Alcohol: organic compound with a hydroxyl group -OH on a saturated carbon.
Fermentation: enzymes in yeast turn aqueous glucose into ethanol and carbon dioxide at 25 to 35 °C in the absence of oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. It gives 10 to 15% ethanol (more kills the yeast). Fractional distillation raises it to 96% (rectified spirit); the last 4% water cannot be removed by distillation, so a dehydrating agent such as conc. H₂SO₄ is used to get 100% ethanol.
Hydration of ethene: C₂H₄ + H₂O ⇌ C₂H₅OH with steam at 300 °C and 6 atm (book value) using phosphoric acid catalyst. About 4% converts per pass, the ethene is recycled until about 96% is converted. (The book states 6 atm, 6000 kPa; learn it as the book gives it.)
Gasohol: petrol blended with 10 to 20% ethanol.
Fermentation against hydration (table, often asked as "one advantage and one disadvantage")
Raw material: fermentation uses glucose from plants (renewable); hydration uses ethene from crude oil (non-renewable).
Process: fermentation is a batch process (slow, all reactants left for days); hydration is continuous (fast, efficient).
Product: fermentation gives impure, dilute ethanol that needs further distillation; hydration gives pure ethanol.
Energy: fermentation needs a moderate temperature and low energy; hydration needs high temperature and pressure, so more energy.
Fuel: ethanol is renewable, has a higher octane number than petrol, reduces carbon monoxide and smog-forming emissions, gives about two-thirds of petrol's energy per litre. Methanol has low energy content, so it reduces driving range. Disadvantage of alcohol fuels: absorb water from air (hygroscopic) and can corrode engine parts. Corn-based ethanol cuts greenhouse gas emissions by about 40 to 50%.
Pharmaceuticals: solvents (herbal extracts, homoeopathy), antiseptic and disinfectant (swabs before injections, kill bacteria, viruses and fungi), isopropanol for sterilising equipment, help drugs pass through the skin.
Cosmetics: deodorants, perfumes, lotions, toners; antibacterial and keep ingredients mixed.
Daily life: solvents in paints, inks and cleaners; hand sanitisers and surface disinfectants (important in COVID-19).
Past-paper questions with model answers
Write the equation for complete combustion of methanol or ethanol and name the products (D, M, 2+1): equation above; products carbon dioxide and water.
Equation for fermentation of glucose (M OR): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (yeast, 25 to 35 °C, no oxygen).
Complete balanced equation for addition of steam to ethene to give ethanol (B): C₂H₄ + H₂O → C₂H₅OH, phosphoric acid, 300 °C, 6 atm.
One advantage and one disadvantage of fermentation compared with catalytic hydration of ethene (D): advantage, uses renewable raw material and low energy; disadvantage, slow batch process giving impure, dilute ethanol.
Any two uses of alcohols in daily life (M): fuel (gasohol), solvent, disinfectant and sanitiser, cosmetics and perfumes.