So in the last Exhibition chemistry we took a deep dive into a microscale
version of the traditional cracking experiments. If you haven't seen that,
make sure you go back and watch that before you watch this video, because
everything we're doing today builds on that. If you have, then stick around, we're
going to be making one of these little catalyst tubes, which is going to open up
a massive range of opportunities for you to demonstrate some really cool chemistry.
Let's have a look.
So about a year ago, I promised that we would be returning to Bruce Mattson's fantastic microscale gas chemistry techniques when
I used it to create some chlorine and then react it with some peroxides
generating the singular oxygen and it's fantastic red chemiluminescence.
This time the lights are staying on and we're actually going to be generating some
propene gas and then hydrogenating it using one of the catalyst tubes that I
was just showing you. This is going to demonstrate a number of things: we'll be
able to burn the product gases and see the difference between the alkene and
the alkane and the coolest thing about this is just how fantastically this
demonstrates Avogadro in action as the volume changes of our gas mixture
exactly in the way that we would predict.
Having followed the tips in the last
video and set up the gas collection apparatus, you should load your micro
test tube with propan-2-ol rather than paraffin. The paraffin will work if
you want to just show the difference in the flames, but if you want to
demonstrate Avogadro's law, it's not so good. Some of the gases that you collect
aren't alkenes and so the volume change is a bit poor. For essentially pure
propene, the dehydration of propan-2-ol will get you there. As I mentioned last
time, the ethanol burner ought to be positioned a little bit further from the
substrate than you had it in the cracking set-up. Don't forget to discard
the first portion of expanded air before the propene starts collecting in earnest.
So let's talk about the catalyst tube. In here I have 0.5 g of pellets.
The pellets are 3.2 mm in diameter and that means they fit perfectly inside the
regular 1/8 inch borosilicate tubing that you're gonna have lying around in
the department already. The initial outlay
I'll admit can be a little bit on the expensive side. So you're looking at the
moment in the UK, if you buy a 50 g bottle of this stuff, it's going to cost
about £82. But you only need 0.5 g of pellets to actually make a tube
and so therefore you're looking at maybe 82 pence per tube and
each tube is virtually endlessly reusable. So if you have the potential to get
together with a group of schools nearby for a group purchase, you can actually
make a ton of these for a relatively small investment.
Having put half a gram of pellets into the tube, you can flame polish the ends.
This has the benefit of locking in the pellet so that they don't get lost and it also
makes it easier to slip on a silicone tube to connect to your syringe.
Now in Bruce's book he mentions that the pellets that he was using actually
arrived kind of tan coloured. Now these ones when they came to me, they
were black and he recommends that when you first use the pellets that
you need to prepare them by heating the tube in a Bunsen flame until
the pellets go black. I found that I didn't need to do that so depending on
where in the world you're getting, if your pellets aren't black
on arrival, you might want to investigate that as a possibility if you find that
there's some aspect of this demonstration that doesn't work for you.
However, I found mine just work straight out of the bottle. I'm using the same
Luer-locked stopcocks as before, but as I mentioned in the last video, in
principle everything here that can be done with them can also be accomplished
with a piece of sticky tack on the end of the tube instead. I've used Bruce's
technique for loading a syringe with hydrogen. You can head back and see me
explaining this in more detail in the singlet oxygen demonstration at the link
below or, of course, if you have a hydrogen cylinder you can load from that.
If not, then 0.5 g of magnesium and 3-5 ml of 1 M HCl will give you enough H2 to work with.
So on the right side, I have 30 ml of propene
and on the left side I have my hydrogen syringe. First I flushed
through the catalyst tube with hydrogen and I'm running the hydrogen
syringe down to 30 ml at the same time.
So I now have 30 ml of gas on each side for a total of 60 ml.
As I pass the gas over the catalyst
the reaction proceeds. You can feel the catalyst tube,
it will be warm to the touch.
With each pass the total volume drops
fast at first, naturally, and then slowly converging in on 30 ml, beautifully
demonstrating Avogadro's law as hydrogen and propene combine and simply give
propane.
For a finale, the product gas will burn with a nice clean flame in comparison to the alkene you started with.
The demonstration can be repeated with ethyne by the way.
Here, of course, the volume drops to a third of where it
started, rather than halfway.
This catalyst tube is awesome.
I'm so grateful for Bob for bringing it into my life. Go and check
out Bruce Mattson's book. You'll see the various range of reactions that you can
undertake with one of these tubes. It really will open up some educational
opportunities for you so if you can, get a team together, go out, grab some of
these pellets and see what they can do for you too.