So comrades, welcome to the lecture. What I am going to do
now, we’re going to begin with an experiment to illustrate
this point that the oxygen molecule, O2, has unpaired
electrons.
Now we’re outside just to show you the reactivity of this
lovely stuff here, liquid oxygen. So you can see wonderful
liquid oxygen is a nice blue colour. So over here on the table
we’ve got a tin tray which we’re going to use to contain the
experiment that I’m going to show you next. So we’ve got
our favourite liquid oxygen. Strongly oxidising; very, very
reactive, and here I’ve got some cotton wool. So this is the
same stuff we use to take make-up off, perhaps our partners
or our girlfriends or our wives, and this is the same stuff we
use to wipe our babies’ bums.
Oxygen is an interesting element because it exists in two
forms: the taught form which we breathe all the time is O2,
two oxygen atoms joined together; and there’s another form
called ozone, and which has three atoms arranged like the
letter V, or letter V this way up.
Oxygen has unpaired electrons within the molecular orbitals,
in the atomic orbitals around the structure, and these
unpaired electrons give rise to colour and that colour is blue.
So blue is often perceived to be the colour of solvated or
unpaired electrons, and you can see here the lovely blue
colour. Now lots of chemists are really scared when they see
the blue of liquid oxygen, because liquid oxygen is very, very
reactive. Generally you only make liquid oxygen by mistake.
Not like today when we’re making some on purpose. And if
you’ve made it by mistake in an uncontrolled way, where
there might be organic molecules perhaps simple organic
chemicals like hydrocarbons, grease or perhaps a highly
elaborate compound then, they tend to detonate and they
tend to cause very energetic experiments.
Yeah it’s very fluffy it’s not very reactive. It doesn’t really
burn, it cinders, might smoulder. But here we go, we’re going
to pop it in the tray.
So we begin with Neil’s very high-tech piece of equipment,
which is a test-tube on a piece of string. What we’re going to
do is to put some liquid nitrogen in here. You can see liquid
nitrogen – well you may not see – is colourless. Ok? So this is
liquid nitrogen, and what I hope you can see is that this
magnet, which is a pretty good magnet. Let me just show
you here with my keys, that they stick pretty strongly to the
magnet. The magnet has absolutely no affect at all on the
liquid nitrogen. I can pull the magnet and nothing happens.
And then we’re going to fill up all of the void space, so all of
the space in the cotton particles with oxygen. So you might
want to stand back after this bit Brady. So here we go, here’s
our liquid oxygen. Do you want to come in and zoom in on
me?
It’s ok, I’ve got it zoomed nicely.
Ok, so here you can see the lovely blue liquid oxygen colour,
going into the cotton wool particles.
Let’s try the same thing with liquid oxygen. Careful of my
shoes…
So here we have a match on a stick, and I think you can see
its probably alight, so we have our fire, we have our oxygen,
and we have our fuel. Now let’s see what happens when we
put them all together.
The liquid oxygen is a very nice blue colour today. So now
let’s try, here’s the magnet again, and if I take the liquid
oxygen you can see I can pull it right up here. It’s not as
magnetic as my keys, but you can see there’s a really big
difference. It sticks to the magnet. So you can see it really is
magnetic.
You ready? Ok. Woohoo and there she blows!
And ozone is much more reactive than oxygen. So near the
earth’s surface, where you and I are at the moment, ozone is
really quite dangerous because if you breathe it in it can start
reacting with any sort of molecule that has bonds between
carbon. But in the upper atmosphere, ozone is extremely
important because it absorbs ultra-violet light that comes
from the sun and stops this ultra-violet light attacking the
molecules in biological species on the surface. If it wasn’t for
the ozone in the upper atmosphere all of us would be, if not
dead, certainly very much less comfortable than we are at the
moment.
So Neil’s now connecting up a piece of tube to oxygen
cylinder and he is going to fill up this gas jar with oxygen and
we are going to do a really classic experiment: the one that
everyone learns at school and tests for oxygen, which is to
relight a glowing splint. So we have a traditional gas jar full of
oxygen. So here you can see we are burning a splint. Ok, so
this is the wood burning in excess oxygen around us, ok. So
what we are going to do is we are going to take the splint out
so it is just glowing. And if I blow on this splint you can see it
gets brighter. Now we are going to try and relight that splint
by putting it into a very oxygen-rich atmosphere. So we’ll
move over to the jar and we’ll put in our glowing splint and
you can see the flame comes back to life. So that’s a test that
everyone learns for an oxygen-rich environment.
What happened? What made that happen?
Well the increased oxygen content/concentration reignites and
reinitiates that oxidation of the wood or the burning of the
wood. So we’ll do that again. So there we go: flame again.