Crude Oil and Hydrocarbons

Crude oil

A finite mixture of hydrocarbons (compounds of hydrogen and carbon only), formed over millions of years from ancient plankton.

Fractional distillation

Separates crude oil by boiling point in a column (hot at the bottom, cool at the top):

Top (low b.p., short chains):  gases, petrol
                               kerosene, diesel
Bottom (high b.p., long chains): fuel oil, bitumen

Alkanes

Saturated hydrocarbons — only single bonds. General formula:

CₙH₂ₙ₊₂

Methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.

Properties down the chain (longer = …)

Higher boiling point, more viscous, less volatile, less flammable.

Combustion

hydrocarbon + oxygen → carbon dioxide + water  (complete)

Incomplete combustion produces carbon monoxide (toxic) and soot.

Cracking

Breaking long chains into smaller, more useful alkenes + short alkanes (heat / catalyst). Alkenes have a C=C double bond (CₙH₂ₙ) and are used to make polymers.

Exam tip

Alkenes decolourise bromine water (orange → colourless) — the standard test that distinguishes them from alkanes.