Alternative Fuel
A concise revision guide to pollutants from fuel combustion, biofuels, alcohol fuels, biodiesel and the advantages and disadvantages of replacing fossil fuels with renewable plant-based fuels.
GCSE Recap: Burning Fuels and Photosynthesis
Answer three quick GCSE questions before you start.
Where This Sits in the Cambridge Specification
Alternative fuels and carbon neutrality are not in Cambridge Topic 14. This page is extension reading around the environmental consequences of burning alkanes (14.1.6).
Why Alternative Fuels Are Needed
Most fossil fuels are obtained from crude oil. Crude oil is a non-renewable resource, so it will eventually be depleted if it continues to be used faster than it forms.
Alternative fuels have been developed to reduce reliance on finite fuel reserves. Important examples include alcohols such as ethanol and biodiesel, both of which can be produced from plant material.
Definition: A renewable fuel is a fuel made from resources that can be replaced or regrown on a short timescale compared with fossil fuels.
Biofuels are produced from recently living plant material, whereas fossil fuels are made from ancient biological material over very long timescales.
Quick Check: Same Molecule, Different Label
Apply the definition of a renewable fuel to two sources of methane.
Pollutants from Fuel Combustion
Burning fuels in engines can release several pollutants. The exact products depend on the conditions inside the engine, including oxygen supply, temperature and whether the fuel burns completely.
| Pollutant source | Pollutant | Environmental consequence |
|---|---|---|
 |
Nitrogen oxides Formed when N2 and O2 from the air react at the very high temperature reached inside the engine. |
NO is toxic and can form smog. NO2 is toxic and acidic and forms acid rain. |
 |
Carbon monoxide | Toxic gas formed during incomplete combustion. |
 |
Carbon dioxide | Contributes towards global warming. |
 |
Unburnt hydrocarbons Formed because not all fuel burns in the engine. |
React with nitrogen oxides in sunlight to form photochemical smog and ground-level ozone. |
 |
Soot and particulates | Cause global dimming and respiratory problems. |
Exam focus: Link each pollutant to both its origin and its consequence. For example, nitrogen oxides form at high engine temperatures and contribute to acid rain and smog.
Quick Check: Name the Pollutant
Read each situation and pick the pollutant responsible as quickly as you can.
Biofuels from Renewable Resources
Biofuels are fuels produced from biological material, especially plant material. They are described as renewable because the raw materials can be regrown.
Biofuels are often considered more carbon neutral than fossil fuels because the carbon dioxide released during combustion can be partially offset by the carbon dioxide absorbed during plant growth.
Key idea: Biofuels are not automatically zero-carbon. The carbon dioxide released when they burn may be partly balanced by photosynthesis during plant growth, but growing, processing and transporting the fuel can still involve emissions.
Fossil fuels
Crude-oil based fuels are non-renewable because the reserves are finite.
Alternative fuels
Plant-based fuels can be renewable if the crops or raw materials are regrown sustainably.
Quick Check: Follow the Carbon
Use three equations to count the carbon dioxide absorbed and released, then judge the result.
Alcohol Fuels and Biodiesel
Alcohol fuels such as ethanol can be produced by the fermentation of sugars obtained from plants. Because the raw materials can be regrown, ethanol is considered a renewable fuel source.
Biodiesel is produced by reacting vegetable oils with methanol in the presence of an alkali catalyst. This converts the oils into a mixture of methyl esters, which is the fuel used in diesel engines in place of fossil-based diesel.
Alcohol fuels
Ethanol can be made by fermentation of sugars from plants.
Biodiesel
Vegetable oils react with methanol using an alkali catalyst to make biodiesel.
Common mistake: Do not describe biodiesel as crude-oil based. It is made from vegetable oils and methanol, so it is an alternative to fossil-based diesel.
Quick Check: Where Does the Methanol Come From?
Use the way biodiesel is made to judge its carbon balance.
