Alkenes (AQA GCSE Chemistry Combined Science): Revision Notes

Alkenes

What are alkenes?

Alkenes are a type of hydrocarbon. This means they contain only hydrogen and carbon atoms. The key feature of alkenes is that they have a carbon-to-carbon double bond ($\ce{C=C}$).

Alkenes are made by cracking long-chain alkanes. Cracking is a process that breaks down larger molecules into smaller, more useful ones.

Key properties of alkenes

Alkenes are unsaturated compounds. This means they have fewer hydrogen atoms than they could possibly have because of the double bond. The double bond takes up space that could have been filled by hydrogen atoms.

General formula: $\ce{C_nH_{2n}}$

For example:

• Ethene: $\ce{C_2H_4}$
• Propene: $\ce{C_3H_6}$

The bromine water test

This is a simple test to tell the difference between alkanes and alkenes.

How to do the test:

1. Shake the substance with bromine water (an orange solution)
2. Watch what happens to the colour

Results:

• If it's an alkene: The mixture turns from orange to colourless
• If it's an alkane: The mixture stays orange (no change)

Why are alkenes more reactive than alkanes?

Alkenes are more reactive than alkanes because of their double bond. The double bond can break easily, allowing alkenes to react with other substances.

Identifying alkenes in molecules

To spot an alkene molecule, look for:

• Only carbon and hydrogen atoms (hydrocarbon)
• At least one $\ce{C=C}$ double bond
• Fewer hydrogen atoms than the equivalent alkane