So sweet
Critically evaluate the extract with reference to ethanol being a ‘carbon-neutral’ fuel. Support your answer with relevant chemical equations. (5 marks)
I remember this was in hsc, I'll pretend that I remember the extract. It was something about a car manufacturer iirc.
Ethanol is a renewable resource as it is naturally obtained from cellulose which is a major component of biomass, equating to 50% of it. Firstly the cellulose is obtained from crops such as sugar cane and corn. The cellulose then undergoes enzymatic/acid digestion with concentrated 5 mol/L sulphuric acid to be broken down into Glucose. The glucose is then fermented (reactions conditions: 37 degrees Celsius, anaerobic environment to prevent the ethanol from oxidising into ethanoic acid [vinegar], presence of sulphate to prevent growth of bacteria,15% ethanol concentration, yeast enzymes: zymase, nitrates and phosphates: yeast nutrients and glucose).
Fermentation: C6H1206(aq) ------yeast enzymes, 37 degC ----> 2CO2(g) + 2C2H5OH(aq)
This produces 15% ethanol concentration which can be distilled to obtained higher concentrations such as ~ 95%.
Ethanol has several advantages for it's use:
1) Ethanol is a carbon neutral fuel because the CO2(g) released upon combustion of ethanol:
C2H5OH(aq) + 3O2(G) -----> 2CO2(g) + 3H2O(L)
is equivalent to the amount of carbon dioxide used in its photosynthesis.
6CO2(g) + 6H2O(l) ----sunlight/chlorophyll -- C6H12O6(aq) + 6O2(g)
2) Ethanol is a renewable resource as it is obtained from biomass, so it is a better alternative than octane which is obtained from non-renwable fossil fuels.
3) Ethanol is more likely to undergo complete combustion which does not produce any pollutants such as carbon (soot) or CO (which binds to haemoglobin 10 times faster than oxygen, preventing oxygen intake): eqn is above
[insert equation for combustion of octane here, releasing pollutants]
Disadvantages of using ethanol:
1) Ethanol at present is not economically viable as a complete replacement because
2) Ethanol to be used in concentrations greater than 10% in engines require engine modifications to prevent knocking.
3) For the plants and crops to be planted in third world countries, it would mean that resources are allocated to planting crops for ethanol production rather than for food in third world countries which could result in famine and is hence unfavourable.
4) Ethanol is not completely a carbon neutral fuel because during the distillation of ethanol, fossil fuels are utilised and this process also releases CO2(g) into the atmosphere, thus it not technically carbon neutral
EVALUATION: Ethanol is a renewable resource, however the extract (since it is written by a leading car manufacturer) fails to make reference to the CO2 released during other processes and thus the ethanol is NOT carbon neutral. The leading car manufacturer’s article is not accurate because it is not a reliable source and thus the statements made are not scientifically correct.
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