Crude oil and alkanes

Crude oil is a finite resource found in rocks, made from the remains of ancient sea plankton. It is a mixture of hydrocarbons, compounds of hydrogen and carbon only, and most of them are alkanes. Fractional distillation separates crude oil into fractions of molecules with similar numbers of carbon atoms, which are used as fuels and as feedstock for the chemical industry. Cracking breaks long, less useful molecules into shorter ones and alkenes.

Key facts

• Alkanes are saturated hydrocarbons (only single bonds) with the general formula CnH2n+2: methane CH4, ethane C2H6, propane C3H8, butane C4H10.
• Fractional distillation: crude oil is heated until it vaporises; the vapour rises up a column that is hot at the bottom and cool at the top, and each fraction condenses at its own height.
• Fractions from the top: refinery gases, petrol, kerosene, diesel oil, heavy fuel oil, then bitumen at the bottom.
• Longer chains: higher boiling point, more viscous, less flammable and less volatile.
• Complete combustion gives carbon dioxide and water: CH4 + 2O2 → CO2 + 2H2O. Incomplete combustion also gives toxic carbon monoxide and soot.
• Cracking breaks long alkanes into a shorter alkane and an alkene, using a hot catalyst (catalytic cracking) or steam at a very high temperature (steam cracking): C10H22 → C8H18 + C2H4.

Worked examples