Chemistry • Year 12 • Module 7 • Lesson 10

Production of Alcohols: Hydration, Substitution & Fermentation

Build HSC Band 5–6 extended-response technique on alcohol production methods — evaluate trade-offs, critique sources, and construct evidence-based arguments.

Master • Extended Response

1. Extended response — evaluate alcohol production methods (Band 5–6)

8 marks   Band 5–6

Stimulus. The Manildra Group operates Australia’s largest industrial ethanol plant at Nowra, New South Wales (Shoalhaven Starches). The facility converts wheat starch (hydrolysed to glucose) via fermentation, then further refines product via distillation. It is the largest single ethanol producer in Australia, supplying industrial and pharmaceutical markets. Separately, petroleum refineries in Australia have produced ethanol via catalytic hydration of ethene. Queensland’s sugarcane belt produces bioethanol for fuel blends, using fermentation of sugarcane juice. The Australian Wine Research Institute (AWRI) based in Adelaide studies and optimises fermentation in the Australian wine industry.

Q1. Evaluate the three methods of alcohol production — alkene hydration, haloalkane substitution, and fermentation — for large-scale industrial production of ethanol in Australia. In your response you must:

  • Write a balanced equation for each method with correct arrow notation and conditions.
  • Compare the methods on at least four criteria (e.g. purity, rate, feedstock sustainability, energy use, scale, cost).
  • Use at least one Australian industrial example (such as Manildra Group Nowra, Queensland sugarcane bioethanol, or AWRI wine fermentation).
  • Explain why a Le Chatelier argument is needed for alkene hydration but not for fermentation or substitution.
  • Reach a context-dependent judgement — not a single “best method” ranking.
Stuck? Plan first: three equations → four criteria in paired comparison format → Australian example → Le Chatelier for hydration only → context-dependent conclusion. Use the comparison table in Content Card 4 as your scaffold.

2. Source critique — identify the error in this media article (Band 5–6)

7 marks   Band 5–6

Source. The following extract is from a fictional news article about Australian biofuel investment:

“Experts say the new Queensland bioethanol plant will significantly reduce Australia’s carbon footprint. The fermentation process is completely carbon neutral because the yeast consumes all of the carbon dioxide produced during the reaction — so no greenhouse gas enters the atmosphere at all. Furthermore, because fermentation operates at a higher temperature than industrial ethanol synthesis, it uses more energy, making it the costlier option per litre. The unbalanced equation for fermentation is: C&sub6;H&sub1;&sub2;O&sub6; → C&sub2;H&sub5;OH + CO&sub2;, showing that one mole of glucose produces one mole of ethanol.”

Q2. The extract above contains three distinct scientific errors. For each error: (i) identify what the source claims; (ii) explain what is scientifically incorrect; (iii) state the correct chemistry. Where relevant, explain how the error could be detected experimentally or with reference to an equation.

Error 1 (identify, explain, correct):

Error 2 (identify, explain, correct):

Error 3 (identify, explain, correct):

Experimental detection: Describe one simple experiment that would directly refute Error 1 or Error 3.

Stuck? Read the source carefully: check (a) what happens to the CO&sub2; in fermentation, (b) whether fermentation is hotter or colder than industrial hydration, and (c) whether the equation is balanced. Each of these is a testable chemical claim.

3. Predict and justify — changing a fermentation condition

6 marks   Band 5–6

Scenario. An Australian winery (monitored by AWRI) is fermenting Chardonnay grape juice. During production, a temperature-control failure causes the fermentation temperature to rise to 52°C for 48 hours before being corrected. The winemaker notices that the fermentation slows dramatically and the rate of CO&sub2; bubble production nearly stops.

Q3. Predict what has happened to the fermentation at 52°C, justify your prediction using lesson chemistry, and evaluate whether the batch can be recovered after the temperature is returned to 35°C. In your response:

  • Identify the mechanism by which high temperature affects this fermentation.
  • Explain the link between the enzyme, temperature, and ethanol production using precise chemistry terminology.
  • Evaluate whether adding fresh yeast would restore the fermentation, and justify why.
  • Predict the CO&sub2; observation if fermentation resumed normally vs. if aerobic respiration began instead.
Stuck? Connect the enzyme-temperature relationship from Content Card 3 (“above ~45°C enzyme denatures”) to what denaturation means for catalytic function, and to whether the fermentation equation can proceed without active zymase.
Answers — Do not peek before attempting

Q1 — Marking criteria (8 marks)

