Higher and foundation tier

Cracking hydrocarbons

The fractional distillation of crude oil turns a pretty useless substance, crude oil, into lots of very valuable hydrocarbons that are in huge demand. However there is a slight problem with the amounts of each fraction produced in the distillation process! The small volatile hydrocarbons such as the petrol and diesel fraction are in large demand but the distillation process does not produce enough of these hydrocarbon molecules to meet demand. Whereas the fractions containing large hydrocarbon molecules such as bitumen/tar and heavy oils are in much lower demand but the fractional distillation process produces large amounts of these heavy fractions. The solution to this problem is not to simply "chuck away" the large unwanted hydrocarbon molecules but to break up or crack the large hydrocarbon molecules into the smaller high demand hydrocarbon molecules. This process is called cracking.

In the science lab it is relatively easy to crack a large chain hydrocarbon using the apparatus below.

cracking hydrocarbons

How to carry out cracking in the lab

Here heat resistant mineral wool has been soaked in liquid paraffin. To crack the paraffin molecule you could simply heat it up to a high temperature. As you heat the paraffin molecules they vibrate faster and faster and eventually if they are heated to approximately 9000C the molecules would simply shake themselves to pieces. This is called thermal cracking. However a catalyst is often used, a catalyst will speed up the cracking process and also allow it to occur at a lower temperature, a catalyst of aluminium oxide or pieces of broken crockery is often used.

To crack the paraffin molecules into smaller more useful molecules heat up the catalyst for a few minutes with a hot Bunsen flame. Once the catalyst is hot move the Bunsen flame back and forward between the paraffin and the catalyst. Once the paraffin vapour hits the hot catalyst it will be adsorbed onto the surface of the catalyst and cracked into smaller hydrocarbon molecules. These smaller molecules are likely to be a gas and they can be collected underwater as shown in the diagram. The cracked gas is not soluble in water so collecting in this way is an efficient way to do it. It is likely there will be a high proportion of unsaturated hydrocarbons in the cracked gas. You could prove this by simply testing with red/orange bromine water.

Steam cracking

steam crackingThe third method of cracking gases is called steam cracking. This is the most common method used to obtain the unsaturated alkenes ethene and propene, which are used to make polymers. Steam cracking is a type of thermal cracking. The hydrocarbon to be cracked, which is called the feed stock, is mixed with high temperature steam. The steam helps reduce unwanted side reactions. The mixture of hydrocarbons to be cracked and the steam are pumped very very quickly through a series of tubes inside a furnace at a high temperature, typically the mixtures of gases are inside the hot tubes for between 0.1-0.5 seconds. Cracking happens inside the tubes and the gases leave the furnace and are quickly cooled by water in a condenser. The mixture of cracked gases and the steam are separated later in the industrial process. A basic outline of the steam cracking is shown opposite.

Cracking equations

The products of cracking depend on the temperature and which type of cracking is used. However cracking generally produces a mixtures of saturated alkanes and unsaturated alkenes. We can show this as:

large molecule to be cracked → alkane + alkene

Examples of cracking equations

1. The cracking of heptane to produce butane and one other product.
heptane(g) butane(g) + one other product
and since butane is C4H10
C7H16(g) C4H10(g) + one other product
Since we started with 7 atoms of carbon and butane has 4 carbon atoms, this means the other product must have 3 atoms of carbon. Similarly heptane has 16 hydrogen atoms and butane has 10, this means there is 6 left for the other product. So the other product is C3H6, propene.

heptane(g) butane(g) + propene(g)
C7H16(g)C4H10(g) + C3H6(g)

Key Points

Practice questions

Check your understanding - Questions on cracking