Chemistry • Year 12 • Module 7 • Lesson 10
Production of Alcohols: Hydration, Substitution & Fermentation
Apply your understanding of the three production methods to real data, cause-and-effect reasoning, and an Australian industrial context.
1. Interpret comparison data — industrial hydration vs fermentation
The table below summarises selected production parameters for industrial ethanol manufacture using alkene hydration (as practised by plants such as the Manildra Group’s Shoalhaven Starches facility at Nowra, NSW, for industrial-grade ethanol) and fermentation (as practised by Australian bioethanol producers and by wine producers studied by the Australian Wine Research Institute, AWRI). 8 marks
| Parameter | Alkene hydration (industrial) | Fermentation (biological) |
|---|---|---|
| Starting material | Ethene (from petroleum cracking) | Glucose / sugars (from sugarcane, grain, or grapes) |
| Catalyst | H&sub3;PO&sub4; absorbed on silica (heterogeneous) | Yeast (zymase enzyme — biological) |
| Temperature | ~300°C | ~30–35°C |
| Pressure | ~65 atm | Atmospheric (~1 atm) |
| Ethanol purity from reactor | ~95% | ~8–15% |
| Production speed | Continuous; fast throughput | Batch process; days to weeks |
| Renewable feedstock? | No (fossil fuel) | Yes (plant biomass) |
| CO&sub2; footprint of process | High (fossil fuel-derived reactant + energy) | Near-neutral (CO&sub2; released offset by crop photosynthesis) |
| Additional purification needed? | Minimal (condense & separate) | Yes — fractional distillation required for industrial grade |
Data adapted from Manildra Group technical reports and AWRI fermentation research summaries. Industrial parameters reflect standard phosphoric acid hydration practice.
1.1 Identify two parameters from the table that show fermentation is a more sustainable choice than industrial hydration, and explain each. 4 marks
1.2 Explain why the Manildra Group’s Nowra plant (which supplies large quantities of industrial-grade ethanol used in pharmaceuticals and cleaning products) would use hydration rather than fermentation for this application. Refer to data in the table. 2 marks
1.3 Using the data, predict what would happen to the cost and energy use at a fermentation plant if it were required to supply ethanol at 95% purity. Justify your prediction. 2 marks
2. Interpret graph — ethanol yield vs temperature in alkene hydration
The figure below models the equilibrium yield of ethanol from the hydration of ethene at constant pressure (~65 atm) as temperature increases from 150°C to 450°C. A separate rate curve shows relative reaction rate over the same temperature range. 6 marks
2.1 Describe the relationship between temperature and equilibrium yield shown by the green curve. 2 marks
2.2 Using Le Chatelier’s Principle, explain the shape of the equilibrium yield curve. State whether the hydration reaction is exothermic or endothermic and how this determines the direction of the equilibrium shift. 2 marks
2.3 Using both curves, explain why 300°C is chosen as the industrial operating temperature rather than 150°C or 450°C. 2 marks
3. Cause-and-effect chain — oxygen enters a fermentation vessel
Use the cause-and-effect chain below to trace what happens when oxygen accidentally enters a sealed fermentation vessel. The first cause is given; fill in the effect boxes. 5 marks
Effect 1: What does the yeast do differently in the presence of oxygen?
Effect 2: What products are formed instead of ethanol?
Effect 3: What happens to the ethanol yield from this batch?
Effect 4: In an Australian wine fermentation facility (such as one monitored by AWRI), what practical step do winemakers take to prevent this?
Overall outcome: Why is anaerobic control the single most critical condition for maintaining ethanol production in fermentation?
4. Apply to a new scenario — Queensland bioethanol from sugarcane
Queensland is Australia’s largest producer of bioethanol. Sugarcane juice (containing sucrose, which is hydrolysed to glucose and fructose) is fermented to produce fuel-grade ethanol. One large facility produces approximately 100 million litres of ethanol per year, which is blended into petrol (E10 and E85 fuel blends) for use in Australian vehicles. 5 marks
4.1 Write the balanced equation for the fermentation step. Include the arrow notation and state all three conditions. 2 marks
4.2 The raw fermented liquid from sugarcane contains approximately 10% ethanol. Explain what additional processing step is required to produce E85 fuel (85% ethanol) and why this step is necessary. 2 marks
4.3 Explain one reason why bioethanol from Queensland sugarcane is considered more sustainable than industrial ethanol produced by alkene hydration from ethene. 1 mark
Q1.1 — Two sustainability parameters (4 marks)
Any two of: (i) Renewable feedstock — fermentation uses plant biomass (sugarcane, grain, grapes) which is grown using solar energy and CO&sub2; from the atmosphere, whereas hydration uses ethene derived from non-renewable petroleum cracking [1 mark for identifying + 1 for explaining]; (ii) CO&sub2; footprint — fermentation is near-carbon-neutral because the CO&sub2; released during fermentation is offset by the CO&sub2; absorbed by the crop during photosynthesis; hydration uses a fossil-fuel-derived reactant with high overall CO&sub2; emissions [1 + 1]; (iii) Temperature — fermentation at 30–35°C uses far less energy than hydration at ~300°C + ~65 atm, reducing energy-related emissions [1 + 1].
