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Balancing Combustion Reactions

As you have learned, there is not always a straightforward way to balance
a reaction. We tend to just go back and forth, balancing elements
on the left and the right, until it works. Combustion reactions are
easier! Balance the elements in the following order: carbon, hydrogen then
oxygen.
Take pentane:

C_{5}H_{12} + O_{2} -> CO_{2}
+ H_{2}O

Step 1 balance carbon: you see 5 carbons
on the left so you know it will produce 5 CO_{2}.

C_{5}H_{12} + O_{2} -> **5**
CO_{2} + H_{2}O

Step 2 balance hydrogen: you see 12 hydrogens
on the left so you know it will produce 6 H_{2}O.

C_{5}H_{12} + O_{2} -> **5**
CO_{2} + **6**
H_{2}O

Step 3 balance oxygen: you see 10+6 or 16
oxygens on the right so you know it will require 8 O_{2}.

C_{5}H_{12} + **8**
O_{2} -> **5** CO_{2}
+ **6** H_{2}O

Wow! It is balanced!

Actually, it doesn't always work out that easily. Sometimes you
get an odd number of oxygens on the right which leads to a fraction in
front of O_{2}. You then have to multiply all of your coefficients
by 2.

(Vocabulary: The numbers subscripted we have been calling subscripts.
These are the formulas of the molecules. The numbers in front of
the formulas (8, 5
& 2), are called coefficients. These
are the numbers we determine when balancing equations.)

Your steps are:

1) balance carbon

2) balance hydrogen

3) balance oxygen

4) Multiply by 2 if you have a fraction.

Lets try a harder problem, hexane:

C_{6}H_{14} + O_{2} -> CO_{2}
+ H_{2}O

Step 1 balance carbon: you see 6 carbons
on the left so you know it will produce 6 CO_{2}.

C_{6}H_{14} + O_{2} -> **6**
CO_{2} + H_{2}O

Step 2 balance hydrogen: you see 14 hydrogens
on the left so you know it will produce 7 H_{2}O.

C_{6}H_{14} + O_{2} -> **6**
CO_{2} + **7**
H_{2}O

(6x2=12 oxygens) (7x1=7
oxygens)

Step 3 balance oxygen: you see 12+7 or
19 oxygens on the right so you know it will require 19/2 or eight and a
half O_{2}.

C_{6}H_{14} + **19/2**
O_{2} -> **6** CO_{2}
+ **7** H_{2}O

Step 4 To get rid of the fraction, we multiply all the coefficients
by 2.

**2** C_{6}H_{14} + **19**
O_{2} -> **12**
CO_{2} + **14**
H_{2}O

Ta da!! Wow, it sure seems like a lot of work but it is really
easy.

Try your hand at it at
Dr Wong's website.