Ethanol can be brewed from sugar by yeast in a warm tank, or made in a chemical plant by adding steam to ethene from crude oil. Each route has its strengths, and the ethanol it makes is burned as a fuel and used as a solvent.
Alcohols contain the –OH functional group; ethanol is C2H5OH. Ethanol is made by fermentation: aqueous glucose with yeast at 25–35 °C in the absence of oxygen: C6H12O6 → 2C2H5OH + 2CO2. It is also made by the catalytic addition of steam to ethene at about 300 °C and 6000 kPa with an acid catalyst (phosphoric acid): C2H4 + H2O → C2H5OH. Extended: fermentation uses a renewable raw material and little energy but is slow and gives impure, dilute ethanol; hydration is fast, continuous and gives pure ethanol but uses non-renewable crude oil and much energy. Ethanol burns completely to carbon dioxide and water and is used as a fuel and a solvent.
Learn the conditions for each route exactly; they are asked for word for word.
| Fermentation | Hydration of ethene | |
|---|---|---|
| raw material | aqueous glucose (from sugar cane, sugar beet, maize) | ethene (from cracking crude oil fractions) and steam |
| conditions | yeast, 25–35 °C, absence of oxygen (anaerobic) | 300 °C, 6000 kPa (60 atm), phosphoric acid catalyst |
| equation | C6H12O6 → 2C2H5OH + 2CO2 | C2H4 + H2O → C2H5OH |
| process | batch | continuous |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which conditions are used to make ethanol by fermentation?
What are the products of fermentation of glucose?
Which substances react to make ethanol in the industrial hydration process?
Which is a use of ethanol?
Extended. The answer depends on what a country has: plenty of farmland and sugar, or a supply of crude oil.
| Fermentation | Hydration of ethene | |
|---|---|---|
| advantages | renewable raw material (crops); low temperature, so low energy cost; simple equipment | fast; continuous; pure ethanol; high yield, the only product is ethanol |
| disadvantages | slow; batch process; dilute, impure product that must be distilled; carbon dioxide is also made; uses land that could grow food | non-renewable raw material (crude oil); high temperature and pressure, so high energy cost and expensive plant |
Explore the reactions of ethanol in the simulator, including what happens when it burns in a good supply of air.
Fermentation needs the absence of oxygen, but combustion of ethanol needs oxygen: do not mix them up. The fermentation temperature is 25–35 °C, not a high temperature: heating kills (denatures the enzymes in) the yeast. In the hydration equation water is steam, and the catalyst is an acid (phosphoric acid), not yeast.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
What is the balanced equation for the complete combustion of ethanol?
(Extended) Which is an advantage of making ethanol by fermentation rather than from ethene?
(Extended) Which is a disadvantage of making ethanol from ethene?
What mass of ethanol (Mr 46) can be made by fermenting 90 g of glucose (Mr 180)?
This skill is now recorded on your profile. Keep the chain going.