This experiment models the industrial catalytic cracking of long-chain alkanes
and shows that some of the products have shorter chains than the starting
material and
some are alkenes (they have carbon-carbon double bonds)
The experiment is carried out in a boiling tube
and you need to start by putting some mineral wool
not cotton wool into the bottom of the tube
the initial source of alkanes is liquid paraffin
which has about twenty carbons
in its chain length.
Use a dropping pipette to add liquid paraffin so that it soaks into the mineral wool. You will need about
2 cm3.
Just help that to soak in a bit.
and just invert it to check if there's any excess
and if there is remove it using a pipette
you don't want to get paraffin all the way down the inside of the tube
if you can avoid it. You put that into the clamp
and clamp it right near the mouth of the tube.
next you're going to put some catalyst
into the tube and we are using pumice stone
but you can also use pieces
of broken plant pot. Both of these are
silica based and they mimic the silica and alumina based catalysts
that are used in industry. Now you want this tilted
but not so much so that all your catalyst
falls to the bottom.
This has got quite an open structure which gives you
a large surface area for the reaction to take place on.
You attach a bung with a delivery tube
to the mouth of your boiling tube. I have already prepared a bowl of water
with about six test tubes also filled with water
and some bungs. Next you need an assistant
who could possibly be a competent student, although actually, I often do this experiment
with two students
and don't get too involve myself at all
and what you are going to do is to heat
using a roaring Bunsen flame. And the hottest part of the flame which
is just beyond
the blue cone. And you are going to heat
the pumice stone. As you do so
some gas will escape and your assistant needs to collect the gas
in a test tube. You want to get the pumice stone
good and hot before you go any further. Once you've been heating for about 10 seconds
then you can flick across and just briefly
heat your paraffin soaked in the mineral wool.
Then go back and keep the catalyst hot.
The whole time keep the catalyst hot and then just
flick across to start the paraffin
evaporating you want to heat the pumice stone for about 10 seconds
for every second that you're heating your paraffin
it is important to keep the boiling tube and its contents hot
if it is allowed to cool water may be sucked back into the boiling tube
and it can cause the boiling tube to crack. It's vital to watch the delivery
tube for signs of suckback. If this seems to be starting
the solution is to continue to heat the boiling tube with the hottest part of the flame
just beyond blue cone and to remove the whole thing from the water.
It is tempting to push to Bunsen closer to the boiling tube
or two remove the heat both of which will increase suckback,
and you can see there, there are some water drops
coming up the tube and if I just keep heating those
then they should come out.
Continue until six test tubes of gas have been collected.
Okay when you come to the end
and you need to remove it from the water and stop heating.
it's really important that you remove from the water first
and carry-on heating for a moment. When you are happy that there is no more water in the tube
you can stop heating. But I wouldn't turn off the Bunsen burner
until you're convinced at that everything has stopped.
Then you need to test the gases.
The first two you can discard because they will just be
air from your boiling tube. Okay the third one
you can pass around the class and they can have a gentle smell
and it should smell something like camping gas, which is a mixture of propane
and butane. The fourth one you can attempt to ignite
to prove that you have made a flammable gas.
To the 5th tube
you can add a few drops of bromine water, which is just bromine
dissolved in water
and shake it up and you can see that the bromine
goes colourless. This shows the presence of an alkene
or carbon-carbon double bond. A similar reaction you can do with acidified
potassium permanganate.
A few drops into the test tube again shake it up
and you can see that this has changed colour and it will go either to
brown or a colourless solution and again that shows the presence of an alkene
or a carbon-carbon double bond.