Difference Between Combustion and Oxidation Reactions of Ethanol

Main Difference – Combustion vs Oxidation Reactions of Ethanol

Ethanol is an alcohol having the molecular formula C₂H₅OH. The chemical formula of ethanol is CH₃CH₂OH. Ethanol is used as fuel since it can undergo combustion reactions. It can also undergo oxidation reactions to form aldehyde forms and carboxylic acid forms. The main difference between combustion and oxidation reactions of ethanol is that combustion reactions of ethanol always produce heat and light whereas oxidation reactions of ethanol do not always produce heat and light.

Key Areas Covered 

1. What are the Combustion Reactions of Ethanol
      – Definition, Properties, Reactions 
2. What are the Oxidation Reactions of Ethanol
      – Definition, Properties, Reactions 
3. What is the Difference Between Combustion and Oxidation Reactions of Ethanol
      – Comparison of Key Differences

Key Terms: Aldehyde, Biofuel, Carbon Dioxide, Carbon Monoxide, Carboxylic Acid, Combustion Reaction, Complete Combustion, Complete Oxidation, Ethanol, Gasoline, Incomplete Combustion, Incomplete Oxidation, Oxidation Reaction

What are the Combustion Reactions of Ethanol

Combustion reactions of ethanol are the reactions that occur when ethanol is burnt. Ethanol is a highly flammable liquid that can be used as a fuel. Burning of ethanol can produce heat and light as energy. Therefore, combustion of ethanol is an exothermic reaction. When Ethanol is burnt in the presence of molecular oxygen (O₂), it forms two final products. They are carbon dioxide (CO₂) and water molecules (H₂O).

The combustion of ethanol is indicated by a blue colored flame. The combustion of ethanol is a simple process which involves the combination of ethanol and oxygen. The combustion of ethanol can occur in two ways.

• Complete Combustion
• Incomplete Combustion

Figure 01: The blue flame indicates the complete combustion of ethanol.

The complete combustion results in CO₂ and H₂O. But incomplete combustion will produce carbon monoxide (CO) or Carbon (C) as a product. Incomplete combustion will take place when there is insufficient oxygen (O₂).  

Complete Combustion of Ethanol

             CH₃CH₂OH_(l)  +  3O_2(g)       →     2CO_2(g)   +    3H₂O_(l)

Incomplete Combustion of Ethanol

              CH₃CH₂OH_(l)  +   2O_2(g)      →     2CO_(g)   +    3H₂O_(l)

              CH₃CH₂OH_(l)   +   O_2(g)        →     C_(s)   +    3H₂O_(l)

The incomplete combustion often results in a mixture of carbon monoxide (CO) gas and carbon (C) dust.

The heat produced by the combustion of ethanol is used to drive the piston of vehicle engines. Ethanol can also be used as a rocket fuel. In addition, ethanol can be produced as a biofuel from the biomass of plant material. Therefore, ethanol shows environmental friendly properties than fossil fuel.

Ethanol is a good additive for gasoline. The mixing of ethanol with gasoline prevents some emissions of air pollutants. However, when burnt, ethanol produces a flame which releases pollutants.

What are Oxidation Reactions of Ethanol

Oxidation reactions of ethanol are chemical reactions that take place when ethanol is oxidized by oxidizing agents. Oxidation of ethanol produces an aldehyde called Ethanal as the first product. It can undergo further oxidation to form the carboxylic acid form, which is known as Ethanoic acid.

Figure 2: Oxidation of Ethanol

However, oxidation of ethanol can also occur in the presence of a catalyst. This catalyst is used to reduce the activation energy of the oxidation reaction. If the activation energy is high, the reaction will not be initiated. Oxidation can occur in two phases:

• Complete Oxidation
• Incomplete Oxidation

The complete oxidation of ethanol forms Ethanoic acid as the end product. Incomplete oxidation of ethanol forms Ethanal as the end product. Both oxidations will produce water molecules (H₂O) as byproducts.

