Jeff Liebermann wrote in message . ..
On 20 Mar 2004 23:28:38 -0800, (John Michael
Williams) wrote:

snipped lots of good stuff

I think if I can see the spark, it can ignite gas vapor,
provided the flame had a path out of the gap.

I beg to differ. The ignition of a gasoline oxygen mixture requires a
specific amount of energy to ignite. Anything less will not produce
the requiste chemical reaction. Think spark plug heat ranges and glow
plugs in model airplanes. I'll grind the numbers if you want, but
it's now midnight, I'm tired of waiting for Windoze update, and I'm
going home.

The ignition of a gaseous oxygen-gasoline mixture, or a (potentially
more sensitive) hydrogen-oxygen mixture does require a specific
minimum amount of energy, which depends on the partial pressures of
the oxygen and the fuel, and - IIRR - the partial pressures of any
inert diluent gases around.

Lesser amounts of energy can induce the requisite chemical reaction,
but the reaction will fizzle out, rather than providing enough energy
to ingnite the surrounding shell of a gas mixture and produce a
self-propagating flame front.

The controlling relationship is between the volume of the sphere in
which the reaction is first initiated, and the surface area of that
sphere - if the intial volume is too small, not enough energy is
released to heat the surrounding shell of gas to the ignition
temperature.

Once you've got the basic idea,the thermodynamics is pretty
straightforward.

I had to work through the equations many years ago for an experiment
intended to monitor the process in which one of the "Dewar benzenes"
converted itself to normal - Kekule's - benzene, which is an
enormously energetic process, involving about an order of magnitude
more energy per molecule than you get out of TNT and PETN. I really
didn't want to blast my experimental apparatus to smithereens.

When I went through the calculations with my supervisor, he pulled a
very long face - the motivation for the experiment had been some
unexpected flashes of light seen when a dumb organic chemist had
released small drops of liquid "Dewar benzene" into a hot cell, and my
calculations made it clear that the flashes of light were just thermal
radiation from a hot plasma, rather than fluorsecence from from an
electronically excited state of Kekule benezene, which is what my
supervisor had been hoping for ...

For the difference between Dewar benzene and Kekule benzene see

http://www.chemsoc.org/exemplarchem/...enzenering.htm

-------
Bill Sloman, Nijmegen