QUESTION 4 (8 marks)
Bioethanol is a renewable energy source made from biomasses such as starch and cellulosic materials. The two-step process for the conversion of starch and cellulose to bioethanol is shown.
Process | Step 1 | Step 2 | Conversion to glucose | Production process |
Starch | Enzymatic hydrolysis (α–amylase) of starch biomass to form glucose | Fermentation of glucose to form bioethanol (yeast) | Easier | Faster |
Cellulose | Acid hydrolysis (H2SO4(aq) at 320 °C and 25 MPa) of cellulose biomass to form glucose | Fermentation of glucose to form bioethanol (yeast) | Harder | Slower |
a) Identify why it is important to control the temperature during the fermentation process to produce bioethanol.
[2 marks]
b) Explain why cellulose is harder to convert to glucose than starch.
[3 marks]
c) After 48 hours of fermentation, a 15% w/v glucose solution produces 37.5 g L-1 of ethanol. Calculate the percentage yield of ethanol. Show your working.
[3 marks]
QCAA guide · typeset solution
QCAA sample response and mark allocation
4a) | The fermentation process requires yeast as a catalyst. Yeast is temperature sensitive. | • identifies fermentation requires yeast as a catalyst [1 mark] • identifies that yeast is temperature-sensitive [1 mark] |
4b) | Cellulose is a linear polymer. The β-glucose monomers in cellulose can pack closely together. This increases hydrogen bonding between adjacent chains, which reduces interactions with water (solvents) and makes hydrolysis of cellulose more difficult than starch. | • identifies cellulose is a linear polymer [1 mark] • identifies monomers can pack closely together [1 mark] • explains increased H-bonding between adjacent chains makes hydrolysis of cellulose more difficult [1 mark] |
4c) | 6126150Moles C H O0.833180.18==3232Moles CH CH OH1.67Mass CH CH OH76.70.8321.6746.083%7.537.5theoretical yield76.72 gEthanol yield48.92×======×= | • determines 150 g glucose can produce 1.67 M of ethanol [1 mark] • calculates theoretical mass of ethanol as 76.72 g [1 mark] • calculates % yield of ethanol is 48.9% [1 mark] |
QCAA sample response and marking criteria reproduced from the official guide.
Compare your working with the guide above.