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Second Example
The next example is Al (aluminum) plus O2 (oxygen) becomes Al2O3
(aluminum oxide). This one is a little bit more involved even though you still have two
elements combining to form a compound.
Let’s start with an inventory. We have one aluminum on the left and
two aluminums on the right. (We could just put a two in front of the aluminum on the left
to balance that, but let's finish the inventory first.) Also, there are two oxygens on the
left and three on the right. Another way of saying that is oxygens come in pairs on the
left and triplets on the right. So, if you have something that starts out in pairs and
ends up in triplets, it is not going to be as easy to balance as what we had before. It is
a lot like trying to add fractions. If you can remember what you had to struggle through
when you learned to add 1/2 to 1/3, you used something called the lowest common
denominator, the least common denominator, or perhaps it was called the least common
multiple or something like that. The least common multiple of 2 and 3 is 6. If we started
with 6 oxygens on the left, we could end up with 6 oxygens on the right and still have
pairs and triplets.
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Al + O2  Al2O3 |
| 1 Al, 2 O |
2 Al, 3 O |
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To do this we could put a three in front of the O2 that gives 6
oxygens, and put a two in front of the Al2O3 and that gives 6
oxygens on the right. That balances the oxygen. Having done that we end up with four
aluminums in the 2 Al2O3's (two formula groups times two aluminum
atoms per Al2O3 formula group).
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| Al + 3
O2 2 Al2O3 |
| 1 Al, 6 O |
4 Al, 6 O |
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If we finish with four aluminums on the right, we must start with four
aluminums on the left, so we put a four in front of the Al on the left side. Thus, the
balanced equation for that reaction is 4 Al + 3 O2 2 Al2O3.
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| 4 Al
+ 3 O2 2
Al2O3 |
| 4 Al, 6 O |
4 Al, 6 O |
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Note that in this particular equation I skipped over the aluminum and did not try to
balance it first. I started with the oxygen. You can start anywhere you want; and as you
keep going back and forth, you will eventually get the equation balanced. However, if you
start with the combination that is going to give you the most problems and get that sorted
out first, then the easier ones fall into place and you don't have to change things so
many times.
It might be useful to start over with that equation and try balancing the aluminum
first and carry it through until you get it balanced. See if it makes a difference to you,
whether you start with aluminum or oxygen.
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E-mail instructor:
Eden Francis
Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender
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