Q1.2 — Manildra Nowra uses hydration (2 marks)
Industrial applications such as pharmaceuticals and cleaning products require very high-purity ethanol. The table shows hydration produces ~95% purity directly from the reactor, whereas fermentation produces only 8–15% [1 mark for identifying purity difference from data]. Hydration is also a continuous process with fast throughput, whereas fermentation is a slow batch process, making hydration more economical for large-scale industrial supply [1 mark for speed/throughput or minimal additional purification needed].
Q1.3 — Fermentation at 95% purity (2 marks)
Energy use and cost would increase substantially [1]. Fractional distillation would be required to concentrate 8–15% ethanol to 95% — this is an energy-intensive process requiring significant heating, which adds infrastructure and operating costs and reduces the energy-efficiency advantage of fermentation [1]. This partly erodes the sustainability advantage of the low-temperature fermentation step.
Q2.1 — Describe equilibrium yield vs temperature (2 marks)
As temperature increases from 150°C to 450°C, the equilibrium yield of ethanol decreases [1 mark]. The relationship is approximately inverse: at 150°C the yield is very high (~80%), falling steeply to ~15–20% at 450°C [1 mark for quantitative description or reference to trend direction].
Q2.2 — Le Chatelier explanation (2 marks)
The hydration reaction (CH&sub2;=CH&sub2; + H&sub2;O ⇌ CH&sub3;CH&sub2;OH) is exothermic in the forward direction [1]. By Le Chatelier’s Principle, increasing temperature supplies heat energy, which opposes the exothermic forward reaction and favours the endothermic reverse reaction (dehydration). The equilibrium therefore shifts left (towards reactants) as temperature rises, producing less ethanol at higher temperatures [1].
Q2.3 — Why 300°C is chosen (2 marks)
At 150°C, the equilibrium yield is high but the reaction rate is too low for economical industrial production (the rate curve is near-zero at 150°C) [1]. At 450°C, the rate is high but the equilibrium yield is very low (~15%), also uneconomical. At ~300°C, both curves are at acceptable values simultaneously — the rate is sufficient for continuous industrial throughput and the yield is high enough to be economically viable with ethene recycling. This is the rate-yield compromise [1].
Q3 — Cause-and-effect chain (5 marks)
Effect 1: In the presence of oxygen, yeast switches from fermentation (anaerobic respiration) to aerobic respiration [1].
Effect 2: Aerobic respiration produces CO&sub2; and water (H&sub2;O) instead of ethanol [1].
Effect 3: Ethanol yield from this batch falls significantly or reaches zero — no ethanol is produced while aerobic conditions persist [1].
Effect 4: Winemakers seal fermentation vessels and often blanket with inert gas (CO&sub2; or nitrogen) to exclude oxygen; gas-release valves allow CO&sub2; to escape without admitting air [1].
Overall outcome: Anaerobic conditions are critical because yeast will always preferentially respire aerobically when oxygen is available, completely diverting glucose away from ethanol production. Without oxygen exclusion, there is no ethanol produced regardless of the quality of other conditions [1].
Q4.1 — Fermentation equation and conditions (2 marks)
C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2; [1 mark — must have coefficient 2 on both products; single arrow → acceptable as goes effectively to completion]. Conditions: yeast (zymase enzyme), ~30–35°C, anaerobic (no oxygen / air excluded) [1 mark — all three required].
Q4.2 — Additional processing (2 marks)
Fractional distillation is required to concentrate the ethanol from ~10% to 85% [1 mark for identifying the process]. This is necessary because the fermentation product is a dilute aqueous ethanol solution — ethanol and water have different boiling points (78.4°C and 100°C respectively), so repeated fractional distillation separates and concentrates the ethanol to the required purity for fuel blending [1 mark for mechanism/why].
Q4.3 — Sustainability advantage (1 mark)
Any one valid reason: sugarcane is a renewable plant biomass grown using solar energy and photosynthesis, whereas ethene comes from non-renewable petroleum. OR: the CO&sub2; released during fermentation is offset by CO&sub2; absorbed during sugarcane growth, making the process near-carbon-neutral, unlike hydration which relies on fossil fuel feedstock. OR: fermentation operates at low temperature (~35°C) vs ~300°C for hydration, using far less energy. [1 mark]