Cracking: GCSE Chemistry Definition

What is cracking?

Cracking is a thermal decomposition reaction where large, less useful hydrocarbon molecules are broken down into smaller, more useful molecules. The smaller hydrocarbon products are a mixture of alkanes and alkenes.

The two types of cracking are thermal (or steam) and catalytic.

What is thermal cracking or steam cracking?

Thermal cracking can also be called steam cracking because hydrocarbons are mixed with steam and heated to a high temperature. This type of cracking produces more ring structures and unsaturated compounds.

Hydrogen and more alkenes form when cracking is performed at higher temperatures and higher pressure

What is catalytic cracking?

Catalytic cracking heats the hydrocarbon molecules to 470 – 550°C to vaporise them. The vapours are then passed over a hot aluminium oxide catalyst to break the covalent bonds in the molecules.

Some exam boards require you to know one type of cracking; while some boards require you to know both.

Make sure you know the correct conditions for cracking as specified by your exam board:

• AQA GCSE Chemistry uses 470 - 550 ^oC for catalytic cracking

• OCR Gateway GCSE Chemistry uses 600 - 700 ^oC for cracking in general

Cracking equations

Symbol equations are used to show cracking. It is important that the number of carbon and hydrogen atoms are equal on both sides of the reaction arrow.

hexane → butane + ethene

C₆H₁₄ ⟶ C₄H₁₀ + C₂H₄ 

Multiple products can be made during cracking but they must be a mixture of alkanes and alkenes.

Why is cracking important?

Fractional distillation produces lots of long chain hydrocarbons and less short chain hydrocarbons. Long chain hydrocarbons, such as bitumen and fuel oil, are in high supply and low demand. Short chain hydrocarbons, such as petrol and diesel, are in low supply and high demand.

Cracking allows long chain hydrocarbons to be converted into short chain hydrocarbons. This solves the supply and demand issue.

Cracking Revision Resources to Ace Your Exams

To explore cracking further, you should check out the revision notes on cracking and practise our exam questions to ensure success. Flashcards on hydrocarbons are also a great way to revise the wider topic of hydrocarbons before an exam. 

Explore our GCSE Chemistry revision resources.