# Cracking

Breaking large hydrocarbon molecules into smaller, more useful ones, using heat and a catalyst. It makes alkanes and alkenes.

Chemistry (school) · Part 08: Carbon chemistry and the Earth · Card 6 of 10 · Last updated 2026-10-19

## Analogy

Cracking is like snapping long spaghetti into short pieces so it fits the pot.

## Example

**Cracking decane**

C10H22 -> C8H18 + C2H4
alkane -> shorter alkane + alkene

## In practice

Refineries crack long, less useful fractions to make more petrol and ethene for plastics.

## Practice

### Flashcards

- **What is cracking?** Breaking long hydrocarbons into shorter, more useful ones.

## Questions people ask

### Why is cracking needed?

Crude oil gives more large molecules than are needed, while demand for small ones like petrol is high. Cracking converts surplus long chains into smaller molecules, balancing supply and demand. It also makes alkenes, which are raw materials for making plastics.

### What conditions are used in cracking?

Long hydrocarbon vapours are heated to a high temperature, around 600 to 700 degrees Celsius, and passed over a hot catalyst such as aluminium oxide. Alternatively, steam is mixed with the vapour at a very high temperature. This is called thermal decomposition.

## Related concepts

- [Crude oil](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-1)
- [Fractional distillation](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-8/chemistry-school-part-8-concept-3)

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