Amen - RIP Glenn.
On Mon, Jan 27, 2020 at 3:05 PM Peter Fairbrother <peter@xxxxxxxxxx> wrote:
On 23/01/2020 20:56, Henry Spencer wrote:
Nitrous/acetylene, alarming though it sounds, actually managed to pass
safety tests for things like handling, which I would definitely have bet
against.
In a way I am not too surprised. Nitrous is unusual stuff.
The molecule is linear N=N=O but with a positive charge on the middle
nitrogen and the negative charge spread out between the nitrogen and the
oxygen on the ends.
The decomposition of nitrous gives N2 and O2 and a heck of a lot of
energy, about 85 kJ/mol when in gaseous form - the nitrogen triple bond
is one of the strongest known. At compressed liquid nitrous densities
the equivalent energy released is much higher, about 180 kJ/mol from a
quick BOTE.
The precise mechanisms of the reaction are not known, but they are known
to be complex (to put it mildly!). There are potentially at least one
first order and three second order rate-determining mechanisms - the
dominant one changes with density and temperature. It is however
generally thought that the most important one is N2O -> N2 + O* where O*
is a lone oxygen atom.
However we do know that the activation energy is very high, above 250
kJ/mol. The bonds in the molecule are strong, and the ends of the
molecules, being partly negatively charged, repel each other and prevent
close contact.
This means that in order to get nitrous to decompose you need a high
concentration of energy in one spot.
However, because the amount of energy given off is so high, and much
higher in mixtures with organic compounds (the C-O bond in carbon
monoxide is the only bond between two atoms which is stronger than the
nitrogen triple bond), the total amount of energy needed to start a
self-sustaining reaction is not large.
The requirement for a high concentration but low total quantity of
required energy to initiate a self-sustaining reaction means that a
nitrous/organic mixture can appear benign in tests where the amount of
energy used is substantial but the concentration of energy is limited;
but not be benign in use.
This is especially true when highly energetic organics like acetylene
are used, and the minimum deflagration radius (and detonation radius,
and DDT length) is tiny.
Nitrous is also a horrible long-lasting ozone-depleting and greenhouse gas.
Frankly, about the only justifiable use for nitrous is in childbirth and
colonoscopies. And dentistry, maybe.
Peter Fairbrother
(who can be a half-century behind the times when talking chemistry)
