Cracking (AQA GCSE Chemistry): Revision Notes
Cracking
What is cracking?
Cracking is a process that breaks down large hydrocarbon molecules into smaller, more useful ones. It uses heat to split apart the big molecules we don't need as much into smaller molecules that are in high demand.
This process is called a thermal decomposition reaction because it uses heat to break down compounds.
The word "cracking" comes from the idea of "cracking apart" or breaking the chemical bonds in large molecules to create smaller fragments.
Why do we need cracking?
There's a problem with crude oil. When we refine it, we get lots of large hydrocarbon molecules, but there's a shortage of the small molecules that people actually want to use as fuels.
Cracking solves this problem by breaking the large molecules down into smaller, more useful ones.
This mismatch between what crude oil naturally provides and what society needs is why cracking is such an important industrial process. Without cracking, we would have an oversupply of heavy oils and a shortage of petrol and other light fuels.
How does cracking work?
Cracking happens when oil fractions are heated until they turn into gas. Then the gas vapours go through one of two processes:
- Method 1: Pass the vapours over a hot catalyst
- Method 2: Mix the vapours with steam and heat to very high temperatures
Both methods break the large hydrocarbon molecules apart.
Products of cracking
When you crack a large alkane molecule, you always get two products:
- A shorter alkane molecule (still has only single bonds)
- An alkene molecule (contains a C=C double bond)
Worked Example: Cracking Octane
Example equation:
In this example:
- C₈H₁₈ is the large alkane we started with
- C₂H₄ is an alkene (notice it has fewer hydrogen atoms)
- C₆H₁₄ is a shorter alkane
Remember that alkenes always have fewer hydrogen atoms than alkanes because they contain a C=C double bond. This is why you can identify which product is the alkene in a cracking equation.
Uses of hydrocarbons in modern life
The products from cracking have many important uses:
- Alkanes are used as fuels
- Alkanes are used as feedstock to make other products like solvents, lubricants and detergents
- Alkenes are used to make polymers (plastics)
- Alkenes are used as feedstock to make many other chemicals
Why smaller hydrocarbons are better
Smaller hydrocarbons make better fuels than larger ones because they are:
- Less thick (less viscous) so they flow better
- More flammable so they burn more easily
- Have lower boiling points so they turn to gas more easily
This makes them much more useful as fuels.
These properties explain why petrol (containing small hydrocarbons) flows easily and ignites readily in car engines, while heavy oils (containing large hydrocarbons) are thick and difficult to burn.
Laboratory cracking experiment
Laboratory Demonstration: Cracking Paraffin
You can crack hydrocarbons in the lab using this setup:
- Paraffin (a hydrocarbon) is soaked onto mineral wool
- The paraffin is warmed to turn it into vapour
- The vapour passes over a hot catalyst
- The strong heat breaks down the molecules
- Smaller hydrocarbon molecules are produced and collected in cold water
This is a thermal decomposition process because heat breaks down the original compound.
Key Points to Remember:
- Cracking uses heat to break large hydrocarbon molecules into smaller ones
- Two methods exist: hot catalyst or steam at high temperatures
- Products are always: one shorter alkane + one alkene (with C=C double bond)
- Smaller molecules make better fuels because they're less thick and more flammable
- Alkenes from cracking are used to make plastics and other chemicals