Specification content

Alcohol production
Biofuels
Classification and reactions

Biofuels

Biofuels are fuels made from recently living organisms. There are broadly three catagories:

1 First-generation biofuels are made largely from edible sugars and starches.
2 Second-generation biofuels are made from non-edible plant materials.
3 Third-generation biofuels are made from algae and other microbes.

"write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel and give reasons why this statement is not valid"

Equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel:

Photosynthesis : 6CO2+6H2O→C6H12O6+6O2
Fermentation : C6H12O6 → 2C2H5OH + 2CO2
Combustion : C2H5OH + 3O2 → 2CO2 + 3H2O

Add the fermentation and combustion reactions together :

(1) : C6H12O6 → 2C2H5OH + 2CO2
(2) : C2H5OH + 3O2 → 2CO2 + 3H2O
(3) : (2) x 2 : 2C2H5OH + 6O2 → 4CO2 + 6H2O

Add (1) and (3)

(4) : C6H12O6 + 2C2H5OH + 6O2 → 2C2H5OH + 2CO2 + 4CO2 + 6H2O
(5) : C6H12O6 + 6O2 → 2CO2 + 4CO2 + 6H2O
(6) : C6H12O6 + 6O2 → 6CO2 + 6H2O

Equation (6) is respiration

The above equations show that during the fermentation process, glucose is converted into ethanol and carbon dioxide. When ethanol is burned as a fuel, it releases carbon dioxide and water, which are taken up by plants during photosynthesis, thus completing the carbon cycle. This suggests that the production and use of ethanol fuel does not add any additional carbon dioxide to the atmosphere, making it a carbon-neutral fuel.

However, this statement is not entirely valid. While it is true that the burning of ethanol produces carbon dioxide and water, the production of ethanol itself requires energy inputs and generates emissions. For instance, the cultivation of crops such as corn and sugarcane for ethanol production requires land, water, fertilizers, and pesticides, which contribute to greenhouse gas emissions. Additionally, the fermentation process requires energy for heating and cooling, and the transportation of the finished product generates emissions as well.

Moreover, some studies have suggested that the use of ethanol fuel may not be entirely carbon-neutral because of indirect land-use change (ILUC). The production of biofuels like ethanol can cause a shift in land use patterns, such as the conversion of forests and grasslands into croplands, which leads to increased carbon emissions. Therefore, while ethanol produced by fermentation can be considered a relatively clean and renewable fuel source, it is not entirely carbon-neutral.