Reaction Types: Combustion Reactions
Combustion
Combustion generally means the reaction of oxygen gas (O2) with a substance.
Other words that describe this are burning, oxidation and respiration.
When we burn fuels in air (oxygen) we generally get carbon dioxide and water vapour.
Most fuels are hydrocarbons - i.e substances that contain carbon and hydrogen.
Carbon can burn in:
 lots of air to form carbon dioxide CO2
 a little air to form carbon monoxide CO,
 if nothing happens carbon (soot ) particles may also be formed.
Hydrogen reacts with the oxygen in the air and always forms H2O, i.e water vapour.
LOOK AT THE FLOW CHARTS BELOW
Here are some examples:
Notice that the products are all the same, in every reaction. Just the amounts of carbon dioxide and
water vapour vary.
CH4
+
2O2
CO2
+
2H2O
2C2H6
+
7O2
4CO2
+
6H2O
C3H8
+
5O2
3CO2
+
4H2O
2C6H6
+
15O2
12CO2
+
6H2O
2C8H18
+
25O2
16CO2
+
18H2O
If the substance all ready contains oxygen it is likely to burn very well - its carrying its own
fuel. If the substances that are burnt contain other elements like nitrogen or sulphur then the gases
nitrogen dioxide and sulphur dioxide are also produced.
ethanol
2C2H5OH
nitroglycerine
4C3H5N3O9 + 11O2
+ 7O2
4CO2
+ 6H2O
12CO2 + 10H2
+ 12NO2
O
ethyl mercaptan
C2H5SH
+ 9O2
4CO2
+ 6H2O
So if we burn in air (oxygen) substances containing atoms :
 C and H (hydrocarbons ) the products are carbon dioxide and water
 C, H and O the products are carbon dioxide and water
 C, H and N the products are carbon dioxide, water and nitrogen dioxide
 C, H and S the products are carbon dioxide, water and sulphur dioxide
+ 2SO2
Balancing these chemical equations.
In all the examples above the chemical equations have all been balanced. In order to do this we
need to ensure that the number of atoms on each side are exactly the same. Look at the simple
example below involving magnesium and oxygen. Magnesium
Mg
+
+
1 magnesium
2 Mg
2 magnesium
Oxygen
O2
2 oxygens
+
O2
2 oxygen
Magnesium
Word
oxide
equation
MgO
Unbalanced
1 magnesium
chemical
1 oxygen
equation
2MgO
balanced
2 magnesium
chemical
2 oxygen
equation