Cracking (AQA GCSE Chemistry Combined Science): Revision Notes
Cracking
What is cracking?
Cracking is a really important process in the oil industry. There's a problem - we have a shortage of small molecules that make good fuels. But crude oil contains lots of big hydrocarbon molecules that aren't as useful.
Cracking solves this problem by breaking down large hydrocarbon molecules into smaller, more useful ones.
The oil industry relies heavily on cracking because crude oil naturally contains mostly large molecules, but we need smaller ones for everyday fuels like petrol and diesel.
How cracking works
Cracking is a thermal decomposition reaction. This means we use heat to break molecules apart.
There are two main ways to do cracking:
- With a catalyst: Oil fractions are passed over a hot catalyst
- With steam: Oil fractions are mixed with steam and heated to very high temperatures
Both methods use high temperatures to break the chemical bonds in large molecules.
High temperatures are essential for cracking - without sufficient heat, the strong covalent bonds in hydrocarbon molecules won't break apart.
What cracking produces
When we crack a large alkane molecule, we always get two products:
- A shorter alkane molecule (useful for fuels)
- A molecule with a C=C double bond (an alkene)
Worked Example: Cracking Reactions
Here are typical cracking reactions:
This could also happen:
In both cases, we get smaller, more useful molecules from the original large alkane.
The alkene (like ) contains a double bond, which makes it very useful for making other chemicals.
Why are smaller hydrocarbons better?
Smaller hydrocarbons make much better fuels than larger ones because they:
- Are less viscous (less thick and sticky)
- Are more flammable
- Have lower boiling points (they tend to be gases or liquids at room temperature)
- Are more useful as fuels
The physical properties that make smaller hydrocarbons better fuels are all related to the weaker intermolecular forces between smaller molecules compared to larger ones.
Uses of cracking products
The products from cracking have many important uses:
- Alkanes: Used as fuels for cars, planes, and heating
- Alkanes: Used as feedstock to make other products like solvents and detergents
- Alkenes: Used to make polymers (plastics)
- Alkenes: Used as feedstock to make many other chemicals
Laboratory cracking
In the lab, we can crack paraffin (a mixture of alkanes) by:
- Soaking paraffin onto mineral wool
- Heating it strongly with a Bunsen burner
- Using a catalyst to help the reaction
- Collecting the gas products in water
The gas produced contains smaller alkanes and alkenes.
Laboratory cracking demonstrates the same principles as industrial cracking, but on a much smaller scale. The products can be tested to show they contain both alkanes and alkenes.
Summary
Key Points to Remember:
- Cracking breaks large hydrocarbon molecules into smaller, more useful ones
- It always produces an alkane AND an alkene
- Smaller hydrocarbons make better fuels because they're less thick and more flammable
- Cracking uses high temperatures, either with a catalyst or steam
- The products are used for fuels and to make other chemicals like plastics