Cracking

Lesson Objective: To be able to explain cracking and its uses.

- Recall what cracking means (C)- Explain why some hydrocarbons are more

useful(B)- Correctly explain the cracking process and

work out the products (A)

Cracking Hydrocarbons

• What is crude oil? • Where does it come from?• What does it contain?

Crude Oil

0

5

10

15

20

25

30

35

40

1 4 8 12 16 20 >24

Length of carbon chain

% in crude oil

Demand

Methane gas

Octane – found in the petrol fraction

Long chains – found in the residue fraction

CrackingWhen we split crude oil up into its different fractions some are more useful than others.

Very high demand!

Very low demand!

Q) Why are the smaller chain alkanes more useful?

Problem

• The long chained hydrocarbons are not useful• They are thick liquids or solids with high

boiling points • They do not catch fire very easily• This means they are not good for fuels

Cracking

• We can break down long hydrocarbon chains into smaller more useful chains

• Would you rather have lots of individual bars of chocolateor only one big bar?

Cracking video

• http://www.youtube.com/watch?v=Ehcq4cARkMI

• http://www.youtube.com/watch?v=7KO3X_8QhsA up to 4:11mins

• Large hydrocarbons are broken into smaller molecules using heat and a catalyst.

• This process is known as catalytic cracking.• The small molecules produced are then separated by

distillation.

CatalyticcrackerHeat to

vaporise

Distillationtower

pressure

Big Molecules

Sm

alle

r m

ole

cule

s

Molecules break up

Cracking allows you to split the longer molecules up into smaller ones.

How does catalytic cracking work?

Cracking Theory

• Long hydrocarbon chains are vaporised and passed over a hot catalyst or mixed with steam

• It is heated to a high temperature• The long chains are ‘cracked’ as thermal

decomposition takes place• The large chains split apart to form smaller

more useful chains

Definitions

• Hydrocarbon - Contains hydrogen and carbon only

• Vaporised - Turned into a gas • Catalyst - Speeds up a reaction• Thermal decomposition - Break down of

substance due to heat

How many carbons?

7

2

5

How many hydrogen?

16

4 12

Task • Produce a flow diagram for cracking alkanes

using ALL the key words

Passed over a catalyst

Vaporised: turned from a liquid to a gas

Thermal decomposition takes place (long chain

breaks down)

Smaller more useful chains produced

Useless long chain hydrocarbon

Common exam questions

1) What is a cracking?

2) Why is it used?

3) Why is a catalyst used?

4) Does the catalyst get used up?