Ethanol Production
Compare fermentation of glucose with hydration of ethene, explain their conditions, and choose a route using stated production priorities.
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Two different starting materials can lead to the same product. Compare glucose fermentation with ethene hydration, then choose a route using the needs of a particular plant.
Fermentation
Yeast enzymes convert glucose into ethanol and carbon dioxide under warm, oxygen-free conditions:
The equation conserves six C, twelve H and six O atoms. The ethanol is initially dissolved in water, rather than collected as a pure liquid.
- Warm conditions support a useful reaction rate. Low temperature slows the process; excessive heat denatures yeast enzymes.
- Exclude incoming oxygen to maintain the intended anaerobic fermentation conditions. An airlock allows carbon dioxide to escape while limiting entry of air.
- After fermentation, distillation raises the ethanol concentration. Ordinary distillation of an ethanol–water mixture does not by itself guarantee completely pure ethanol. See Distillation and Fractional Distillation.
Glucose obtained from crops can be a renewable feedstock if the crops are regrown. That does not make the whole process emission-free. Farming, processing and distillation also use resources and energy; Biofuels and Carbon Emissions considers the complete carbon balance.
Hydration of ethene
Ethene reacts with steam at high temperature and pressure, with a phosphoric(V) acid catalyst, H₃PO₄:
This is a reversible addition reaction. The equation shows gases in the hot reactor. Ethanol is collected as a liquid after cooling, with further processing of the ethanol–water product mixture. Unreacted material can be recycled through a continuous plant.
Conventional ethene feedstock comes from crude oil and is non-renewable. Feedstock origin matters: the word “hydration” describes a reaction, not whether its carbon source is renewable. Exact temperature and pressure values are not required for this course.
Compare the routes against a purpose
| Feature | Typical crop-glucose fermentation | Conventional industrial ethene hydration |
|---|---|---|
| Starting materials | Aqueous glucose | Ethene and steam |
| Key conditions | Yeast; warm; oxygen excluded | High temperature and pressure; phosphoric(V) acid catalyst |
| Process comparison used here | Slower batch setup | Faster continuous setup |
| Product handling | Dilute ethanol needs separation | Ethanol–water stream also needs processing; unused reactants can be recycled |
| Feedstock origin | Regrown crops can replenish the glucose source | Crude oil is a finite source of conventional ethene |
A choice needs criteria: speed, existing equipment, feedstock supply, energy use and environmental impact. “Renewable” alone does not prove that a route is best for every factory. The comparison concerns these setups; batch or continuous describes how a plant operates, not the definition of the chemical reaction.
Explain conditions and choose a route
Modelled example 1
Oxygen exclusion (fermentation)
Problem
Study the worked solution
Identify the condition being controlled
Method
Keep the vessel anaerobic by excluding oxygen in air.Reason
Yeast produces ethanol from glucose under anaerobic fermentation conditions.Working
No oxygen → conditions favour ethanol fermentation rather than aerobic respiration.Link the condition to yield
Method
Explain that oxygen can reduce the recovered ethanol yield.Reason
Oxygen permits aerobic respiration and can support bacteria that oxidise ethanol to ethanoic acid. Maintaining the intended anaerobic conditions limits those competing processes.Working
Exclude incoming air while allowing CO2 to escape through an airlock.
Guided practice 2
Compare production methods
Problem
Match the process to the factory priorities
Hints
Hint 1: priority words
Hint 2: product stream
View solution step by step
Choose hydration
Method
Select hydration of ethene.Reason
Hydration can operate continuously and produces ethanol quickly.Working
Factory requirement → hydration.Justify by comparison
Method
Contrast hydration with fermentation.Reason
The given fermentation design is slower and operates in batches; its dilute product also needs separation.Working
Choose the continuous hydration design for these priorities. Renewable feedstock or local crop supply could favour fermentation in a different decision.
Try without prompts
Mind stretcher 1: A different factory, a different choiceExtension
A small producer has a reliable local supply of crop-derived glucose and low-pressure fermentation vessels. Its priority is to use this local feedstock; it accepts slower production. It has no high-pressure hydration plant. Choose a route, explain two operating conditions, and identify a separation step it will still need. Why is “renewable feedstock means no carbon emissions” an incomplete conclusion?
Show answer
Fermentation suits the given feedstock and equipment. Use yeast under warm, oxygen-free conditions, allowing carbon dioxide to escape through an airlock. Excessive heat denatures the enzymes. Distillation is needed to concentrate the initially dilute ethanol. Crop regrowth can replace the glucose feedstock, but fermentation releases CO2 and processing uses energy; the whole life cycle must be considered before comparing net carbon emissions.
Mind stretcher 2: Why did fermentation fail?Extension
A learner ferments glucose at 60 °C and obtains very little ethanol. Give the likely reason and explain why more heating does not always give more product.
Show answer
The temperature is too high for the yeast enzymes, so they denature and fermentation slows or stops. Raising temperature can increase reaction rate within a suitable range, but excessive heat destroys the enzymes’ ability to catalyse this process.
Bring the routes together
Fermentation starts with glucose and yeast under warm, oxygen-free conditions. Hydration starts with ethene and steam at high temperature and pressure with a phosphoric(V) acid catalyst. Both make ethanol, and both require product processing. Match a choice to stated priorities rather than declaring one route best in every situation.
Practise and check
Use the Organic Chemistry topic check to practise and check your understanding.
Syllabus and review details
- SEC G3 Pure Chemistry 2027 · 2027
Content structure and subject content, PDF pages 9–24
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