Hello, in this video,
I'm going to talk about chain reactions. I'm actually going to start by just making
a general comment about reacting systems and then talk about chain reactions. So it's rare that a chemically reacting
system involves only one reaction. More typically,
there are numerous numerous reactions, in some cases thousands. And I have one reaction mechanism I work
with that has almost 25,000 reactions. However, there's certain
common characteristics among reacting systems that are helpful
in understanding overall behavior, and we'll say something about some of them. In this video, we'll talk specifically
about chain reactions because I'm, well and I should back up and
say I'm mostly interested in combustion. So I'll give some examples
from that field, and I'll start in this video
with chain reactions. Then I'll talk a little bit
about unimolecular reactions and then say something about
hydrogen oxidation, which is one of the simplest
reaction mechanisms in combustion. And then general hydrocarbon oxidation,
which is considerably more complex. So some chemical species are much less
stable or more reactive than others. Highly reactive species
are called radicals, and there were some examples
in the previous two videos, H atom, Molecule, oxygen atom. These are highly reactive and they play a very important
role in chemical mechanisms. And complex mechanisms involving radicals will sometimes display
chain reaction behavior. And most combustion reactions,
combustion and oxidation reactions
involve chain reactions. And we'll illustrate that with a simple
example, which is not from combustion. It's chlorine and hydrogen. But the reason that I'll illustrate it
with this is that you may have seen this in one of your chemistry classes,
your high school chemistry class or undergraduate chemistry class. I certainly learned about it
in undergraduate chemistry. And that's the chlorine hydrogen reaction. So if you take hydrogen molecule and
chlorine molecule and mix them together, they're gases
at normal room temperature and pressure, mix them together in a jar for
example, nothing will happen. But if you excite them
with a UV light source and just a flash lamp, like the flash on
your camera, they'll react explosively, meaning very, very rapidly,
and they'll convert into HCl. So it's UV light that causes the
dissociation of the chlorine initially. And so, if the chlorine dissociates,
it produces two chlorine atoms. And chlorine atoms are radicals,
highly reactive. And this is called a chain reaction,
a chain initiation reaction, because it produces radicals. We start with a stable molecule, and
all of a sudden we produce radicals. So once you have a chlorine, or
once you have chlorine atoms available, then they can attack hydrogen molecule. And they can remove a hydrogen and
replace it with a chlorine. So you've gone from one radical and one stable molecule into a product
molecule HCl and another radical. And then once you've produced a hydrogen, it can attack chlorine and
replace one of the chlorines, and you end up with a product molecule HCl and
chlorine, another radical. And these two reactions are called chain
carrier or chain propagation reactions, because the number of radicals
is unaffected by them. They keep producing, one radical goes in,
one radical comes out, maybe a different radical, but
you're still producing a radical. This whole process is ended eventually
by what's called recombination or chain breaking reactions. In this case, it's the recombination
of chlorine with chlorine, chlorine atom with chlorine atom,
to produce a stable chlorine. And it's a three body reaction
because if two chlorine radicals collide with each other, they won't
stick together unless some of the energy of the collision is removed by
involving another molecule. So this removes a radical and
slows things down. There are additional reactions in that,
but that's basically the picture. You have a chain initiation
reaction of some kind or reactions. You have some chain carrying reactions
that very rapidly produce products or tend to produce products. And then you have something
that ends all that. In combustion, ignition is controlled
by the chain initiation reactions. That's how a combustion reaction starts. The chain carrying reactions
are generally very, very fast. And as a result,
the radical concentrations are small, but they're important. Even though they have
small concentrations, they're important because their
reaction rates are very fast. And then there's chain
termination reactions. These are so-called
rate-limiting reactions, which means that the overall rate of
the global process is controlled by them. So that's it on this video. Thanks for listening and have a great day.