Alcohols
Section: 11. Organic Chemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Manufacture of Ethanol: Fermentation
Ethanol is manufactured by the fermentation of aqueous glucose, using yeast, at 25-35°C, in the absence of oxygen.
C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
After fermentation, the dilute ethanol solution is concentrated by (fractional) distillation, which separates ethanol (boiling point 78°C) from water (boiling point 100°C).
Worked Example
- Question (based on a real exam question): (i) state two conditions needed for the fermentation of aqueous glucose. (ii) Name the method used to separate ethanol from the reaction mixture afterwards.
- Answer: (i) any two of: temperature 25-35°C; yeast; anaerobic conditions/absence of oxygen. (ii) (fractional) distillation.
Manufacture of Ethanol: Catalytic Addition of Steam to Ethene
Ethanol is also manufactured by the catalytic addition of steam to ethene, at 300°C and 6000 kPa/60 atm, in the presence of an acid catalyst (phosphoric acid).
C₂H₄ + H₂O → C₂H₅OH
Comparing the Two Methods (Supplement)
| Factor | Fermentation | Catalytic addition of steam to ethene |
|---|---|---|
| Raw material | glucose - renewable | ethene, from crude oil - non-renewable |
| Rate of reaction | slow (batch process, takes days) | fast (continuous process) |
| Temperature | low, 25-35°C | high, 300°C |
| Purity of product | dilute ethanol - needs distillation to concentrate | pure, concentrated ethanol directly |
| Energy cost | low | high |
Combustion and Uses of Ethanol
Ethanol burns in air, releasing energy (an exothermic reaction):
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Uses of ethanol:
- as a solvent, e.g. in perfumes, inks and cosmetics
- as a fuel, e.g. in camping stoves and as a petrol additive
Common Mistakes
- ❌ Saying ethanol burns to produce carbon dioxide and hydrogen, or that combustion is endothermic
- ✓ complete combustion of ethanol produces carbon dioxide and water (not hydrogen), and combustion is always exothermic - it releases energy, which is why ethanol is useful as a fuel
- ❌ Mixing up the two manufacturing methods' conditions, e.g. saying fermentation uses 300°C, or that the catalytic route uses yeast
- ✓ fermentation is the low-temperature (25-35°C), yeast-catalysed method; catalytic addition of steam to ethene is the high-temperature (300°C), acid-catalysed method
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