You have certainly watched several fires on television, witnessed an area burning or set fire to some material. What actually happened? What is a burn or combustion? What is needed for combustion to take place? Why can materials be burned? Is all material burning? Let's now answer these questions and learn more about the combustion phenomenon.
Burning, fire, cremation are synonyms that can be used to represent the chemical phenomenon called combustion. Combustion is the chemical reaction in which new substances are produced together with light and heat (wood burning) or just heat (methanol burning, a type of alcohol used as fuel).
As it is a chemical process (the materials used before will be transformed into new materials), we can represent the combustion through an equation, but it is necessary to inform that for a combustion reaction to occur, the presence of gas is required oxygen2). See below the equation that represents combustion:
Fuel + O2 → CO2 + H2O
For the combustion reaction to take place, four fundamental elements must be present, they are:
Fuel: material that will undergo transformation and fuel combustion. Example: gasoline.
Oxidising: substance that gives life and sustains combustion. Example: oxygen gas.
Activation energy: it is an energy that will cause the combustion reaction.
Chain reaction: we can say that it is the combination of the three elements above, as one is related to the other, forming a cycle. Example of activation energy: a spark.
The materials that can be burned are varied, but not every material can be used as fuel. The greatest examples of fuels are organic compounds (substances that have the elements hydrogen and carbon in their constitution), such as:
gasoline (C8H18)
ethanol (C2H6O)
methanol (CH4O)
glucose (C6H12O6)
But it is interesting to note that some metals also undergo combustion, such as:
iron (Fe)
sodium (Na)
magnesium (Mg)
Pure metal magnesium
Observation:Therefore, for the reaction to take place it is necessary that the fuels have chemical affinity with the oxidizer, otherwise the process will not happen. For example, sand has no affinity for oxygen, so it does not burn, on the contrary, it is used to put out a fire.
There are some types of combustion, the most common being the following:
Complete combustion:is the one in which water is produced (H2O) and carbon dioxide (CO2). Its flame is blue. Example: lighting a lighter or new stove flame.
CH4O + 3/2 O2 → CO + 2 H2O
Example of a complete methanol (CH) combustion equation4O)
Incomplete combustion:is one in which water and carbon monoxide (CO) are produced, with a smaller amount of carbon dioxide being produced as well. In addition to these products, this combustion produces soot (coal or carbon). Its flame is yellowish. Example: burning wood.
CH4O + 3/2 O2 → CO + 2 H2O
Example of an Incomplete Combustion Equation
Slow combustion:it is the one in which light and heat are not produced, in addition to happening very slowly, it is the model of combustion that occurs with iron. It undergoes combustion, but it does not produce carbon dioxide and water, it produces rust. Example: Rust formation (Fe2O3) steel wool (used for washing dishes).
2 FE + 3 O2 → 2 Fe2O3
Example of a slow combustion equation
Rust on a padlock, an example of slow combustion
By Me. Diogo Lopes Dias
