energy 
than the 3 single bonds it could be if it rearranges.
So 100% Acetylene is always very dangerous, whether as liquid or gas.
That's why Acetylene in a welder's bottle is diluted with Acetone and 
perhaps a solid absorbent.

Personally, I have not seen or thought through what this self-reaction 
looks like when it happens.
Perhaps it can look like fire or an explosion.   Certainly it is no help 
to a fireman.

Cordially,
Jim Swenson
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