4.5.3Organic Chemistry
Manufacture of Ethanol — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 15 of 18 Unit quiz
Section 1 of 5Hydration of Ethene
Ethene comes from cracking of crude oil fractions.
Reaction
- C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g); a water molecule adds across the C=C double bond.
Conditions
- The reaction uses about 300 °C, a pressure of 60 to 70 atm and a concentrated phosphoric acid catalyst.
Separation
- Ethene boils at −103 °C so is easily separated from ethanol (78 °C) and water (100 °C) in a condenser.
Recycling
- Unreacted ethene is recycled, and ethanol and water are separated by fractional distillation.
Section 2 of 5Fermentation
Fermentation
Reaction
- C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂; yeast enzymes convert glucose to ethanol and carbon dioxide.
Conditions
- The mixture is kept warm at about 25 to 35 °C, with no oxygen, for several days.
Temperature control
- If too cold, the reaction is too slow; if too hot, the enzymes denature.
Product
- Fermentation stops at about 15% ethanol because the yeast is killed, so fractional distillation is needed.
| Condition | Hydration | Fermentation |
|---|---|---|
| Temperature | About 300 °C | About 30 °C |
| Pressure | 60–70 atm | Atmospheric |
| Catalyst | Phosphoric acid | Enzymes in yeast |
Section 3 of 5Comparing the Methods
Comparing the Methods
Raw material
- Hydration uses ethene from crude oil, which is non-renewable, while fermentation uses sugar or starch from plants, which is renewable.
Rate
- Hydration is fast, whereas fermentation is slow, taking days.
Type of process
- Hydration is continuous, whereas fermentation is a batch process.
Purity
- Hydration gives fairly pure ethanol; fermentation gives a dilute mixture needing distillation.
Energy
- Hydration needs high temperature and pressure, so equipment and energy costs are high.
Section 4 of 5Fermentation in Steps
1
Sugar or starch is dissolved in water and yeast is added.
2
The mixture is kept at about 30 °C in the absence of oxygen.
3
Yeast respires anaerobically, forming ethanol and carbon dioxide.
4
Once the ethanol reaches about 15%, the yeast dies and the batch ends.
5
The ethanol is separated from the water by fractional distillation.
Section 5 of 5Exam Checklist & Tips
4.5.3 – Key Points for the Exam
- Hydration A reaction in which water adds to a molecule.
- Fermentation Breakdown of sugar into ethanol and carbon dioxide by yeast in the absence of oxygen.
- Batch process A process where reactants are added and the product is removed in separate lots.
- Continuous process A process that runs without stopping, with reactants entering and products leaving.
Common Mistakes
- Saying fermentation needs oxygen; it is an anaerobic process.
- Writing the catalyst for hydration as sulfuric acid rather than phosphoric acid.
- Forgetting that yeast enzymes are denatured at high temperatures.
- Describing ethanol from fermentation as pure when it must be distilled.
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