Ethanol Fuel - Chemistry

Chemistry

Glucose (a simple sugar) is created in the plant by photosynthesis.

6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ + 6 O₂

During ethanol fermentation, glucose is decomposed into ethanol and carbon dioxide.

C₆H₁₂O₆ → 2 C₂H₅OH+ 2 CO₂ + heat

During combustion ethanol reacts with oxygen to produce carbon dioxide, water, and heat:

C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O + heat

After doubling the combustion reaction because two molecules of ethanol are produced for each glucose molecule, and adding all three reactions together, there are equal numbers of each type of atom on each side of the equation, and the net reaction for the overall production and consumption of ethanol is just:

light → heat

The heat of the combustion of ethanol is used to drive the piston in the engine by expanding heated gases. It can be said that sunlight is used to run the engine (as is the case with any combustion-based energy source, as sunlight is the only way energy is added to the planet, and geothermal energy, which comes from the heat already present inside the earth).

Glucose itself is not the only substance in the plant that is fermented. The simple sugar fructose also undergoes fermentation. Three other compounds in the plant can be fermented after breaking them up by hydrolysis into the glucose or fructose molecules that compose them. Starch and cellulose are molecules that are strings of glucose molecules, and sucrose (ordinary table sugar) is a molecule of glucose bonded to a molecule of fructose. The energy to create fructose in the plant ultimately comes from the metabolism of glucose created by photosynthesis, and so sunlight also provides the energy generated by the fermentation of these other molecules.

Ethanol may also be produced industrially from ethene (ethylene). Addition of water to the double bond converts ethene to ethanol:

C₂H₄ + H₂O → CH₃CH₂OH

This is done in the presence of an acid which catalyzes the reaction, but is not consumed. The ethene is produced from petroleum by steam cracking.

When ethanol is burned in the atmosphere rather than in pure oxygen, other chemical reactions occur with different components of the atmosphere such as nitrogen (N₂). This leads to the production of nitrous oxides, a major air pollutant.