Difference Between Complete Combustion and Incomplete Combustion

The main essence of a combustion reaction is that it involves O-oxygen and that it is an exothermic reaction. Combustion reactions release energy in the form of heat and light.


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An example of a combustion reaction involving Propane and oxygen is as below:


Combustion reactions can be of two types depending on the amount of oxygen that is available for the reaction.

Key Takeaways

  1. Complete combustion occurs when a fuel burns completely in the presence of an adequate supply of oxygen, producing carbon dioxide and water as byproducts; incomplete combustion occurs when the oxygen supply is insufficient, leading to the formation of carbon monoxide, soot, or other harmful byproducts.
  2. Complete combustion releases more energy than incomplete combustion, making it more efficient.
  3. Incomplete combustion can have negative consequences, such as increased air pollution and potential health risks due to carbon monoxide exposure.

Complete Combustion vs Incomplete Combustion

The difference between Complete Combustion and Incomplete Combustion is the amount of oxygen available for it. If the amount is sufficient or more, it is a Complete Combustion reaction and if it is less, it is an Incomplete Combustion reaction.

Complete combustion vs Incomplete combustion

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When there is a sufficient or an abundant amount of oxygen available during the process of combustion, then the reaction is known as a Complete Combustion reaction.

When the amount of oxygen is insufficient for the process of combustion, then the reaction is known as an Incomplete Combustion reaction.

Comparison Table

Parameters of ComparisonComplete CombustionIncomplete Combustion
DefinitionThe combustion reaction which takes place in the presence of a sufficient or abundant amount of oxygen. Also known as Complete Combustion.The combustion reaction that takes place in the presence of an insufficient amount of oxygen.
Smoke-typeNo smokeSooty
ProductsUsually produces CO2 (Carbon Dioxide) as a primary product.Usually produces CO (Carbon Monoxide) as a primary product.
Energy ProductionProduces more energy as compared to Incomplete Combustion when combusting the same reactants.Produces lesser energy when compared to Complete Combustion when combusting same reactants.

What is Complete Combustion?

Complete combustion is the combustion process where the amount of oxygen involved in the relationship is sufficient in amount or more than required.

Usually, these reactions take place with hydrocarbons being on the reactant side as reducing agents. Hydrocarbons and oxygen react together to form water and carbon dioxide.

This is the reason, whenever we burn stuff like wood, paper, and similar other items which contain hydrocarbons in them, we see a yellow flame, which is a sign of Incomplete Combustion, rather than a Complete Combustion reaction.

A few examples of Complete Combustion reactions are given below:

Complete Combustion of methane:

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)

Methane here is the reducing agent that reacts with oxygen, which is the oxidizing agent. This gives us carbon dioxide and Hydrogen as the final products. That is the minimum amount of oxygen needed by methane.

Complete Combustion of methanol:

2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(g)

Similar to the above example, methanol too here is the reducing agent and oxygen acts as the oxidizing agent. Methanol is a more complex hydrocarbon and hence requires more molecules of oxygen and produces more water and carbon dioxide.

Complete Combustion of a substance gives the highest amount of energy possible which can be extracted. This is because all of the substance is successfully combusted.

This type of combustion is also called ‘clean combustion’ as the products formed by this combustion does not pollute or harm the environment in any way, since it is just carbon dioxide and water.

A very common example that we see of clean combustion is the burning of LPG in our homes, as it usually produces a clear blue flame and no smoke.

complete combustion

What is Incomplete Combustion?

An Incomplete Combustion reaction is a reaction where the amount of oxygen present in the reaction is insufficient then the required amount of oxygen needed to carry out the reaction in a complete manner.

Like Complete Combustion reactions, the reactants play the same role, where oxygen is an oxidizing agent, and the hydrocarbons are reducing agents.

This type of reaction is mostly undesirable as it releases very little energy compared to complete reactions of the same substances.

This reaction is mainly characterized by a yellow flame with sooty smoke. The primary products of this reaction are water and carbon monoxide (CO).

When household appliances get burnt or catch fire, they undergo an Incomplete Combustion reaction. The toxic carbon monoxide hence produced is colorless and odorless.

Some reaction examples of Incomplete Combustion are:

Incomplete Combustion of Propane-LPG

2 C3H8 + 9 O2 → 4 CO2 + 2 CO + 8 H2O + Heat

Some people might state the fact that there is carbon dioxide present on the product side, and hence this will be a Complete Combustion reaction.

A very common example of Incomplete Combustion is the burning of coal. This produces a lot of soot and smoke and hence causes a lot of environmental degradation.

incomplete combustion

Main Differences Between Complete Combustion and Incomplete Combustion

  1. The products of a complete combustion reaction are environment friendly and do not cause pollution whereas the products of an Incomplete Combustion reaction are major pollutants in today’s world.
  2. A complete combustion reaction produces more energy than an Incomplete Combustion reaction with the same product.
  1. https://pubs.acs.org/doi/abs/10.1021/acscatal.7b02414
  2. https://www.sciencedirect.com/science/article/abs/pii/S0883292701000610
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