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4.3.5Additional lessons

Biofuels — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 83 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Types of Biofuel

Biofuels are produced from recently living material rather than from ancient fossilised remains.

Bioethanol
  • Bioethanol Ethanol made by fermenting sugars from crops such as sugar cane or maize: C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g).
Biodiesel
  • Biodiesel Made by reacting plant oils or animal fats with an alcohol in the presence of a catalyst, in a process called transesterification.
Renewable source
  • Renewable source The raw materials are regrown each year, unlike crude oil, which takes millions of years to form.
Blending
  • Blending Bioethanol can be mixed with petrol, and biodiesel with diesel, so existing engines can often use them.

Section 2 of 5Carbon Balance

1
Growing plants absorb carbon dioxide from the atmosphere through photosynthesis.
2
The plant material is converted into fuel using energy and processing steps.
3
Burning the biofuel releases carbon dioxide back into the atmosphere.
4
If the crops are replanted, the cycle can appear nearly carbon neutral over a full growth period.
5
Fertiliser, farming machinery, transport and distillation use fossil energy, so real net emissions are not zero.

Section 3 of 5Advantages and Disadvantages

Advantages and Disadvantages
Advantages
  • Advantages Renewable, may cut net CO₂ emissions, and can reduce dependence on imported oil.
Land use
  • Land use Fuel crops compete with food crops and may lead to deforestation, raising food prices.
Energy content
  • Energy content Ethanol releases less energy per gram than petrol, so a larger mass is required for the same energy.
Water and chemicals
  • Water and chemicals Growing crops needs water and fertilisers, which can pollute rivers and soils.

Section 4 of 5Worked Example

Question

  • Calculate the theoretical mass of ethanol obtained by fermenting 180 g of glucose (M = 180.16 g mol⁻¹;
    • M of C₂H₅OH = 46.07 g mol⁻¹).

  1. Moles of glucose = 180 ÷ 180.16 = 0.999 mol.
  2. Mole ratio C₆H₁₂O₆C₂H₅OH = 1 : 2, so ethanol = 1.998 mol.
  3. Mass of ethanol = 1.998 mol × 46.07 g mol⁻¹ = 92.0 g.

Answer 92.0 g of ethanol (theoretical)

Section 5 of 5Exam Checklist & Tips

4.3.5 – Key Points for the Exam

  • Biofuel A fuel produced from recently living biological material.
  • Fermentation Anaerobic breakdown of sugars by yeast, producing ethanol and carbon dioxide.
  • Transesterification Reaction of a triglyceride with an alcohol to make esters (biodiesel) and glycerol.
  • Carbon neutral Releasing no more carbon to the atmosphere than was absorbed in producing the fuel.

Common Mistakes

  • Stating that biofuels are always carbon neutral, ignoring emissions from farming and processing.
  • Saying fermentation needs oxygen, when it is anaerobic.
  • Forgetting that fermentation also produces CO₂.
  • Treating biodiesel and ethanol as the same fuel made by the same method.

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