There are several types of combustion reactions in chemistry, with the two most common being complete and incomplete combustion. The type or reaction varies depending on several factors, such as the:
- Type of combustible material
- Surface area of the material
- Vapour pressure
- Amount of oxygen
- Atmospheric pressure
Any chemical reaction that involves oxygen as one of the reactants and produces heat and/or light energy is called a combustion reaction. In some instances, such as the explosive reaction of oxygen with hydrogen, combustion can also be classified as a combination reaction.
In this post:
What’s the Difference Between Complete and Incomplete Combustion?
The final products of combustion always include an oxide and the release of heat energy. If the other reactant (aside from oxygen) is an organic compound, the products of a complete combustion reaction are water and carbon dioxide.
Example: Complete combustion of propane gas
As the name suggests, a complete combustion reaction fully utilises oxygen. In this reaction, the other reactant is fully consumed as it combines with oxygen. As you can see in the example, no carbon residue is left. However, in an incomplete combustion, carbon monoxide is produced and some carbon residue is left in the form of soot.
Example: Incomplete combustion of propane gas
Incomplete combustion is bad for the environment because it forms additional pollutants like carbon soot. It’s also dangerous because carbon monoxide is a toxic substance that can kill people in closed spaces, such as inside a garage or in tunnels.
Additionally, incomplete combustion is inefficient because the fuel isn’t completely burned. For an engine, when fuel isn’t completely combusted, it means that the engine will not perform very well in terms of producing mechanical power. It may also lead to engine failure or damage.
What is a Combustible Substance?
A combustible substance is any material that can easily react with oxygen at a fast rate while producing heat and light energy. Combustible substances also come in various states. They may either be gas, liquid, or solid.
Flammable materials, on the other hand, are a category of combustible substances that have lower flashpoints. The technical definition of a combustible material has a flashpoint of above 37.8ºC and below 93.3ºC. Meanwhile, a flammable material has a flashpoint below 37.8ºC.
Many substances can react with oxygen, but usually at very slow rates. For example, the rusting of iron is a reaction with oxygen (known as oxidation) that takes a very long time, making it barely perceptible to the human eye. It may take several days or even months before any noticeable difference is observed. An iron nail will not rust naturally in just a few minutes or hours. With this in mind, iron and many other metals are non-combustible.
On the other hand, hydrogen, methane, alcohol, gasoline, paper, and gunpowder are some examples of highly combustible materials. Unlike iron, these substances react easily with oxygen when they’re ignited by friction, a spark, an increase in ambient temperature, or by a flame.
Some highly volatile liquids will similarly auto-ignite if there is sufficient oxygen and vapour pressure. Combustible liquids like kerosene, diesel, and gasoline have specific flashpoint ranges that are defined by the minimum temperature that can ignite them at a certain vapour pressure. This is the same for flammable liquids, like ethanol and acetone. This table compares common fuels and their flashpoints:
What’s the Difference Between a Combustible and Non Combustible Substance?
Combustible materials rapidly react with oxygen and produce heat and light energy in the process. Meanwhile, a non combustible substance is either too inert to react with oxygen, such as the noble gases, or it reacts very slowly with oxygen, such as iron.
For example, the reaction of iron with oxygen is an oxidation reaction, but not an example combustion. As we touched on earlier, it’s too slow and the energy released is too low. Here’s it’s balance chemical equation:
Fe + O2 ⟶ Fe2O3
Similar oxidation reactions occur with other metals like copper and zinc. At room temperature and normal pressure, most metals do not combust. One of the exceptions is magnesium. You can easily ignite a magnesium strip by exposing it to a flame with relatively low temperature, such as a candle flame:
2Mg + O2 ⟶ 2MgO
Most other metals, like aluminium, have such high melting points that they can only combust in vapour form at very high temperatures.
5 Examples of Combustion
Combustion reactions can be classified into several types depending on the energy and rate of reaction, the chemical byproducts, and the physical properties or combustion behaviour.
- Complete combustion
The burning of petrol inside engines can generally be considered as complete combustion.
Complete combustion completely consumes the combustible material. It usually happens if the conditions are right, such as in highly efficient car engines where the amount of oxygen and temperature are sufficient to sustain the combustion.
- Incomplete combustion
The burning of wood is an example of incomplete combustion. Aside from the non-combustible components of wood, such as minerals, the process of burning wood creates carbon residues.
- Smouldering combustion
A flameless wood or coal ember is an example of smouldering combustion. This is relatively slow and has low heat and light energy.
- Rapid combustion
The explosion of fireworks can be classified as an example of rapid combustion. Most fireworks have oxidisers as part of their ingredients. Once triggered, the gunpowder can rapidly combust and produce high heat and bright lights.
- Turbulent combustion
The burning of fuel by gas turbines, such as those found in jet planes, is turbulent because the gas is channeled through a narrow nozzle.
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