Reactions of the alkanes

1. Combustion

Alkanes have a relatively low carbon to hydrogen ratio and so usually burn with a clean, blue flame. The products of complete combustion are carbon dioxide and water. Write down a balanced equation for the complete combustion of octane (found in petrol).

Methane gas from a gas cooker burns with a clean, blue flame

2. Action of bromine in sunlight

The use of bromine in an inert solvent allows it to mix with the non-polar hexane. The reaction is attempted in sunlight and in the dark. Any fumes which were evolved were tested with ammonia gas. Deduce the products of any reaction, and write a balanced equation for it.

Video - the bromination of hexane in ultra-violet light

3. Cracking of alkanes

Here an alkane is heated strongly in the absence of air causing it to break down. We have used liquid paraffin as the alkane and an aluminium oxide catalyst. Liquid paraffin is a mixture of alkanes containing around ten to twelve carbon atoms. The gases that are produced are collected over water, remembering that the first few bubbles will be air. We test the collected gases with bromine water. From your observations try to identify a product of this reaction, and then suggest a balanced equation for the reaction with decane, C₁₀H₂₂. Strong heating is needed, so care needs to be taken to avoid suck back.

Video - cracking of alkanes

Reactions of the alkanes - results and equations