I'd probably go down the root of cellulose to ethene/ethanol and starch to ethanol, and then maybe just mentioning biopolymers briefly.
I'd describe briefly how cellulose is formed from the linking of b-glucose monomers that form a 1,4-glycosidic linkage, and results in the elimination of a small molecule (water). Thus, its a condensation polymer (equation). Then I'd describe how due it containing 6 carbons in its chain it can theoretically be converted down into ethene and ethanol.
Describe the conversion process: 1. cellulose to glucose (cellulase enzymes causing hydrolysis - equation). 2. Fermentation (equation) producing 15% v/v ethanol. 3. Fractional distillation and molecular seiving for 15%v/v ethanol to 100%v/v. (you can use the ethanol here as a fuel - describe the combustion of ethanol and list like 2 of these advantages of it:Theoretically carbon neutral:
Photosynthesis: 6CO2(g) + 6H2O(l) → 6O2(g) + C6H12O6(aq) (consumes 6 moles of CO2)
Fermentation of Glucose: C6H12O6(aq) → 2CO2(g) + 2C2H5OH(aq) (releases 2 moles of CO2)
Combustion: 2C2H5OH(l)+ 6O2(g) → 4CO2(g) + 6H2O(l) (releases 4 moles of CO2)
Thus, from these equations there is no net release of CO2. However, in reality it is not as fuels are burnt in fractional distillation prior to step 3 (required as fermentation only produces 15% v/v), during growth and harvesting and also during transportation of fuels, but it is still much more environmentally friendly than octane.
It burns cleaner than octane: due to its shorter carbon chain length and the fact it already contains an oxygen atom, so less oxygen is required per mole of fuel. Therefore, it more readily undergoes complete combustion which reduces toxic pollutants such as C (soot - carcinogenic) and CO (binds irreversibly with hemoglobin in red blood cells, restricting oxygen leading to hypoxia) produced from incomplete combustion.
Also increases fuel efficiency as heat of reaction given by ∆H = bonds broken - bonds formed, reduces in incomplete combustion as CO is less stable than CO2, so less bonds are formed and thus it is less exothermic.
Ethanol Complete Combustion: C2H5OH(l)+ 3O2(g) → 2CO2(g) + 3H2O(l)
Octane Complete Combustion: C8H18(l)+ 252O2(g) → 8CO2(g) + 9H2O(l)
Octane Incomplete Combustion: C8H18(l)+ 6O2(g) → CO2(g) + 6C(s) + CO(g) + 9H2O(l)
Mention briefly alternatively you can produce the ethanol from starch and just like list all the equations within 2 lines of the page)
Mentioning 2 disadvantages from these:
Engine modifications: When more than 15% v/v of ethanol is used as an additive to regular petrol known as gasohol is used in car engines, expensive engine modifications are required as ethanol is a polar molecule and thus attracts water causing corrosion.
Large areas of arable land are required to grow crops: which has an ethical issue in the use of crops for fuel and also large environmental problems such as soil erosion, deforestation and salinity issues.
Lower molar heat of combustion: produces less energy per mole than octane, so this means that more fuel is needed for same energy (i.e. to travel the same distance).
Disposing large amounts of waste fermentation liquors: after ethanol removal can be environmentally damaging.)
However, if ethene is being produced instead of ethanol, this can be achieved through concentrated sulufic acid (dehydration - equation)).
This can then undergo addiition reactions such as addition polymerisation (to produce polyethylene a plastic - equation).
The advantages of using cellulose to ethene/ ethanol are:
It is a major component of biomass so readily available
This is a renewable resource (compared to the current source for petrochemicals being crude oil).
Technical problems/disadvantages:
Problems:
It is difficult to break cellulose into glucose. This is because cellulose contains hydrogen bonding to form very compact fibres. This makes it difficult for chemicals to access the 1,4 -glycosidic linkage.
In theory, it can be converted but it is too expensive and inefficient with current technology, while fossil fuels used to extract ethene through cracking are currently a much cheaper and faster process.
Land required for agriculture can lead to soil erosion, salinity and deforestation problems.
Mention some stuff about biopolymers such as reduces the plastic pollution waste problem and are more environmentally friendly, but cost more to make and are less stronger than fossil fuel derived polymers (will depend on what one you chose to study for more detail )
Evaluation: currently using cellulose is not economically sustainable (however, as techniques improve this may change), ethanol production from starch has limitations but is currently being used in countries like Brazil (but hasn't had a widespread impact as it could). The issue in terms of plastics however, may be solved from the use of biopolymers as the prices decrease in the future.
Rough sort of answer (with too much info as you can't fit into a 5 marker), but using some of this stuff would be good