Advantages and Disadvantages of Biofuels
Biofuels can reduce dependence on fossil fuels, but their production can also compete with food production and land use.
| Advantages | Disadvantages |
|---|---|
| Reduction in use of fossil fuels Biofuels reduce reliance on finite fossil fuel resources. |
Fewer food crops may be grown Land and crops may be diverted from food production to fuel production. |
| Biofuels are renewable Plant material can be regrown if managed sustainably. |
Engines may need modifying Ethanol-rich fuels attack the seals and fuel lines of older engines, and ethanol releases less energy per litre than petrol. |
| Biofuels are more carbon neutral The CO2 released when bioethanol or biodiesel burns may be partly offset by the CO2 absorbed as the crop grew. |
Rainforests may be cut down to provide land Deforestation damages habitats and can increase net carbon emissions. |
| Crude oil is conserved as a chemical feedstock Burning less of it as fuel leaves more crude oil for making organic compounds. |
Shortage of fertile soils Large-scale fuel crop production can increase pressure on fertile land. |
| Lower risk of large-scale fossil fuel pollution Less extraction and transport of fossil fuels can reduce the risk of major pollution events. |
Environmental pressure Intensive farming can cause water stress, habitat loss and soil degradation. |
Reduced fossil fuel use
Using biofuels can decrease dependence on finite crude-oil resources.
Food crop competition
Growing fuel crops can reduce the land available for food crops.
Deforestation risk
Rainforests may be cleared to provide land for biofuel crop production.
Land and water pressure
Large-scale crop growth can put pressure on fertile soils and water supplies.
Quick Check: Judging Green Claims
In each round, pick the one statement that is accurate.
Common Exam Mistakes
- Do not say biofuels are completely carbon neutral. It is more precise to say they may be more carbon neutral because plant growth can offset some CO2 released during combustion.
- Do not confuse carbon monoxide with carbon dioxide. Carbon monoxide is toxic, while carbon dioxide contributes towards global warming.
- Do not forget that nitrogen oxides form from nitrogen in the air at high engine temperatures, not from nitrogen inside the fuel.
- Do not list only advantages of biofuels. Examiners often expect a balanced evaluation including food crop competition, land use and deforestation.
Alternative Fuel Summary
Fossil fuels
Crude-oil fuels are non-renewable and will eventually be depleted.
Biofuels
Alcohols and biodiesel can be made from plant-based renewable resources.
Pollutants
Combustion can produce CO, CO2, nitrogen oxides, unburnt hydrocarbons and particulates.
Trade-offs
Biofuels reduce fossil fuel use but may increase pressure on food crops, forests and fertile soils.
Master Introduction to Organic Chemistry and Alkanes for Cambridge International AS & A Level Chemistry
Continue from these free revision notes into the full Topics 13 and 14.1 Introduction to Organic Chemistry and Alkanes course. The guided video lessons are ready now; the Cambridge International MCQ bank, teacher-marked short-answer questions and KASP spec-point report are being written, and the course opens for enrolment as soon as they are complete.
Guided video teaching
Learn the chemistry and exam technique through structured video lessons with worked examples and walkthroughs.
Instant MCQ feedback
Auto-marked MCQ quizzes provide immediate diagnostic feedback for every answer choice.
Teacher-marked SAQs
Submit written exam responses and receive chemistry specialist feedback with improvement guidance.
Progress tracking
Identify strengths and weaknesses across the full Topics 13 and 14.1 specification with targeted reporting.
See how the course works
Click play to start the course preview animation.
Some ionic radii are shown.
| Ion | Ionic radius / nm |
|---|---|
| Na+ | 0.102 |
| K+ | 0.138 |
| F− | 0.133 |
| Cl− | 0.180 |
Which compound has the strongest ionic bonding?
Explain why the metallic bonding in magnesium is much stronger than that in sodium.
Alternative Fuel FAQ
Use these questions to check the main exam ideas about pollutants and plant-based fuels.
Why are fossil fuels described as non-renewable?
Fossil fuels are non-renewable because they are formed over very long timescales and are being used faster than they can be replaced.
Why are biofuels described as renewable?
Biofuels are described as renewable because their raw materials, such as plant crops, can be regrown.
Why can biofuels be more carbon neutral than fossil fuels?
The carbon dioxide released during biofuel combustion can be partly offset by carbon dioxide absorbed during plant growth.
How is biodiesel made?
Biodiesel is produced by reacting vegetable oils with methanol in the presence of an alkali catalyst.
What are the main disadvantages of growing crops for biofuels?
Less land may be available for food crops, rainforests may be cut down and there may be greater pressure on fertile soils and water supplies.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Cambridge topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in