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 exothermicenergy 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 flamethe 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 to haemoglobin in red blood cells with an affinity approximately 200 times greater than oxygen, preventing oxygen delivery to tissues. Even at 200 ppm in air, CO causes headaches; at 1600 ppm, loss of consciousness within 2 hours; at higher concentrations, death. 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 flamethe 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.

Every combustion reaction that produces CO₂ contributes to atmospheric CO₂ concentration. The chemistry is straightforward: for every mole of carbon in the fuel, one mole of CO₂ is produced. One mole of octane (C₈H₁₈, 114 g) produces 8 moles of CO₂ (352 g). A standard car tank holds about 50 litres of petrol (approximately 400 mol of octane), burning it produces roughly 3,200 mol = 140 kg of CO₂. The global vehicle fleet burns approximately 100 billion litres of petroleum per day, releasing billions of tonnes of CO₂ annually.

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.