Chemical equations: Balancing chemical equations

When graphite burns, the carbon atoms react with the oxygen in the air. If there is plenty of oxygen, carbon dioxide is formed... ... and if there is less oxygen available, the poisonous gas carbon monoxide is formed. When we describe the reaction where carbon dioxide is formed, it looks like this: with a word formula... ... with images of the atoms... ...

and with chemical symbols. It's enough with one carbon atom and one oxygen molecule to form one molecule of carbon dioxide. In the other reaction, where carbon monoxide is formed, it's not quite that simple. The word formula looks almost the same. The only difference is the name of the compound that's formed.

What does the reaction look like when we draw the atoms? The oxygen atoms are attached two by two, in oxygen molecules. However, the product formed - carbon monoxide - only has one oxygen atom. Where did the other go? It can't just disappear.

We can think of it like this: Since we start with two oxygen atoms, two carbon monoxide molecules can form. Let's draw one more molecule! Now the number of oxygen atoms match, two on each side of the equation. But then the number of carbon atoms doesn't match. We've got two carbon atoms after the reaction, one in each carbon monoxide molecule.

The second carbon atom can't have appeared out of nowhere! There must have been two carbon atoms before the reaction as well. So let's draw one more carbon atom! Now there's the same number of carbon atoms on the left side as on the right: two of them. And the same amount of oxygen atoms before the reaction, as after.

Two of them. The two sides match. The equation is balanced. Now there's just one thing left: writing down the balanced chemical equation, with chemical symbols. We can use the image that we've drawn.

Carbon atoms don't attach to each other in any specific number, so we write these as two independent carbon atoms: two C. This number telling us how many carbon atoms we have, is called the coefficient for this substance. Oxygen atoms come two by two, so they are written as O-two, with the two as a subscript. Only one oxygen molecule is needed. So we don't write a coefficient in front.

The carbon monoxide molecule is written as C - O, one carbon and one oxygen. Two of those were formed, so we write the coefficient 'two' before the carbon monoxide molecule. By putting these numbers in - the coefficients - we've balanced the chemical equation. The equation needs to be balanced, if we want to calculate how much is needed of each substance. In this case, we see that the combustion needs twice as many carbon atoms, as oxygen molecules.

If we start with 20 carbon atoms, we're going to need ten oxygen molecules. If we have a thousand oxygen molecules to start with, two thousand carbon monoxide molecules will form, and so on. The coefficients in a chemical equation show the proportions of the substances. It's usually easier to count the number of atoms of each kind, if you draw the molecules involved in a reaction. This can help a lot when the examples get a bit more complicated.