Combustion: Complete and incomplete

This is Michael’s new camping stove. Let’s use it for an experiment! Open the gas valve to allow the gas to flow from the tank to the burner. There is no flame yet. Take a heat source, such as a lit match, and light the burner.

The flame appears! But if you use a lid to cover the camping stove to cut off the air supply, the flame dies. And one more thing: lift off the cover, turn off the gas supply, and bring another lit match to the burner. The fire doesn’t start! A fire needs three things: heat to start and keep the fire going, fuel that can burn, and oxygen in the air.

If you remove any of these things, the fire dies. Why? Fire, or burning, is a chemical reaction, called combustion. As with every chemical reaction, combustion needs substances to undergo the reaction — reactants, it requires the right conditions to occur, and it produces new substances — products. In our experiment, the reactant is fuel, specifically methane.

When enough heat is supplied for methane to reach the ignition temperature, methane starts reacting with oxygen in the air. Combustion is a rapid oxidation reaction and it produces substances that are high in oxygen, such as oxides. If there is plenty of oxygen to react with, the products of methane combustion are carbon dioxide and water. The chemical equation of this reaction goes like this: one molecule of methane, containing one atom of carbon and four atoms of hydrogen reacts with two molecules of oxygen, containing two oxygen atoms each, producing one molecule of carbon dioxide, containing one atom of carbon and two oxygen atoms, and two molecules of water, with two hydrogen atoms and one oxygen atom each. This is an example of complete combustion that occurs with a sufficient supply of oxygen.

What happens if there isn’t enough oxygen, like when the camping stove burner was covered? When there is too little oxygen for each methane molecule to react with two oxygen molecules, carbon dioxide and water can’t both be produced. An incomplete combustion takes place. Let’s say there is just enough oxygen for each methane molecule to react with three oxygen atoms, that is one and a half oxygen molecules. In a chemical equation we can’t have half a molecule, so let’s double the amount of both reactants: two methane molecules, reacting with three oxygen molecules.

That’s two carbon atoms, eight hydrogen atoms and six oxygen atoms that can recombine. Four oxygen atoms combine with two hydrogen atoms each, forming four water molecules. This leaves two carbon atoms and two oxygen atoms. Each carbon atom combines with one oxygen atom, forming two carbon monoxide molecules. What if there is only one oxygen molecule available per each methane molecule?

Then, hydrogen still combines with oxygen to form water molecules, but there isn’t any oxygen left for the carbon atom to combine with, to form an oxide! This is incomplete combustion too. The products are water, and carbon that deposits as soot. Both complete and incomplete combustion reactions also produce energy in the form of heat and light: complete combustion does so more than incomplete combustion. Heat produced in the reaction, makes it possible for the combustion to continue.

Combustion is a chemical reaction initiated by heat, during which a substance — fuel — reacts with oxygen. It is a high-temperature rapid oxidation reaction that releases energy in the form of heat and light. The fuel undergoes complete combustion if the oxygen supply is plentiful. Fuel undergoes incomplete combustion if the oxygen supply is insufficient.