In math one of the first things you learn is addition and subtraction. In chemistry you also need to learn to add and subtract. Instead of using numbers however, we will use atoms and molecules. This is the part of chemistry known as stoichiometry. Learning how to write a balanced chemical equation is fundamental to doing good chemistry.

Here is an example of a balanced chemical reaction for the formation of carbon dioxide :

stoichiometry 1a

The reason the above equation is balanced is because there is the same number of oxygen and carbon atoms on either side of the equation. Please note that in a chemical equation, all the items on the left are known as the reactants and all the items on the right are called products.

If you see a number in front of an atom or molecule this is how many of the atoms or molecules there are. Yet they will each be free floating and not bonded. See the left side of the illustration above where the 2 carbon monoxides are not bonded to each other. Here is another example in which we have the number 2 in front of Chlorine, this indicates that there are two Chlorine atoms that are not bonded:

equation chlorine not bondedIf the number is a subscript to the right of the atom, this means there are two atoms bonded together:

equation chlorine bondedA subscript is always for individual atoms alone and never for the molecule as a whole:

equation chlorine barium bonded

Balancing Equations

Now, if we look back at the original equation at the top of the page, you can see 2 oxygen atoms bonded together as O2. If we count up all the atoms on the left side of the equation we get a total of 2 carbon and 4 oxygen (and remember that these are the reactants because they are on the left side of the equation). Now if we check the right side of the equation we see that the equation is balanced because there are still 2 carbon atoms and 4 oxygen atoms (and these are the products because they are on the right side).

You may be wondering why we need to use 2 carbon monoxide molecules to create carbon dioxide. After all, would it not be simpler to use 1 carbon monoxide and then add just 1 oxygen atom to create 1 carbon dioxide? Yes it would be, but we cannot do this because in real life, sources of oxygen come bonded as a molecule of O2. So if we have only 1 carbon monoxide (CO) molecule and added O2, we would have 3 oxygens. But there is only room for 2 oxygens in CO2. We’d have an extra oxygen atom which would unbalance our equation. To solve for this we use 2 carbon monoxide molecules and this balances everything out.

Before moving on to the next tutorial please see our sample questions for this tutorial.


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