As you have learned, there is not always a straightforward way to balance a reaction. We apply try and error, until it works.
Combustion reactions are easier!
The only thing you need to do is to follow an order for balancing the reaction.
The correct order is: “CHO”, carbon, hydrogen and then, oxygen.
We will balance pentane:
C5H12 + O2 -> CO2 + H2O
Step 1 balance carbon: you see 5 carbons on the left so you know it will produce 5 CO2.
C5H12 + O2 -> 5 CO2 + H2O
Step 2 balance hydrogen: you see 12 hydrogens on the left so you know it will produce 6 H2O.
C5H12 + O2 -> 5 CO2 + 6 H2O
Step 3 balance oxygen: you see 10+6 or 16 oxygens on the right so you know it will require 8 O2.
C5H12 + 8 O2 -> 5 CO2 + 6 H2O
Sometimes you get an odd number of oxygens on the right which leads to a fraction in front of O2.
Your steps then will be:
1) balance carbon
2) balance hydrogen
3) balance oxygen
4) Multiply by 2 if you have a fraction.
Lets try a harder problem, hexane:
C6H14 + O2 -> CO2 + H2O
Step 1 balance carbon: you see 6 carbons on the left so you know it will produce 6 CO2.
C6H14 + O2 -> 6 CO2 + H2O
Step 2 balance hydrogen: you see 14 hydrogens on the left so you know it will produce 7 H2O.
C6H14 + O2 -> 6 CO2 + 7 H2O
(6×2=12 oxygens) (7×1=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 O2.
C6H14 + 19/2 O2 -> 6 CO2 + 7 H2O
Step 4 To get rid of the fraction, we multiply all the coefficients by 2.
2 C6H14 + 19 O2 -> 12 CO2 + 14 H2O
Now you try it…………
Combustion reactions Worksheet
- Copy the following exercises,
- Write the equation for the complete combustion of the substances in oxygen
- Balance the equations
- Methane (CH4) + Oxygen
- Ethane (C2H6) + oxygen
- Propane (C3H8) + oxygen
- Butane (C4H10) + oxygen
- Pentane (C5H12) + oxygen
- Hexane (C6H14) + oxygen
- Ethene (C2H4) + oxygen
- Ethyne (C2H2) + oxygen
- Benzene (C6H6) + oxygen
- Ethanol (C2H5OH)
- Propanol (C3H7OH)
- Acetone (C3H6O)
- Toluene (C6H5CH3)
- Octanol (C8H15OH)
- Decane (C10H22)
How to balance combustion equations
Combustion reactions can also be balanced using the formulas below. After you balance the equations at the top, verify that your results agree with the following general formulas.
CxHy + (x + y/4) O2 → x CO2 + y/2 H2O
• Compounds containing oxygen too