10.2.2Chemistry of the Environment
Ethanol as a Fuel — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 7 Unit quiz
Part 1 of 5Uses of Ethanol
Ethanol is an important alcohol with several applications, and it can be made in two different ways.
Fuel
- Ethanol is used as a fuel for vehicles in some countries, either on its own or mixed with petrol.
Solvent
- Ethanol dissolves many substances that water cannot, so it is used as a solvent.
Drinks
- Ethanol is the alcohol in beers, wines and spirits, which is why it is often made by fermentation.
Laboratory fuel
- Spirit burners in school laboratories use ethanol because it burns cleanly with little odour.
Part 2 of 5Two Ways to Make Ethanol
The two production methods differ in raw materials, conditions and the purity of the product.
Two Ways to Make Ethanol
Hydration
- Ethene reacts with steamC₂H₄ + H₂O → C₂H₅OH, at about 300 °C and 60–70 atm with a phosphoric acid catalyst.
Fermentation
- Yeast converts glucose into ethanol and carbon dioxide without oxygen: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
Separation
- Both methods produce ethanol mixed with water, which is separated by fractional distillation.
Yeast limit
- Fermentation stops at about 15% alcohol, because higher concentrations kill the yeast.
| Feature | Hydration of ethene | Fermentation |
|---|---|---|
| Raw material | Ethene from crude oil and steam | Sugar and water |
| Renewable? | No | Yes |
| Type of process | Continuous | Batch |
| Conditions | About 300 °C, 60–70 atm, phosphoric acid catalyst | 20–35 °C, no oxygen, yeast |
| Purity of product | High | Low |
| Energy requirement | High | Low |
Part 3 of 5Ethanol as a Biofuel
Biofuels are made from plant material and are often described as carbon neutral.
1
Ethanol biofuel is produced from a dilute solution made by fermenting plant sugars.
2
Plants absorb carbon dioxide during photosynthesis, and this is released again when the biofuel is burned.
3
Because the carbon released matches the carbon absorbed by the plants, biofuels are described as carbon neutral.
4
Biofuels produce no sulfur dioxide, but growing crops uses land that could grow food and takes a long time.
Part 4 of 5Worked Example
Question Calculate the maximum mass of ethanol that can be made from 180 g of glucose by fermentation.
- EquationC₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
- Mr of glucose C₆H₁₂O₆ = 6(12) + 12(1) + 6(16) = 180.
- Mr of ethanol C₂H₅OH = 2(12) + 6(1) + 16 = 46, so two molecules of ethanol have a total mass of 92.
- Mass of ethanol = 180 g × 92 ÷ 180 = 92 g.
Answer The maximum mass of ethanol is 92 g, assuming all the glucose is converted.
Part 5 of 5Exam Checklist & Tips
10.2.2 – Key Points for the Exam
- Hydration The addition of water across a carbon–carbon double bond, as in the reaction of ethene and steam.
- Fermentation The anaerobic breakdown of glucose by yeast to form ethanol and carbon dioxide.
- Biofuel A fuel produced from plant or animal material.
- Carbon neutral Describes a fuel whose burning releases no net extra carbon dioxide into the atmosphere.
- Batch process A process that is run in separate stages, with the reaction vessel emptied and refilled each time.
- Continuous process A process that runs without stopping, with reactants added and products removed as it goes.
Common Mistakes
- Saying that fermentation needs oxygen; it takes place in the absence of oxygen.
- Calling yeast a reactant that is used up; yeast provides enzymes that act as catalysts.
- Describing biofuels as carbon free;
- they release carbon dioxide when burned, but this is balanced by photosynthesis.
- Assuming fermentation can run at high temperatures; the enzymes in yeast are denatured if it gets too hot.
Last updated:
\n