QUESTION 155 (8 marks)
An ethanol plant compares two routes. Route F ferments 90.0 g of glucose at 80.0% ethanol yield, then recovers 90.0% of the ethanol formed. Route H hydrates 1.50 mol of ethene at 60.0% ethanol yield, then recovers 95.0% of the ethanol formed. Water is in excess. The fermentation enzymes lose catalytic activity when heated far above their operating range. Use M(glucose) = 180.0 g mol⁻¹ and M(ethanol) = 46.0 g mol⁻¹.
(a) Write the balanced reaction equation for each route. [2 marks]
(b) Calculate the recovered ethanol mass from each route and determine which is greater for these feed amounts. [4 marks]
(c) Explain why increasing the fermentation temperature far above the enzyme’s operating range need not increase its production rate. [2 marks]
Practice marking scheme
Answer
Route F recovers 33.1 g ethanol; route H recovers 39.3 g. Route H gives more recovered ethanol for these specified feed amounts. Excessive temperature can denature the fermentation enzymes and lower their activity.
Working
Fermentation: . Hydration: . F has theoretical ethanol amount mol, giving recovered mass g. H has theoretical amount 1.50 mol, giving g. Reaction yield and recovery are separate fractions. Enzyme denaturation can reduce active sites despite an increase in collision frequency; this is a supplied fermentation operating constraint, not an assessment of protein structural hierarchy.
Marking criteria
- Writes balanced glucose fermentation equation. [1 mark]
- Writes balanced ethene hydration equation. [1 mark]
- Calculates theoretical ethanol amount 1.00 mol for F. [1 mark]
- Applies both yield and recovery to obtain F mass ≈ 33.1 g. [1 mark]
- Applies both fractions to obtain H mass ≈ 39.3 g. [1 mark]
- Concludes H has the greater recovered mass for the supplied feeds. [1 mark]
- Identifies loss of enzyme activity/denaturation at excessive temperature. [1 mark]
- Links reduced enzyme activity to a possible reduction in rate despite higher collision frequency. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
View the QCAA syllabusCompare your working with the guide above.