4.5.3Electrochemistry
Manufacture of Ethanol — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 20 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Hydration of Ethene
1
Ethene from cracking is reacted with steam at about 300 °C and 60–70 atm over a concentrated phosphoric acid catalyst.
2
A water molecule adds across the C=C double bond, forming ethanol: C₂H₄ + H₂O → C₂H₅OH.
3
Unreacted ethene is separated by condensing the mixture, because ethene has a much lower boiling point than ethanol and water.
4
The ethanol and water are then separated by fractional distillation, and the unreacted ethene is recycled.
Section 2 of 5Fermentation
Feedstock
- Feedstock Sugar or starch is dissolved in water and yeast is added to provide the enzymes.
Conditions
- Conditions The mixture is kept at about 25–35 °C, with an optimum near 30 °C, and oxygen must be excluded.
Equation
- Equation C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH, a form of anaerobic respiration by the yeast.
Batch process
- Batch process The yeast is killed once the alcohol concentration reaches about 15%, so the process is stopped and restarted in batches.
Temperature control
- Temperature control If the temperature is too low the reaction is slow, and if it is too high the enzymes are denatured.
Section 3 of 5Comparing the Two Methods
Comparing the Two Methods
Renewability
- Renewability Hydration uses a non-renewable feedstock, whereas fermentation uses a renewable source of sugar.
Separation
- Separation Both methods require fractional distillation to obtain pure ethanol from the water.
Energy
- Energy Hydration needs high temperature and pressure, whereas fermentation needs only mild warmth.
| Feature | Hydration of ethene | Fermentation |
|---|---|---|
| Starting material | Ethene from cracking, from crude oil | Sugar or starch, from crops |
| Conditions | About 300 °C, 60–70 atm, phosphoric acid catalyst | 25–35 °C, no oxygen, yeast enzymes |
| Process type | Continuous | Batch |
| Concentration of product | Ethanol obtained with water after reaction | Dilute mixture of ethanol and water |
Section 4 of 5Worked Example
Question
- Glucose, C₆H₁₂O₆ (Mr = 180), is fermented to ethanol (Mr = 46).
- Calculate the maximum mass of ethanol that can be made from 90 g of glucose.
- Moles of glucose = 90 ÷ 180 = 0.50 mol
- From C₆H₁₂O₆ → 2C₂H₅OH, 1 mol of glucose gives 2 mol of ethanol, so 0.50 mol gives 1.0 mol of ethanol
- Mass of ethanol = 1.0 × 46 = 46 g
Answer 46 g (the theoretical maximum, assuming complete reaction).
Section 5 of 5Exam Checklist & Tips
4.5.3 – Key Points for the Exam
- Hydration The addition of water across a C=C double bond.
- Fermentation The anaerobic conversion of sugar into ethanol and carbon dioxide by yeast enzymes.
- Anaerobic Taking place without oxygen.
- Batch process A process in which a fixed amount is made in one cycle, then stopped and restarted.
- Phosphoric acid catalyst The concentrated acid used to speed up the hydration of ethene.
Common Mistakes
- Stating that fermentation needs oxygen; it is anaerobic.
- Giving the hydration conditions as room temperature; hydration needs about 300 °C and high pressure.
- Describing fermentation as a continuous process; it is a batch process.
- Forgetting that yeast enzymes are denatured at high temperatures, so the temperature must be controlled.
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