Chemical equations: Propane combustion reaction

A chemical reaction can be described with a chemical equation. It tells us which substances are reacting, and which are formed. A chemical equation also shows us the ratios between the substances: how much is needed of each substance so that the two sides match. Because of course, atoms can't disappear in the reaction; and no new atoms can form. The atoms present before the reaction, have to be there afterwards.

Figuring out how many atoms or molecules we need of each kind, is called balancing the chemical equation. Now, let's balance a chemical equation that involves four different substances. Two reactants, that turn into two products. We'll start with the gas propane, the substance found in bunsen burners. When propane burns, it reacts with the oxygen in the air.

The substances formed in the combustion are carbon dioxide and water. With a word formula, it'll be: Propane plus oxygen becomes carbon dioxide plus water. The word formula doesn't tell us anything about the amount of oxygen needed to burn a certain amount of propane, or how much carbon dioxide is formed. But we will be able to see that, when we have balanced the equation. Here's how to do it: The molecules in propane are made of three carbon atoms and eight hydrogen atoms.

They look like this. The propane burns in oxygen gas. Oxygen molecules consist of two oxygen atoms attached to each other... ... and carbon dioxide and water are formed. The molecules in carbon dioxide consist of one carbon atom and two oxygen atoms.

The water molecules are made of one oxygen atom and two hydrogen atoms. The shape of the molecules isn't what's important here, just the number of atoms of each kind, of each element. To make it clearer, we can draw them like this. The carbon atoms on one row, the hydrogen atoms on another row, and the oxygen atoms on a third row. This'll make it easier to count one type of atom at a time.

We can start with the carbon atoms. There are three carbon atoms before the reaction. In order to have three carbon atoms after the reaction, more carbon dioxide molecules are needed. And every time you add one, you have to add both its carbon atoms and its oxygen atoms into the table. Now there are the same number of carbon atoms on the right side as on the left.

The number of hydrogen atoms before the reaction is eight. How many water molecules will then be formed? A total of four, since there are two hydrogen atoms in each water molecule. Now, the numbers of hydrogen atoms match as well! Then there's just the oxygen atoms left.

How many are there on the right-hand side? Six oxygen atoms in the carbon dioxide molecules, and four in the water molecules - a total of ten. How many oxygen molecules must have reacted, in order for there to be ten atoms? That's right, five molecules, with two atoms in each. Let's make sure it totals five oxygen molecules on the left side.

It's probably best to double check our calculations. Three carbon atoms before; and three after the reaction. Eight hydrogen atoms on the left; and eight on the right. Ten oxygen atoms in the reactants; and ten in the products. The equation is balanced!

One propane molecule plus five oxygen molecules form: three carbon dioxide molecules, plus four water molecules. If we write that as a chemical equation, with the chemical formula for each substance, it'll be one C-3-H-8 plus five O-2, reaction arrow, Three C-O-2 plus four H-2-O. We drew the atoms in a table, so that it would be easier to count them. You could of course count them directly in the chemical equation, for instance: "four H-2-O" contains four times two... ... hydrogen atoms.

Eight. Which is the same number as in one molecule of propane. Both methods work just as well when balancing the chemical equation. Use the method that works best for you.