Kitchen chemistry: burning propane

Continuing my kitchen chemistry series, I thought I’d talk about the combustion of natural gas. Many kitchens have a gas stove. Indeed, “we’re cooking with gas” is a colloquial phrase meaning that things are going well.

One common type of natural gas is propane. While propane is a gas at room temperature and atmospheric pressure, it becomes liquid at high pressure. It is shipped in liquid form, in pressurised containers (photo of truck below by J. Smith):

Each molecule of propane consists of three carbon atoms (black in the model below), and eight hydrogen atoms (white in the model below), and propane therefore has the chemical formula C3H8. As always, each carbon atom has four connections (chemical bonds) to other atoms:

In reality, the atoms are closer together, so that a more realistic shape for the propane molecule is this:

When propane burns, each propane molecule combines with five oxygen molecules in the air. Each oxygen molecule is a pair of oxygen atoms (O2), connected by a double bond:

The result of the combustion is a complete rearrangement of the 21 atoms, giving three molecules of carbon dioxide (CO2) and four molecules of water (H2O) in vapour form:

Combustion of propane

In symbolic form, C3H8 + 5 O2 → 3 CO2 + 4 H2O. A somewhat similar reaction happens when our bodies “burn” food, but it is slower and more controlled. Fortunately!