University Federico II of Naples, Italy

European Chemistry Thematic Network

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The combustion reaction

Consider the combustion of wood.

The chemical nature of wood is closely related to sugars. To make things simpler, let's consider wood to be composed just of Sugar, whose formula is C₆H₁₂O₆

Actually, wood is composed mainly of Cellulose, that is a polymer made up by repetition of Glucose residues. Glucose is a sugar, and cellulose formula is C₆H₁₀O₅

When burning, wood reacts with Oxygen, which is contained in air:

C6H12O6	+	6 O2	=	6 CO2	+	6 H2O

The two reaction products are Carbon dioxide CO₂ and Water H₂O. Both are released as gases in air. Carbon dioxide is a normal component of air; water is generally known as a liquid, but it is a gas at the high temperature of flame. Indeed, gaseous water is a normal component of air (atmospheric humidity).

So in wood combustion we observe solid wood disappear and be converted to gas products, leaving only some ashes. These are formed by minor components of wood that cannot burn and remain solid.

The other major effect of combustion is production of heat and light. See the page on energy balance for a discussion.

A chemical reaction is a transformation where some molecules are destroyed and new molecules are formed. In the above example, sugar and oxygen molecules disappear and water and carbon dioxyde molecules appear. Molecules are not conserved across chemical reactions.

The product molecules, however, are formed by exactly the same atoms that made up the reactant molecules. Atoms are conserved across chemical reactions, but are rearranged in different molecules.

The combustion reaction of methane is an example of a simpler reaction, that will be discussed in more detail.