11.6 Alcohols
- Syllabus
- 0620–2026–2027
- Topic
- 11.6
- Level
- —
Learning objectives
11.6.1Ethanol manufacture• Describe ethanol manufacture by: (a) fermentation of aqueous glucose at 25-35 °C with yeast and no oxygen (b) catalytic steam addition to ethene at 300 °C and 6000 kPa/60 atm with acid catalyst11.6.2Combustion of ethanol• Describe combustion of ethanol11.6.3Uses of ethanol as: (a) a solvent (b)• State the uses of ethanol as: (a) a solvent (b) a fuel11.6.4Advantages and disadvantages of the• Describe advantages and disadvantages of the manufacture of ethanol by: (a) fermentation (b) catalytic addition of steam to ethene
Compare the two ways to manufacture ethanol
| Route | Reactants | Conditions | Equation |
|---|---|---|---|
| fermentation | aqueous glucose | yeast, 25–35 °C, absence of oxygen | C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ |
| catalytic addition of steam | ethene + steam | acid catalyst, 300 °C, 6000 kPa / 60 atm | C₂H₄ + H₂O → C₂H₅OH |
Yeast provides enzymes for fermentation. The glucose must be in aqueous solution, the temperature must be warm enough for enzyme activity, and oxygen is excluded so fermentation rather than aerobic respiration occurs.
Steam adds across the C=C bond in ethene to make ethanol. The reaction is an acid-catalysed addition process carried out at high temperature and pressure.
Fermentation produces dilute aqueous ethanol, so fractional distillation is used when a more concentrated or pure product is required.
Do not swap the conditions: fermentation uses 25–35 °C and no oxygen; ethene hydration uses 300 °C, 6000 kPa/60 atm and an acid catalyst.
Describe the combustion of ethanol
Ethanol burns in oxygen and releases energy, so it can be used as a fuel.
Complete combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.
The balanced equation contains 2 carbon atoms, 6 hydrogen atoms and 7 oxygen atoms on each side.
Combustion is exothermic. In a plentiful supply of oxygen, the carbon and hydrogen in ethanol form carbon dioxide and water.
Ethanol already contains oxygen, but oxygen gas is still required for complete combustion. Do not omit O₂ from the reactants.
State the two uses of ethanol
| Use | Why ethanol is suitable |
|---|---|
| solvent | dissolves many substances used in products and laboratory mixtures |
| fuel | burns exothermically in oxygen and releases useful energy |
If a question asks for the syllabus uses, state both: ethanol is used as a solvent and as a fuel.
Its solvent use is a physical application; its fuel use depends on the chemical combustion reaction.
Do not replace the named uses with how ethanol is manufactured. Fermentation and hydration are production routes, not uses.
Evaluate fermentation and ethene hydration
| Route | Advantages | Disadvantages |
|---|---|---|
| fermentation | renewable plant-derived glucose; low temperature and pressure | slow; batch process; dilute/impure ethanol needs distillation; land may be needed for crops |
| catalytic addition of steam to ethene | fast; continuous process; purer product | ethene comes from non-renewable petroleum; high temperature and pressure require energy; equilibrium limits conversion per pass |
Make each comparison relative: fermentation uses renewable feedstock and gentler conditions, whereas hydration is faster, continuous and gives purer ethanol.
No route is simply 'better'. The decision balances feedstock renewability and energy conditions against rate, continuity, purity and separation needs.
Low temperature and renewable feedstock are fermentation advantages, not hydration advantages. Pure product and continuous operation belong to ethene hydration.