1. L.O: To understand how cracking works, and what it is used for.
Cracking Crude Oil
L.O: To understand how cracking works, and what it is used for.
Wednesday, 21 November 2018

2. Starter: Complete the fractional distillation diagram.
R_f_n_r_ G_s
_e_r_l
N_p_h_
K_r_s_n_
D_e_e_
_i_
_i_u_e_

3. What is Cracking?
Long-chain hydrocarbons form thick gloopy liquids like tar which aren't all that useful, so a lot of the longer molecules produced from fractional distillation are turned into smaller ones by a process called cracking.
Some of the products of cracking are useful as fuels, like petrol for cars and paraffin for jet fuel. Cracking also produces substances like ethene, which are needed for making plastics.

5. How does Cracking work?
Obviously, we don’t use a knife to crack hydrocarbon molecules!
Cracking is, in fact, a thermal decomposition reaction — breaking molecules down by heating them.

7. Alternatively, the vapour can be mixed with steam and heated to a very high temperature. This will also lead to thermal decomposition of long-chain hydrocarbon molecules to form smaller ones.
How does Cracking work?
The first step is to heat the long-chain hydrocarbon to vaporise it (turn it into a gas).
The vapour can then be passed over a powdered catalyst at a temperature of about 400 °C °C.
Aluminium oxide is one of the catalysts used. The long-chain molecules split apart or “crack” on the surface of the specks of catalyst

8. Products of Cracking
Most of the products of cracking are alkanes and unsaturated hydrocarbons called alkenes.

9. For example…
Kerosene is a long-chain hydrocarbon molecule (it has 10 carbon atoms).
There's lots of kerosene in crude oil, but kerosene itself isn't that useful.
Cracking is used to break kerosene down into octane and ethene.
Octane is a shorter-chain alkane which is useful for making petrol.
Ethene is an alkene which is useful for making plastics.

10. Quick Questions What is cracking and why is it useful?
Give two things that the products of cracking can be used for.
What type of reaction is cracking?
Describe how an aluminium oxide catalyst can be used to crack a long-chain hydrocarbon.
How can cracking be performed without using a catalyst?
Name two types of hydrocarbon that are produced in cracking. 1. 3.

11. Answers E.g. alkanes and alkenes.
Cracking is the process used to break long‑chain hydrocarbons down into smaller ones. It is useful because shorter‑chain hydrocarbons are usually more useful than longer‑chain hydrocarbons.
E.g. fuel and making plastics.
A thermal decomposition reaction.
The long‑chain hydrocarbon is heated so that it vaporises. The vapour is then passed over a powdered aluminium oxide catalyst at a temperature of about 400 – 700 °C. The long-chain molecules will crack on the surface of the catalyst.
By mixing the vaporised hydrocarbon with steam at a very high temperature.
E.g. alkanes and alkenes. 1. 3.