Difference Between Complete Combustion and Incomplete Combustion

Main Difference – Complete Combustion vs Incomplete Combustion

A combustion reaction is a chemical reaction that releases energy by the oxidation of a fuel. The chemical reactions which release energy are called exothermic reactions. Thus, combustion reactions are exothermic. A fuel can be oxidized with an oxidizing agent. The oxidizing agent for most combustion reactions is atmospheric oxygen. The energy that is released from combustion reactions can be either heat or light. Energy is mainly released in the form of heat; light energy is also released as a flame. Combustion can occur in two ways as complete combustion and incomplete combustion. The main difference between complete combustion and incomplete combustion is that in complete combustion, carbon dioxide is the only product that includes carbon whereas, in incomplete combustion, carbon monoxide and carbon dust are formed as products.

 Key Areas Formed

1. What is Complete Combustion
      – Definition, Properties, Examples
2. What is Incomplete Combustion
      – Definition, Properties, Examples
3. What are the Similarities Between Complete Combustion and Incomplete Combustion
      – Outline of Common Features
4. What is the Difference Between Complete Combustion and Incomplete Combustion
      – Comparison of Key Differences

Key Terms: Carbon Dioxide, Carbon Monoxide, Combustion, Exothermic Reaction, Flame, Fuel, Oxidation, Oxidizing Agent

What is Complete Combustion

Complete combustion is the complete oxidation of fuel. This reaction is highly exothermic and produces a high amount of energy and a limited number of products. In fuel burning or combustion, hydrocarbons in the fuel are oxidized by atmospheric oxygen, giving carbon dioxide (CO₂) and water (H₂O) as products. The complete combustion occurs where there is a sufficient amount of oxygen present. In the presence of oxygen, carbon atoms in hydrocarbons can get oxidized into carbon dioxide and hydrogen is oxidized into water. The general reaction for complete combustion is given below.

Hydrocarbon   +    Oxygen         →            Carbon dioxide    +    Water

For a fuel like ethanol, the complete combustion can be given as,

C₂H₅OH_(l)        +     3O_2(g)            →              2CO_2(g)      +     3H₂O_(l)

The complete combustion reactions result in oxides of carbon, sulfur and other elements present in the fuel. Carbon is oxidized to carbon dioxide whereas sulfur is oxidized into sulfur dioxide. Complete combustion results in less amount of air pollutants. The complete combustion can be generally characterized by a blue flame.

Figure 01: A blue flame is created in complete combustion.

Since the atmosphere is composed of only 21% oxygen by volume, a lot of air is needed for complete combustion to take place. Even though the amount of by products that result from complete combustion is low, it still adds unfavourable emissions. For example, carbon dioxide is a greenhouse gas which causes global warming.

What is Incomplete Combustion

Incomplete combustion is a chemical reaction that involves the partial oxidation of a fuel. The incomplete combustion occurs where there is an insufficient amount of oxygen. Here, the fuel is incompletely oxidized. Hence, the incomplete combustion forms a number of byproducts. But the amount of energy released from this combustion is comparatively low. The major products of incomplete combustion include Carbon monoxide (CO), carbon dust (we call it “soot”) and water (H₂O). The general formula for incomplete combustion is shown below.

Hydrocarbon   +     Oxygen          →        Carbon monoxide    +    Carbon     +    Water

The byproducts may vary according to the amount of oxygen that is involved in the combustion. For example, sometimes it yields only carbon monoxide or soot. However, it commonly gives a mixture of carbon monoxide and soot along with water.

For example, incomplete combustion of ethylene can result in carbon and water as byproducts.

C₂H_4(l)           +         O_2(g)       →       2C_(s)    +     2H₂O_(g)

Incomplete combustion of ethanol can form carbon monoxide and carbon dust along with water.

C₂H₅OH_(l)     +     2O_2(g)     →    2CO_(g)    +    3H₂O_(l)

C₂H₅OH_(l)     +     O_2(g)       →    C_(s)          +    3H₂O_(l)

Figure 2: A yellow flame is produced with incomplete combustion.

Incomplete combustion is characterized by a yellow flame. Since the amount of energy released from incomplete combustion is low, it is undesirable. Moreover, carbon monoxide produced by this combustion is an air pollutant and is deadly for the human body. Carbon monoxide can bind with hemoglobin in our blood and limit the oxygen transportation inside the body.

Similarities Between Complete Combustion and Incomplete Combustion

• Both complete combustion and incomplete combustion are exothermic.
• They produce heat and light as energy forms.
• Both reactions give water as a byproduct.
• Both combustion types involve the oxidation of a fuel.
• These reactions involve the combination of molecular oxygen with fuel.
• Both these combustion reactions result in unfavorable gas emissions.
• They can form flames while burning.

Difference Between Complete Combustion and Incomplete Combustion

Definition

Complete Combustion: Complete combustion is the complete oxidation of fuel.

Incomplete Combustion: Incomplete combustion is the partial oxidation of fuel.

Energy Released

Complete Combustion: Complete combustion produces a high amount of energy.

Incomplete Combustion: Incomplete combustion produces a low amount of energy.

Amount of Oxygen Involved

Complete Combustion: Complete combustion occurs where there is enough oxygen present.

Incomplete Combustion: Incomplete combustion occurs where there isn’t enough oxygen present.

Byproducts

Complete Combustion: Complete Combustion produces carbon dioxide and water as major byproducts.

Incomplete Combustion: Incomplete combustion produces carbon monoxide, carbon dust and water as major byproducts.

Flame

Complete Combustion: Complete combustion creates a blue colored flame.

Incomplete Combustion: Incomplete combustion creates a yellow colored flame.

Effect on the Environment

Complete Combustion: Complete Combustion produces carbon dioxide which can cause global warming.

Incomplete Combustion: Incomplete combustion produces carbon monoxide which is an air pollutant.

Conclusion

Combustion reactions are exothermic reactions that release energy when fuel is burnt. Complete combustion of a fuel yields a high amount of energy whereas incomplete combustion yields a less amount of energy. This is the main difference between complete combustion and incomplete combustion. Complete combustion is very important in industrial scale applications. Incomplete combustion is used in household needs such as burning wood to make heat energy for cooking, etc. Even though there are a number of uses in combustion, it causes the emission of unfavorable gases to the environment that can act as air pollutants.

References:

1.”GCSE Bitesize: Combustion.” BBC. BBC, n.d. Web. Available here. 24 July 2017. 
2.”Combustion.” Combustion. N.p., n.d. Web. Available here. 24 July 2017. 

Image Courtesy:

1. “1938298” (Public Domain) via Pixabay 
2. “1707042” (Public Domain) via Pixabay