Complete Oxidation of Ethanol

Ethanol   +     Oxygen          →            Ethanal        +     Water     →          Ethanoic Acid    +    Water

CH₃CH₂OH_(l)   +   [O]         →           CH₃CHO_(l)   +   H₂O_(l)       →           CH₃COOH_(l)    +     H₂O_(l)      

The complete oxidation of ethanol results in Ethanoic acid at the end of the reaction. But ethanol oxidation first forms Ethanal and then Ethanal is further oxidized into Ethanoic acid.

Incomplete Oxidation of Ethanol

  Ethanol   +      Oxygen             →           Ethanal     +     Water

CH₃CH₂OH_(l)   +   [O]              →           CH₃CHO_(l)   +   H₂O_(l)

In the above equation, [O] indicates the atomic oxygen that comes from the oxidizing agent. As an example, let’s consider Sodium dichromate (Na₂Cr₂O₇) was used as the oxidizing agent along with sulphuric acid (H₂SO₄).

CH₃CH₂OH_(l)    +      Na₂Cr₂O_7(aq)    +   H₂SO_4(aq)      →       CH₃CHO_(l)   +   2NaCrO_4(aq)  +   2H₂O_(l)

Oxidation of Ethanol needs either a catalyst or an oxidizing agent in order to complete the reaction. However, oxidation of ethanol does not produce heat or light as energy forms. Another way of oxidation of ethanol is through catalysts. Silver catalyst is such a catalyst. Ethanol can be oxidized by passing a mixture of ethanol vapor and air over a silver catalyst at 500^oC. This results in Ethanal as the product along with water (H₂O).

Difference Between Combustion and Oxidation Reactions of Ethanol

Definition

Combustion Reactions of Ethanol: Combustion reactions of ethanol are the reactions that occur when ethanol is burnt.

Oxidation Reactions of Ethanol: Oxidation reactions of ethanol are chemical reactions that take place when ethanol is oxidized by oxidizing agents.

Oxidizing Agents

Combustion Reactions of Ethanol: Combustion reactions of ethanol do not require oxidizing agents.

Oxidation Reactions of Ethanol: Oxidation reactions of ethanol require the presence of oxidizing agents.

Catalysts

Combustion Reactions of Ethanol: Combustion reactions of ethanol do not need catalysts.

Oxidation Reactions of Ethanol: Oxidation reactions of ethanol can occur in the presence of catalysts.

End Products

Combustion Reactions of Ethanol: The end products of oxidation reactions of ethanol can be CO₂, CO, C, and H₂O.

Oxidation Reactions of Ethanol: The end products of combustion reactions of ethanol can be either Ethanal or Ethanoic acid along with H₂O.

Energy Forms

Combustion Reactions of Ethanol: Combustion reactions of ethanol can produce heat and light.

Oxidation Reactions of Ethanol: Oxidation reactions of ethanol cannot produce heat and light.

Conclusion

Combustion is also an oxidation reaction since the end product of combustion is always an oxidized species. Moreover, combustion involves the combination of oxygen with the starting material. This also indicates that combustion is an oxidation reaction. Although there are similarities between these two phenomena, there are distinct properties that allow us to distinguish the difference between combustion and oxidation reactions of Ethanol.

References:

1.”Oxidation of alcohols.” Chemguide. N.p., n.d. Web. Available here. 21 July 2017. 
2.”Combustion of Ethanol.” ABHSscience. N.p., n.d. Web. Available here. 21 July 2017. 
3.”Oxidation of Ethanol.” Oxidation of Ethanol: Easy exam revision notes for GSCE Chemistry. N.p., n.d. Web. Available here. 21 July 2017. 

Image Courtesy:

“Water cooler bottle ignition” By Chocolateoak – Сопствено дело (CC BY-SA 3.0) via Commons Wikimedia
“Oxidation of ethanol” By Richtom80 at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia