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.

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 absorbed 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

The 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.

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

• heptane is C₇H₁₆, so the products of cracking heptane must contain 7 carbon atoms and 16 hydrogen atoms.
• so far we have

heptane_(g) → butane_(g) + one other product

c₇H_16(g) → C₄H_10(g) + one other product

heptane_(g) → butane_(g) + propene_(g)

c₇H_16(g) → C₄H_10(g) + C₃H_6(g)

Key Points

• Hydrocarbon molecules are cracked to produce smaller more useful products.
• Cracking generally produces an saturated alkane and an unsaturated alkene molecule. The alkanes are mostly used as fuels and the alkenes are used to manufacture polymers and as precursors for making many useful substances.
• Molecules can be cracked by heat alone, this is aclled thermal cracking or a catalyst maybe used- this is called catalytic cracking. The final method you need to be aware of is called steam cracking. Steam cracking is a form of thermal cracking.

Check your understanding - Questions on cracking

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