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The following extract was taken from the website of a leading car manufacturer - HSC - SSCE Chemistry - Question 22 - 2008 - Paper 1
Question 22
The following extract was taken from the website of a leading car manufacturer. Critically evaluate the extract with reference to ethanol being a ‘carbon-neutral’ f... show full transcript
Worked Solution & Example Answer:The following extract was taken from the website of a leading car manufacturer - HSC - SSCE Chemistry - Question 22 - 2008 - Paper 1
Step 1
Critically evaluate ethanol's carbon neutrality
Answer
Ethanol is often promoted as a carbon-neutral fuel because the carbon dioxide (CO₂) released during its combustion is theoretically equal to the CO₂ absorbed by the plants during their growth phase.
The combustion of ethanol can be represented by the chemical equation:
ightarrow 2CO_2 + 3H_2O$$ In this process, 2 moles of carbon dioxide are produced for every mole of ethanol burned. However, it's essential to consider that the net carbon neutrality also depends on the cultivation practices of the crops used to produce ethanol, such as corn or sugarcane. The energy inputs and CO₂ emissions involved in cultivating, harvesting, and processing the crops must also be evaluated.Step 2
Support your answer with relevant chemical equations
Answer
During the fermentation process to produce ethanol, sugars are converted into ethanol and carbon dioxide by yeast. The balanced equation for this fermentation can be written as:
ightarrow 2C_2H_5OH + 2CO_2$$ This reaction indicates that one mole of glucose ( $C_6H_{12}O_6$) produces two moles of ethanol ( $C_2H_5OH$) and two moles of carbon dioxide. It's important to note that while the combustion of ethanol yields CO₂, the lifecycle analysis must consider all inputs, including fossil fuels used in agriculture, transportation, and processing, which may offset the carbon neutrality claim. Thus, while ethanol shows potential as a renewable fuel, the overall impact on carbon emissions can vary significantly based on agricultural practices and energy sources used.