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

Methane gas Octane – found in the petrol fraction Long chains – found in the residue fraction

Cracking When 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 chocolate or only one big bar?

Cracking video kMI kMI QhsA up to 4:11mins QhsA

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. Catalytic cracker Heat to vaporise Distillation tower pressure Big Molecules Smaller molecules 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? How many hydrogen?

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?