Making Polymers (GCSE Chemistry)


Uploaded by freeeschool on 04.09.2012

Transcript:
GCSE Chemistry – Making Polymers
Hello and welcome to this video about Making Polymers. This really follows on from the
other three videos done about Crude oil, Cracking and the Alkenes. What we are going to look
at is how the alkenes can be used to make polymers and by polymers, we often refer to
plastics for this particular topic. Let’s have a look and see how this works.
Here, I have got two molecules. Each one of these is an alkene that we have met before.
The alkene is actually ethenes. We got two ethene molecules side by side. Now, we remember
from a previous video that they have double bonds within their structure. These black
dots are carbon atoms and you can see the two carbon atoms are joined by a double bond.
Now, this double bond is quite important because it allows the ethene molecules to react with
other ethene molecules. The way this happens is, under the right conditions, if you bring
these molecules close together by increasing pressure and increasing temperature to increase
the speed of reaction, what happens is this double bonds can split and neighboring ethene
molecules can join.
What happens is you end up with a longer chain and if you have enough ethene molecules, they
can react to make much longer chains. This we described as a polymer. The individual
ethene molecule that we had to start off with is described as a monomer. A monomer basically
means single unit. A polymer means many units joined together. This process is called “polymerization”.
So you are polymerizing ethene to make a polymer and for this particular example, ethene becomes
polyethene. That’s a type of plastic that this becomes when you make this. Now, the
way we explain this reaction is as shown in this diagram, but we don’t actually just
two or three ethene monomers or two ethene molecules reacting. We often have up to hundreds.
But first, to your right, described to me if I have 200 ethene monomers, what would
happen? How would they crack? You could actually go to the process of drawing out 200 ethene
molecules and joining them up in the second diagram, but obviously, that would be a waste
of time. What we can do is we can just summarize it like this.
What we can say is here’s our ethene. If I have n number of ethene and any number of
this ethene molecules, what happens is they can join together in a long chain and these
single units as shown in brackets will appear n number of times. So just to give a number
in there to make it a lot easier to understand. Imagine I said up to two hundred ethene monomers.
You can draw out 200 and then draw a big long chain with 200 ethene monomers in it, but
a simpler way is just to split up this double bond in here and show what each part looks
like and we know that it repeats 200 times. So we end up with a long chain of these repeating
units that repeat 200 times. This is just slightly more technical way of showing how
this reaction happens. You should be aware what these numbers and what this n means in
these diagrams.
The only thing that we didn’t cover was that you might sometimes see monomers that
are different and might scale a little bit to say that although there are many, I haven’t
dealt with a monomer that has a different group. But if it is a monomer, we know that
it can be a repeating unit when we make the polymer.
Here I got three of them. So this is actually propene, I think. We got three propene molecules
and they can join together and the polymer would look like this. We’d have single unit
there. As you can see, it is repeating over and over. It is just the CH3 at the top there
that you need to highlight. All you need to know really is that it repeats, just as in
the last one here. It was simpler to have simple hydrogens and carbons, but sometimes,
you do get different groups on the ends of the bonds here. In a similar way, we can say,
“Well, this can react n number of times if we have n units of this monomer and we
polymerize, then we’ll end up with a long polymer chain which will have that number
of units in its structure.
That’s a brief outline of polymerization. It’s used to make plastics and those kinds
of materials. The substances that you get, the raw materials often come from crude oil
and you probably need to think about the implications of using these polymers and where they come
from and how you can get rid of them and whether they’re renewable or non-renewable and all
that kind of stuff.
That’s really a topic for another video. But for now, I hope you understood what’s
going on and see you again soon.
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