Combustion of Hydrocarbons

Light a gas stove and notice the crisp blue flame; light a candle and notice the flickering yellow-orange one: both are burning hydrocarbons, but the gas stove has abundant oxygen while the candle's wick is oxygen-starved, and that single difference changes the products completely. Complete combustion occurs when a hydrocarbon burns with sufficient oxygen to convert all carbon to CO₂ and all hydrogen to H₂O. The general word equation is: hydrocarbon + oxygen → carbon dioxide + water. The reaction is always exothermic energy is released as heat and light. For methane: CH₄ + 2O₂ → CO₂ + 2H₂O. For octane (petrol): 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O. The products, CO₂ and H₂O, are both thermodynamically stable; the system has released the maximum possible energy.

To balance a combustion equation, count C atoms first (each → one CO₂), then H atoms (every 2 H → one H₂O), then count the O₂ molecules needed to supply all the oxygen. The reaction produces a blue flame the characteristic colour of complete combustion. A gas stove burns with a blue flame; a candle burns with a yellow-orange flame (incomplete combustion because oxygen is limited near the wick). Engineers design combustion chambers to ensure excess oxygen is always present to achieve complete combustion and maximum energy output.

Incomplete combustion occurs when oxygen supply is insufficient to fully oxidise all carbon atoms in the fuel. Instead of CO₂, the carbon partially oxidises to produce carbon monoxide (CO) a colourless, odourless gas that is extremely toxic. CO binds reversibly to haemoglobin in red blood cells with an affinity about 200 times greater than oxygen. This reduces the blood's oxygen-carrying capacity and makes the remaining oxygen harder for haemoglobin to release to tissues. Exposure can cause headache, nausea, vomiting or weakness; higher exposure can cause dizziness, loss of consciousness, long-term brain injury or death. Effects depend on exposure level and duration and on the person. Incomplete combustion also produces soot, fine carbon particles (particulate matter), when oxygen is severely limited.

The visual signal of incomplete combustion is the yellow or orange flame the colour comes from glowing carbon soot particles in the flame. A yellow gas burner flame is dangerous: it signals incomplete combustion inside the appliance, producing CO in the living space. Gas appliances must be serviced regularly to maintain the blue complete-combustion flame. Diesel engine exhaust contains both CO and fine carbon particulates, the black soot visible from trucks, and these are the reason diesel vehicles are subject to strict emission standards under Australian Design Rules.

Complete combustion of carbon-containing fuel produces CO₂ and contributes to atmospheric CO₂ concentration: each mole of carbon atoms in the fuel forms one mole of CO₂. For a pure-octane model, one mole of octane (C₈H₁₈, 114.23 g) produces 8 moles of CO₂ (352.1 g) in complete combustion. Petrol is a mixture, so its exact emissions per litre depend on its composition and density. The U.S. Energy Information Administration reports world consumption of petroleum and other liquid fuels at 102.8 million barrels per day in 2024 across all uses. Using 158.987 litres per barrel, this is about 16.3 billion litres per day; it is a global total, not a vehicle-fuel estimate.

CO₂ is a greenhouse gas: it absorbs infrared radiation emitted by Earth's surface and re-radiates it back to Earth, trapping heat. Since 1850, atmospheric CO₂ has risen from 280 ppm to over 420 ppm, a 50% increase directly attributable to combustion. Australia is the world's largest per-capita CO₂ emitter among developed nations, largely because of its heavy coal and gas use. The connection between the chemistry of combustion and the physics of the greenhouse effect is one of the most important applied chemistry concepts of the 21st century.