4.3.5Additional lessons
Biofuels — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 57 of 98
Part 1 of 6What Are Biofuels
Biofuels aim to reduce reliance on fossil fuels.
Definition
- A biofuel is a fuel made from recently living material, such as crops or waste, instead of fossil deposits.
Bioethanol
- Ethanol made by fermenting sugars from crops such as sugar cane or maize, used alone or blended with petrol.
Biodiesel
- A mixture of esters made from vegetable oils or animal fats, used in diesel engines.
Renewable
- Biofuel feedstocks can be regrown within a short time, whereas fossil fuels take millions of years to form.
Part 2 of 6Making Biofuels
1
For bioethanol, crops are crushed to release sugars, and yeast ferments glucose without oxygen: C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g).
2
The dilute ethanol solution is then purified by fractional distillation to raise the ethanol concentration.
3
For biodiesel, vegetable oils react with an alcohol such as methanol in the presence of a catalyst in a transesterification reaction.
4
The reaction forms esters, called biodiesel, and glycerol as a by-product, which are then separated.
| Biofuel | Raw material | Main process |
|---|---|---|
| Bioethanol | Sugar cane, maize | Fermentation then distillation |
| Biodiesel | Vegetable oils, fats | Transesterification with an alcohol |
Part 3 of 6Carbon Neutrality
Carbon Neutrality
Idea
- Plants absorb CO₂ for photosynthesis as they grow, which is released again when the fuel burns, so net emissions can be lower.
Not fully neutral
- Energy is needed to grow, harvest, transport and process crops, and this often comes from fossil fuels.
Land use
- Clearing forests for crops releases stored carbon and can cancel the benefit of the fuel.
Comparison
- Overall, a biofuel can reduce net greenhouse gas emissions, but the amount depends heavily on how it is produced.
Part 4 of 6Advantages and Disadvantages
Advantages and Disadvantages
Advantages
- Renewable, can reduce net CO₂ emissions and can be used in existing engines when blended with fossil fuels.
Food competition
- Growing fuel crops may use land and water needed for food, which can raise food prices.
Energy density
- Ethanol releases less energy per gram than petrol, so a given mass of fuel does less work.
Environmental impact
- Monocultures can reduce biodiversity, and fertilisers can pollute water.
Part 5 of 6Worked Example
Question
- 100 g of glucose is fermented to ethanol.
- Calculate the theoretical mass of ethanol.
- (Mr glucose = 180.18, Mr ethanol = 46.08)
- C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g).
- Amount of glucose = 100 ÷ 180.18 = 0.555 mol.
- Amount of ethanol = 2 × 0.555 = 1.110 mol.
- Mass of ethanol = 1.110 × 46.08 = 51.1 g.
Answer 51.1 g of ethanol (theoretical)
Part 6 of 6Exam Checklist & Tips
4.3.5 – Key Points for the Exam
- Biofuel A fuel produced from recently living biological material.
- Fermentation The anaerobic breakdown of sugar by yeast into ethanol and carbon dioxide.
- Transesterification A reaction in which an ester reacts with an alcohol to produce a different ester and alcohol.
- Renewable A resource which can be replaced by natural processes on a human timescale.
- Carbon neutral A process with no net addition of CO₂ to the atmosphere.
Common Mistakes
- Describing biofuels as completely carbon neutral without allowing for energy used in production.
- Saying that biodiesel and petrol diesel are the same compounds;
- biodiesel is a mixture of esters, whereas diesel is made of hydrocarbons.
- Forgetting that fermentation also releases CO₂.
- Assuming biofuel production never competes with food.
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