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:

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!