1.9.4Unit 1. Structure, Bonding and Introduction to Organic Chemistry
Alternative Fuels — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 34 of 36 Unit quiz
Part 1 of 5Why Alternatives Are Needed
Biofuels and hydrogen are the most common alternatives studied at this level.
Finite supply
- Finite supply Fossil fuels are non-renewable and will become more scarce and more expensive over time.
Greenhouse gas
- Greenhouse gas Burning them transfers carbon locked away for millions of years into the atmosphere as carbon dioxide.
Energy security
- Energy security Many countries depend on imports of oil and gas, so renewable fuels reduce this dependence.
Aim
- Aim A good alternative should release little net CO₂, be renewable and have a high energy output per unit mass or volume.
Part 2 of 5Biofuels
Biofuels
Bioethanol
- Bioethanol Made by fermenting sugars from plants, C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂, using yeast enzymes at about 30–40 °C without oxygen.
Combustion of ethanol
- Combustion of ethanol C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O releases heat and can be blended with petrol.
Biodiesel
- Biodiesel Made from vegetable oils or waste cooking oil and used in diesel engines.
Carbon neutral idea
- Carbon neutral idea The CO₂ released on burning is roughly equal to that absorbed by the plants as they grew, so there is little net increase.
Drawbacks
- Drawbacks Growing crops uses land that could grow food, and farming, fertiliser and processing use energy that may come from fossil fuels.
Part 3 of 5Hydrogen
1
Hydrogen burns to form only water, 2H₂ + O₂ → 2H₂O, so no carbon dioxide is released at the point of use.
2
It can be used directly in combustion or in a fuel cell to generate electricity.
3
Most hydrogen is currently made from methane by steam reforming, which uses fossil fuels and releases CO₂.
4
Hydrogen can also be made by electrolysis of water, which is only low-carbon if the electricity is from renewable sources.
5
Storage is difficult because hydrogen is a gas of very low density and is highly flammable.
Part 4 of 5Making an Evaluation
Making an Evaluation
Compare energy
- Compare energy Hydrogen releases far more energy per gram than petrol, but a given volume of the gas stores little energy.
Sustainability
- Sustainability Biofuels are renewable on a human timescale, whereas fossil fuels are not.
Practicalities
- Practicalities Existing engines can use blends of ethanol, while hydrogen needs new vehicles and refuelling infrastructure.
Balanced view
- Balanced view State both advantages and disadvantages and use evidence such as carbon cost and land use in conclusions.
| Fuel | Main advantage | Main drawback |
|---|---|---|
| Bioethanol | Renewable, near carbon neutral | Land use |
| Biodiesel | Uses waste oils | Land and energy for production |
| Hydrogen | Burns to water only | Storage and production |
Part 5 of 5Exam Checklist & Tips
1.9.4 – Key Points for the Exam
- Biofuel A fuel made from recently living plant or animal material.
- Carbon neutral Releasing no more carbon dioxide overall than is removed from the atmosphere.
- Fermentation Breakdown of sugars by yeast enzymes into ethanol and carbon dioxide in the absence of oxygen.
- Fuel cell A device that converts the chemical energy of a fuel directly into electrical energy.
- Renewable Able to be replaced on a human timescale.
Common Mistakes
- Claiming biofuels are always carbon neutral without considering production energy.
- Saying hydrogen is a source of energy rather than a fuel that must be produced.
- Writing the fermentation equation unbalanced or with oxygen as a reactant.
- Ignoring that hydrogen made from methane produces carbon dioxide.
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