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Fifth Example

This fifth example represents the burning of propane. It has C3H8 + O2 rtarrow.gif (850 bytes) CO2 + H2O. This equation introduces two new features. Look where those elements are going.

Notice that the oxygen from the O2 on the left ends in two different compounds on the right. When something like that happens, wait until the very last to figure out what the coefficient of the O2 should be. The amount of oxygen needed on the left will depend on the amount of carbon dioxide and water together, so we won't be able to determine the amount of oxygen until we have figured out the others.

Also notice that the carbon and the hydrogen from the C3H8 go to two different places. The carbon goes to the CO2 and the hydrogen goes to the H2O.

Equations of this kind are easiest to balance by focusing on the C3H8 and noting that the three carbons go to the CO2 and the eight hydrogens go to the H2O. So the place to start is with the C3H8. Figure out how much CO2 and H2O will be made from the C3H8, then figure out how much oxygen we will need to do it.
C3H8 +   O2 rtarrow.gif (850 bytes)   CO2 +   H2O
3 C, 8 H, 2 O 1 C, 2 H, 3 O
OK, let's do it. Three carbons from C3H8 will give 3 CO2's. Three carbons on the left have to give three carbons on the right.
C3H8 +   O2 rtarrow.gif (850 bytes)  3 CO2 +   H2O
3 C, 8 H, 2 O 3 C, 2 H, 7 O
Also, eight hydrogens from C3H8 will have to give eight hydrogens on the right. To get eight hydrogens, we will need four waters, each with two hydrogens. That gives eight hydrogens.
C3H8 +   O2 rtarrow.gif (850 bytes)  3 CO2 +  4 H2O
3 C, 8 H, 2 O 3 C, 8 H, 10 O
Now, we have three CO2 and four H2O. How many oxygens is that? Let's see. There are six oxygens from the CO2 and four oxygens each from the 4 H2O's. That makes a total of ten oxygen atoms on the right, so we need to start with ten on the left. They come in pairs, so we need five of them.
C3H8 +  5 O2 rtarrow.gif (850 bytes)  3 CO2 +  4 H2O
3 C, 8 H, 10 O 3 C, 8 H, 10 O

Now we should double check to see that everything is balanced. You have three carbons on each side, eight hydrogens on each side, and ten oxygens on each side; so the balanced equation becomes C3H8 + 5 O2 rtarrow.gif (850 bytes) 3 CO2 + 4 H2O.

 

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