Fossil fuels and alkanes
The main fossil fuels are coal, petroleum (crude oil) and natural gas (mainly methane). Crude oil is a mixture of hydrocarbons separated by fractional distillation. Most of these hydrocarbons are alkanes.
Key idea
Alkanes are saturated hydrocarbons (only single C–C bonds). They form a homologous series with the general formula CnH2n+2: methane CH4, ethane C2H6, propane C3H8, butane C4H10.
Combustion
Complete combustion (plenty of air) gives carbon dioxide and water. Incomplete combustion (poor air supply) gives toxic carbon monoxide and soot (carbon).
Worked example
Balance the complete combustion of methane. CH4 + O2 → CO2 + H2O. Balance H first (2 H2O), then O (2 O2): CH4 + 2O2 → CO2 + 2H2O. So 1 mol methane needs 2 mol oxygen.
Reactions of alkanes
Alkanes are fairly unreactive. In ultraviolet light they undergo substitution with halogens, e.g. CH4 + Cl2 → CH3Cl + HCl (one H replaced by Cl).
Remember
- Alkanes: general formula CnH2n+2, saturated.
- Complete combustion → CO2 + H2O; incomplete → CO + C.
- Alkane + halogen (UV) = substitution.