re: HSC Chemistry Marathon Archive
Assess the potential of cellulose in being used as a raw material for the production of petrochemicals. (5 marks)
Petrochemicals are currently mainly being produced from non-renewable resources / crude oil, which is estimated to run out within the next few decades. In addition to this not being sustainable the use of crude oil also means an increasing cost as supply runs lower, an unstable supply from producers and unclean source of petrochemicals. One alternative to this is cellulose is a naturally occurring condensation polymer made of Beta-Glucose monomers. The procedure to produce ethanol from cellulose is to first break it into glucose monomers by using enzymatic or strong acid digestion and then seperating the glucose into CO2 and ethanol. (C6H12O6(aq) > C2H6O(aq) + 2CO2(g))
It is seen as a possible replacement because it is abundant, it contains all the carbon-chain structures needed for the production of petrochemicals and is produced by plants, via photosynthesis in biomass (50% of biomass is cellulose) - hence its' abundance and also its' renewable nature, unlike crude oil. However, there are also multiple issues concerning the use of cellulose, this includes the fact that it is far more expensive than crude oil and also that the process of extracting and cracking cellulose into petrochemicals uses more oil than is received than petrochemicals received. Hence, currently cellulose is not a viable alternative due to its' cost, however, due to the unsuitability of crude oil it may be looked into further in the future and hence the issues may be overcome, however, it is very unlikely to run out unlike crude oil.
You really didn't need to include lengthy statements about the current non-renewable sources of petrochemicals from crude oil. Would have sufficed to say something like, "with our rapidly depleting reserves of fossil fuels, alternative sources of the products from the petrochemical industry must be found"..... then assess cellulose. Depletion of fossil fuels is more of an indirect advantage for switching to cellulose.
There was no point in mentioning the structure of cellulose as it isn't relevant to the question, unless you somehow elaborate on it e.g why it may affect it potential use as a replacement for the petrochemical industry.
You could include the equation for acid hydrolysis. Also be careful with your equation for fermentation, it's not balanced.
Cellulose possesses C-C bonds specifically that are desired to eventually produce things like ethene (petrochemical). Cellulose is derived from plants, which can be regrown (renewable), but much arable land is required (disadvantage).
You said since cellulose is derived from plants and since plants undergo photosynthesis, hence it is an abundant and renewable resource, which really isn't a valid statement (you don't even need to explain it, just say the crops can be regrown and are renewable unlike fossil fuels).
Could include that this process is theoretically greenhouse neutral, as there is no net CO2 emission since fermentation releases CO2, but it will be recycled by plants for photosynthesis.
A little confused by the next point, cellulose is fairly easy/cheap to produce, but the expense/inefficency comes in the processes (acid hydrolysis/fermentation/dehydration) required to get it to ethanol/plastics. May also want to include the distillation of C2H60(aq)-->C2H60(l) and it's difficulties, then dehyration of ethanol into ethene which requires certain conditions that i can't remember and H2SO4(conc) as a catalyst.
There was no point in repeating"cellulose is not a viable alternative due to its cost" since you already stated this. Your connective word "however" was again kind of misused, in the end you can just say something like "from the above mentioned arguments (or something like that), cellulose's role in the petrochemical industry may be given more usage in the future if methods become more efficient/economically viable.". Overall, you have the assess part down, but you should kind of condense/shorten your response. You won't be able to write in such detail/perfect grammar in the actual exam, so just get straight to the point with chemistry/assess and include the relevant chemical equations. You'll decide to include or not include some of the information depending on the question and number of marks allocated. Enigma_1 had a pretty good response for the question.
