Combustion of alkanes

Hydrocarbons ability to combust, or burn, depends on the amount of oxygen available.

They can undergo complete combustion or incomplete combustion.

Complete combustion

Complete combustion occurs when there is a plentiful supply of oxygen. The carbon and hydrogen atoms react with oxygen in an exothermic reaction.

  • Carbon dioxide and water are produced.
  • The maximum amount of energy is given out.
Sections of a Bunsen burner blue flameYou can observe this when you open the air hole on a Bunsen burner.

In general:

hydrocarbon + oxygen → carbon dioxide + water

Here are the equations for the complete combustion of propane, which is used in bottled gas.

propane + oxygen → carbon dioxide + water

C3H8 + 5O2 → 3CO2 + 4H2O

Incomplete combustion

Incomplete combustion occurs when there is a limited supply of oxygen.

  • Carbon (soot), carbon monoxide, and water are produced.
  • Less energy is released, compared to complete combustion.
Bunsen burner yellow flameYou can see this in a Bunsen burner, when the air hole is closed.

Problems with incomplete combustion

Carbon monoxide is a poisonous gas. It binds with haemoglobin in red blood cells and prevents them from carrying oxygen. It is colourless and odourless, so it is difficult to tell if you are breathing it in – electronic detectors are often fitted near home boilers and gas fires.

Particulate (minute, separate particles) carbon can cause health problems. It irritates the lungs’ lining, making asthma worse and causing cancer. It also causes global dimming, which may reduce rainfall and limits light.