Ethanol as a FuelOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is ethanol and why is it used as a fuel?
Ethanol (C2H5OH) is an alcohol - a compound containing the -OH functional group. It burns in air releasing energy, so it can be used as a fuel on its own or blended with petrol. Because ethanol can be produced from plant material, it is classed as a biofuel - a fuel made from recently living matter rather than fossil fuels.
- Ethanol burns to produce carbon dioxide and water, like other fuels containing carbon and hydrogen
- Two industrial methods exist for manufacturing ethanol: hydration of ethene and fermentation of sugar
- Each method uses a different raw material and type of process
Section 2
How is ethanol made by hydration of ethene?
Ethanol can be manufactured by reacting ethene with steam:
C2H4 + H2O -> C2H5OH
- The catalyst used is phosphoric acid
- This is a continuous process: raw materials are fed in and product is removed constantly, with no need to stop and restart
- Ethene is obtained from crude oil by cracking, so this method depends on crude oil - a non-renewable resource
- The reaction reaches equilibrium, so not all the ethene is converted; unreacted gases can be recycled
Examiners want the exact catalyst named: phosphoric acid. An acid catalyst alone will not get the mark.
Section 3
How is ethanol made by fermentation?
Ethanol can also be made by fermenting sugar using yeast:
C6H12O6 -> 2C2H5OH + 2CO2
- Yeast acts as a biological catalyst (it contains the enzymes needed)
- Sugar is a renewable resource, usually from crops such as sugar cane or maize
- This is a batch process: a container of sugar solution and yeast is left to react, then the ethanol is separated off (usually by distillation) before the next batch is started
- Fermentation works best at a warm temperature (around 30 degC) - too hot and the yeast enzymes are denatured
Do not say fermentation burns the sugar - it is a biological breakdown by yeast enzymes, not combustion.
Section 4
How do the two methods compare?
| Feature | Hydration of ethene | Fermentation |
|---|---|---|
| Raw material | Ethene (from crude oil) | Sugar (from crops) |
| Renewable? | No | Yes |
| Process type | Continuous | Batch |
| Labour needed | Low (automated) | Higher (batches must be set up and emptied) |
| Rate of reaction | Fast | Slow |
| Conditions | High temperature and pressure, catalyst needed | Warm temperature, no pressure needed |
| Product purity | High purity, few impurities | Lower purity, needs distillation to concentrate |
| Energy requirement | High (heating and pressurising) | Lower, but distillation adds energy cost |
Section 5
Are biofuels a good alternative to hydrocarbon fuels?
Biofuels, including bioethanol and biodiesel, are made from renewable plant material rather than crude oil.
- Advantage: they use renewable resources, so supplies do not run out in the way crude oil will
- Advantage: growing crops absorbs carbon dioxide, which can offset some of the CO2 released on combustion, giving a lower overall carbon footprint than fossil fuels
- Disadvantage: large areas of land are needed to grow fuel crops, which can compete with land needed for growing food, and can lead to deforestation
- Disadvantage: growing, harvesting and processing crops still uses energy and produces some emissions, so biofuels are not completely carbon-neutral in practice
A sugar cane farm that grows fuel crops on land previously used for food crops illustrates the land-use trade-off examiners often ask about.
Must Know
- Ethene + steam -> ethanol, using a phosphoric acid catalyst; continuous process; non-renewable (ethene from crude oil)
- Sugar -> ethanol + carbon dioxide, using yeast; batch process; renewable
- Fermentation is slower and gives a less pure product than hydration of ethene
- Biofuels use renewable resources and can lower carbon footprint, but need large areas of land, which can compete with food production
- Ethanol, like other fuels, burns to release energy, carbon dioxide and water
That's the notes covered.
Carry on to the next subtopic.