  • 1 mark — Writes balanced equation for alkene hydration with correct reversible arrow (⇌) and all conditions: CH&sub2;=CH&sub2; + H&sub2;O ⇌ CH&sub3;CH&sub2;OH; H&sub3;PO&sub4; catalyst, ~300°C, ~65 atm.
  • 1 mark — Writes balanced equation for haloalkane substitution with correct single arrow (→) and conditions: R—X + NaOH(aq) → R—OH + NaX; aqueous NaOH, heat under reflux.
  • 1 mark — Writes balanced fermentation equation with correct coefficients and arrow (→): C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2;; yeast/zymase, ~35°C, anaerobic.
  • 1 mark — Compares on at least two of: purity (hydration ~95% > fermentation ~15%), rate (hydration continuous > fermentation slow batch), energy (fermentation 35°C vs hydration 300°C + 65 atm), feedstock (fermentation renewable; hydration fossil fuel). Must be stated as a comparison, not a separate description of each.
  • 1 mark — Compares on at least two further criteria (for a total of four criteria across this and the previous mark).
  • 1 mark — Names and correctly contextualises at least one Australian industrial example (e.g. Manildra Group Nowra, Qld bioethanol from sugarcane, AWRI wine fermentation).
  • 1 mark — Explains that Le Chatelier applies to alkene hydration because it is a reversible equilibrium (⇌); fermentation and substitution use single arrows (→) because they go to completion (fermentation stops due to ethanol toxicity to yeast, not equilibrium; substitution goes essentially to completion). Le Chatelier does not apply to irreversible reactions.
  • 1 mark — Reaches a context-dependent conclusion: no single “best” method; hydration preferred for industrial purity and throughput (e.g. pharmaceutical use); fermentation preferred for renewable/sustainability or small-scale / beverage context; substitution primarily for laboratory synthesis not large-scale. Must explicitly acknowledge that the best method depends on application and context.

Q2 — Source critique marking criteria (7 marks)

Error 1 — “yeast consumes all CO&sub2; produced” (2 marks)

Claim: the source says yeast consumes the CO&sub2;, so no greenhouse gas enters the atmosphere. Correction: yeast does not consume CO&sub2; — CO&sub2; is a product of fermentation (C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2;) and is released from the fermentation vessel. The near-carbon-neutral claim for fermentation is based on the CO&sub2; released being offset by CO&sub2; absorbed during crop growth via photosynthesis — not because yeast re-absorbs the CO&sub2;. [1 identify + 1 correct]

Error 2 — “fermentation operates at higher temperature than industrial synthesis” (2 marks)

Claim: fermentation is hotter than industrial ethanol synthesis. Correction: fermentation operates at ~30–35°C, which is far lower than industrial alkene hydration (~300°C + ~65 atm). Fermentation is therefore lower-energy, not higher-energy. Fermentation is less costly per unit energy input because of the low temperature — the opposite of what the source claims. [1 identify + 1 correct]

Error 3 — Unbalanced fermentation equation (2 marks)

Claim: the equation C&sub6;H&sub1;&sub2;O&sub6; → C&sub2;H&sub5;OH + CO&sub2; is presented as the fermentation equation. Correction: this equation is not balanced. The balanced equation requires coefficient 2 on both ethanol and CO&sub2;: C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2;. Carbon balance check: 6C on left; 4C (ethanol) + 2C (CO&sub2;) = 6C on right. The unbalanced version implies only half the carbon is accounted for. [1 identify + 1 correct]

Experimental detection (1 mark)

For Error 1: set up a sealed fermentation vessel connected to limewater (Ca(OH)&sub2; solution). If CO&sub2; is released from the vessel, the limewater will turn milky (calcium carbonate precipitate forms). This directly demonstrates CO&sub2; is released, not consumed by yeast. For Error 3: measure the volume of CO&sub2; gas produced per mole of glucose fermented — if the equation is C&sub6;H&sub1;&sub2;O&sub6; → 2CO&sub2; + 2C&sub2;H&sub5;OH, 2 moles of CO&sub2; should be produced per mole of glucose, which can be verified by gas collection. [1 mark for either valid approach]

Q3 — Temperature failure at 52°C (6 marks)

Prediction (1 mark): At 52°C, the zymase enzyme (produced by yeast) has been denatured — the enzyme’s active site loses its specific 3D shape, so it can no longer bind glucose and catalyse the fermentation reaction. The fermentation slows and effectively stops.

Mechanism / enzyme chemistry (2 marks): Zymase is a biological catalyst — a protein enzyme with a specific tertiary structure. Optimal activity is at ~30–35°C. Above ~45°C, excess thermal energy disrupts hydrogen bonds and other non-covalent interactions maintaining the enzyme’s active-site shape. This is denaturation: irreversible unfolding of the protein. Without a functional active site, glucose cannot be bound and converted to ethanol and CO&sub2;, so the fermentation equation C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2; cannot proceed. [1 for denaturation mechanism; 1 for irreversibility and active-site connection]

Adding fresh yeast (1 mark): Yes, adding fresh yeast would restore fermentation (assuming the vessel is still anaerobic and glucose remains). The new yeast cells contain functional, non-denatured zymase at the correct temperature (35°C after the temperature is corrected). Denaturation is irreversible, but the old denatured enzyme does not prevent new yeast from functioning — only the new cells’ fresh zymase matters.

CO&sub2; observation (2 marks): If fermentation resumes normally after fresh yeast is added and anaerobic conditions are maintained: CO&sub2; bubbles will be produced steadily (C&sub6;H&sub1;&sub2;O&sub6; → 2C&sub2;H&sub5;OH + 2CO&sub2;) and limewater connected to the outlet would turn milky, confirming CO&sub2; release [1]. If oxygen has entered the vessel and aerobic respiration begins: CO&sub2; is also produced (glucose + O&sub2; → CO&sub2; + H&sub2;O), so bubbling would resume, but no ethanol would be produced — the winemaker could distinguish this by testing the liquid for ethanol content (or smell) — aerobic respiration would give no increase in ethanol concentration despite CO&sub2; being released [1].

HSCScience • Chemistry • Year 12 • Module 7 • Lesson 10 • Worksheet 3 (Master)