Cracking hydrocarbons | A microscale version

The exact composition of the crude oil varies depending on where it comes from, but most oil contains more of the larger molecules than the smaller ones. The smaller ones, however, are more useful and therefore more valuable. To increase the profit that can be made from a barrel of oil, the larger hydrocarbons are broken down into smaller ones. You are going to carry out a small scale version of this conversion, which is performed in industry every day.

What you need

• Comboplates® x 2
• 10 cm³ syringe
• 10–15 cm piece straight glass tube
• L-shaped pieces of glass tube x 2
• 1–2 cm length of silicone tubing x 4
• Plastic pipette
• Microburner filled with ethanol
• Lid number 2 for the comboplate® (with one long and one short port) x 2
• Mineral wool
• Aluminium oxide
• Liquid paraffin – about 0.5 cm³
• Bromine water (less than 1%) – about 3 cm³ (Harmful and Irritant)
• Eye protection.

Timing

This experiment is far quicker than the traditional version. How long it takes a class will depend on how familiar they are with the microscale equipment, but students should be able to complete it easily within 30 minutes. This leaves time for the model making and theory work in the same lesson.

Health and safety

• Read our standard health and safety guidance
• Wear eye protection at all times during this experiment, including during set-up and dismantling.
• Ethanol is highly flammable. See CLEAPSS Hazcard HC040a
• Always keep the microburner upright to prevent spills.
• Bromine water is harmful and an irritant. Avoid contact with the skin and do not breathe in the fumes. See CLEAPSS Hazcard HC015b
• If you get bromine on your hands, wash them straight away.

Setting up

• Put on your eye protection.
• Pull the plunger of the syringe until it is fully extended. Fit the syringe to one of the lids. Attach the lid to well F3 on one of the comboplates®. The syringe and the well should be empty.
• Half fill well F3 on the other comboplate® with bromine water and attach the other lid.
• Attach a piece of silicone tubing to each end of the L-shaped glass tubes. Hold the glass tube near the end you are attaching to the silicone tubing to reduce the likelihood of breaking the glass and cutting yourself.
• Attach the long end of each L-shaped tube to the lids on the comboplates®.
• Put some glass wool in one end of the straight glass tube. Push it at least 2 or 3 cm down the tube (a piece of wire or small stick helps). Add 5 or 6 drops of liquid paraffin to the glass wool using a pipette. About 0.5 cm of the tube should be filled with paraffin.
• Holding the tube as horizontal as possible so as not to lose your paraffin, put 1 microspatula of aluminium oxide in the other end.
• Attach the straight glass tube between the L-shaped tubes, making sure the paraffin is at the end with the syringe.
• Get your set-up checked by your teacher before you go any further.

Cracking the hydrocarbons

• Wear eye protection.
• Light your microburner, pick it up and begin to heat the aluminium oxide. Make sure the oxide is very hot before you go any further. As you heat, very slowly depress the plunger on the syringe. If at any stage the plunger gets to the bottom, remove the syringe, pull the plunger out, re-attach it and carry on pressing it down slowly.
• Begin to flick your microburner over the paraffin to heat that up too – make sure you keep heating the aluminium oxide enough to keep it hot. You can see when the paraffin is boiling.
• As it turns into a vapour it will pass through the glass wool and over the catalyst (the aluminium oxide).
• Watch what happens to the bromine water. When you see no more changes or you have run out of paraffin, stop heating.
• Put a few drops of paraffin in one of the other large wells in your plate and add a few drops of bromine water. Observe what happens.
• Wait until the apparatus has cooled before you dismantle it.

Notes

The equipment could be set up for the students if they are likely to find putting all the pieces together very difficult.

As the hole and therefore the drop size of a pipette can vary, it is a good idea to trial this practical before the students do it to ensure that sufficient ethene is generated to discolour the bromine water. If the bromine water is dilute (0.1%) then this is less of a problem.

The catalyst should be heated strongly initially. Once it is hot, the paraffin can be heated: the flame should remain under the catalyst and the occasional flick of the burner used to heat the paraffin – it is possible to see when the paraffin is boiling.

Continual gentle depression of the syringe will ensure that no suck-back takes place and the gases flow through the system into the bromine water. When the syringe is empty, simply remove it, pull the plunger back, re-attach and continue depressing it.

The disadvantage of this experiment is that it is not possible to collect enough product to fill a container and show that it is a gas at room temperature. However, at the end of the reaction any remaining ethene can be lit as it comes out of the open port hole on the lid over the bromine. This should be done with care, but animpressive 10–15 cm flame can be achieved. You could compare the behaviour of paraffin by trying to set fire to it. Put a few drops in a watch glass and attempt to light it using a splint NOT the microburner. It does not light easily. An extra column can be added to the table in the students’ notes to allow for observations relating to this additional aspect of the experiment.