3.5.2Physical Chemistry
Alcohol Production — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 83 of 102
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.
Part 1 of 6Routes to Alcohols
Fermentation
- Fermentation Yeast enzymes convert sugar into ethanol and carbon dioxide in the absence of oxygen.
Hydration of alkenes
- Hydration of alkenes Steam adds across the C=C bond of an alkene in the presence of an acid catalyst such as H₃PO₄.
Haloalkane substitution
- Haloalkane substitution Heating a haloalkane with aqueous NaOH replaces the halogen with –OH.
Reducing carbonyls
- Reducing carbonyls NaBH₄ or LiAlH₄ reduces aldehydes to primary alcohols and ketones to secondary alcohols.
Reducing acids
- Reducing acids Carboxylic acids are reduced to primary alcohols by LiAlH₄, or by hydrogen with a nickel catalyst.
Part 2 of 6Fermentation of Glucose
The overall equation is C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g).
1
Sugar or starch is dissolved in water and yeast is added to the mixture.
2
The mixture is kept warm, at a moderate temperature of roughly 30 °C, and air is excluded so respiration is anaerobic.
3
Enzymes in the yeast break the glucose down into ethanol and carbon dioxide over several days.
4
If the temperature is too low the rate is slow, and if it is too high the enzymes are denatured.
5
The reaction stops when the ethanol concentration reaches about 15%, because this kills the yeast, so distillation is used to purify the ethanol.
Part 3 of 6Fermentation Compared with Hydration
Fermentation Compared with Hydration
Raw material
- Raw material Fermentation uses renewable sugar crops, while hydration uses ethene from crude oil, which is finite.
Type of process
- Type of process Fermentation is a slow batch process, whereas hydration is a fast continuous process.
Product purity
- Product purity Fermentation gives dilute, impure ethanol needing distillation, but hydration gives much purer ethanol.
Energy and cost
- Energy and cost Fermentation needs a mild temperature, but hydration needs about 300 °C and high pressure.
| Feature | Fermentation | Hydration of ethene |
|---|---|---|
| Raw material | Sugar (renewable) | Ethene (non-renewable) |
| Rate | Slow | Fast |
| Process | Batch | Continuous |
| Product | Dilute ethanol | Pure ethanol |
Part 4 of 6Is Bioethanol Carbon Neutral?
A carbon-neutral fuel gives no net emission of CO₂ to the atmosphere over its life cycle.
Is Bioethanol Carbon Neutral?
Photosynthesis
- Photosynthesis 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ removes six moles of CO₂ for each glucose made by the crop.
Fermentation
- Fermentation C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ releases two moles of CO₂ for each glucose.
Combustion
- Combustion C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O releases two moles of CO₂ per ethanol, so the two ethanol from one glucose release four moles.
Why the claim fails
- Why the claim fails Farm machinery, transport and distillation burn fossil fuels, and growing the crop can replace forests or food land.
Part 5 of 6Worked Example
Question Calculate the atom economy for making ethanol from glucose by fermentation, using C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ and relative molecular masses of 180.2 for glucose and 46.1 for ethanol.
- Mass of desired product = 2 × 46.1 = 92.2.
- Mass of all reactants = 180.2 (only glucose is a reactant).
- Atom economy = (92.2 ÷ 180.2) × 100.
- 92.2 ÷ 180.2 = 0.5117, so the value is about 51%.
Answer The atom economy is about 51%, since carbon dioxide is a second product that is not wanted.
Part 6 of 6Exam Checklist & Tips
3.5.2 – Key Points for the Exam
- Fermentation Anaerobic breakdown of sugar by enzymes in yeast to give ethanol and carbon dioxide.
- Hydration The addition of water, here as steam, across a double bond.
- Biofuel A fuel made from recently grown biological material (biomass).
- Carbon neutral Giving no net emission of CO₂ to the atmosphere, because the CO₂ absorbed equals the CO₂ released.
Common Mistakes
- Saying fermentation needs oxygen; it is anaerobic, and oxygen would oxidise the ethanol.
- Defining carbon neutral as producing no carbon rather than no net carbon dioxide emission to the atmosphere.
- Ignoring the fossil fuels used to grow, harvest, transport and distil the crop.
- Forgetting that the yeast stops working at about 15% ethanol, so fractional distillation is needed.
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