Cracking
Cracking is the process by which hydrocarbons with relatively high molecular mass are chemically converted to hydrocarbons with lower molecular mass. This process makes greater use of the saturated hydrocarbons found in crude oil. Unsaturated hydrocarbons, with small molecular mass, such as ethene, propene and butadiene are of greater use to modern society as fuels and as raw materials for the production of plastics.

Two types of cracking processes are used in industry.

The first is called thermal cracking. In the absence of air, steam and raw material are mixed and heated to temperatures around 800oC inside a furnace. During this process, lighter unsaturated hydrocarbons are produced from the thermal decomposition of heavier saturated hydrocarbons. Two examples are given below. The gases are in the furnace for less than one second in order to prevent continued decomposition of ethene into carbon (coke). As the products leave the furnace they are quickly liquefied at temperatures around -100oC and separated by fractional distillation. Carbon dioxide and hydrogen sulfide are unwanted byproducts of thermal cracking. These gases must be removed before they escape into the atmosphere by reaction with sodium hydroxide solution.
2NaOH(aq) + H2S(g) => Na2S(aq) + 2H2O(l)
2NaOH(aq) + CO2(g) => Na2CO3(aq) + H2O(l)

Ethane is decomposed to ethene and hydrogen gas. This reaction is exothermic and the heat given off from this reaction is used to keep the furnace at approximately 800oC .

C2H6(g) => C2H4(g) + H2(g)
Not only ethane but heavier hydrocarbons obtained from fractional distillation, such as naphtha (carbon compounds between 6 and 10 carbon atoms long), can be cracked. In the animation on the right, propane is cracked to form ethene and methane gas. The conditions can be modified according to the product that is desired and the raw material used.

C3H8(g) => C2H4(g) + CH4(g)

The second method is known as catalytic cracking. In this process a catalyst, zeolite, is used to crack heavy fractions at temperatures around 500oC. The products are a mixture of saturated and unsaturated hydrocarbons. Octane is a valuable product and is used as a major component of petrol. The other products are used as feed stock for thermal cracking to yield more ethene.
The benefits of catalytic cracking as opposed to thermal cracking include:

- greater control of the range of end product;
- lower temperatures are used (450oC)

Below is a diagram for the overall cracking process.

Fractional distillation is used to separate the products of thermal and catalytic cracking.

You can crack hydrocarbons at home using a candle. Click to see a 120 kb video. The paraffin wax molecules are too large to vaporise easily at the temperature of the flame. Something else must have been present in the smoke that was more flammable than paraffin wax vapour. Cracking occurs to produce smaller hydrocarbons that ignite in the flame of the candle.

 
As the candles burn, what is reacting with the oxygen in the air?
What is the purpose of the flame?