Biofuels (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Biofuels

What is the difference between renewable and non-renewable resources?

Renewable resources

  • Renewable resources will not run out in the foreseeable future because they can be replaced over a relatively short period of time

  • They can also be called infinite 

  • They can be considered sustainable as they can be produced at the same rate, or faster, than they are being used

  • Biofuels are an example of a renewable resource

Non-renewable resources

  • Non-renewable resources will run out in the foreseeable future as there are limited supplies

    • They cannot be replaced within a short time period

  • They can also be called finite 

  • They are not a sustainable resource

  • Fossil fuels are an example of a non-renewable resource

Reasons for the development of biofuels and other renewable resources:

  • Reduce pollution from the combustion of fossil fuels 

  • Have positive impacts on global warming and climate change

  • The finite supply / limited amount and depletion of fossil fuels, which are non-renewable resources  

What is a biofuel?

  • The three main biofuels are:

    • Bioethanol

    • Biodiesel

    • Biogas

  • Biofuels are made from organic compounds 

  • These organic compounds are produced by biological carbon fixation

Bioethanol

  • Green plants absorb carbon dioxide and convert it into glucose by photosynthesis

carbon dioxide + water → glucose + oxygen

6CO2 (g) + 6H2O (l) → C6H12O6 (aq) + 6O2 (g)

  • Glucose is then converted into ethanol by fermentation.

    • The term bioethanol simply refers to ethanol made from a biological source

    If bioethanol is made this way, it can be described as carbon neutral:

    • The CO2 absorbed during photosynthesis is equal to the CO2 released when the ethanol is burned

Biodiesel

  • Biodiesel is made from renewable vegetable oils, not non-renewable petrochemicals

  • Natural triglyceride oils are converted into esters of methanol, which are less viscous and easier to use as fuel

  • This process is called transesterification

  • This can be catalysed by:

    • Acid, which protonates the carbonyl group

    • Alkali, which deprotonates the methanol or alcohol

    • Alkaline catalysts (e.g. NaOH or KOH) are more commonly used

  • Both acids and alkalis can be used to catalyse the reaction

  • The reaction is reversible, so excess methanol is used to shift the equilibrium towards the products

  • Under ideal conditions, the process can give a yield of up to 98%

How to make biodiesel

Chemical reaction diagram of triglyceride and methanol converting to fatty acid methyl esters and glycerol, catalysed by NaOH.
Transesterification forming methyl esters

Biogas

  • Biogas is a renewable fuel made when organic matter (like food or animal waste) is broken down by microorganisms in the absence of oxygen (anaerobic conditions)

  • It is mostly made up of methane (CH4) and carbon dioxide (CO2), but may also contain small amounts of other gases like hydrogen sulfide

  • The proportions of gases in biogas depend on the type of waste used

  • Carbohydrates (e.g. glucose) produce biogas with 50.0% methane:

C6H12O6 (s) → 3CO2 (g) + 3CH4 (g) 

  • Fatty acids (e.g. heptadecanoic acid or oleic acid) decompose in moist conditions to give biogas with 69.4% methane:

4C17H33COOH (l) + 34H2O (l) → 21CO2 (g) + 51CH4 (g) 

Advantages of biofuel

  • Carbon neutral, renewable, and sustainable if crops/trees are replanted

  • Help reduce greenhouse gas emissions and pollution

  • Reduce landfill waste

    • Biodiesel and biogas can be made from organic waste

  • Can generate income in less developed countries with available land for crops

  • Create jobs in agriculture and energy sectors

Disadvantages of biofuel

  • High costs:

    • Expensive to convert engines/machinery from petrol or diesel

    • Harvesting and transporting crops is costly

  • Land use conflict:

    • Land used for fuel crops may be needed for food

    • May lead to deforestation to meet demand

  • Environmental concerns:

    • Crops can deplete soil nutrients

    • Often require fertilisers and pesticides

  • Generally have lower specific energy than fossil fuels (less energy per kg)

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Richard Boole

Author: Richard Boole

Expertise: Chemistry Content Creator

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

Philippa Platt

Reviewer: Philippa Platt

Expertise: Chemistry Content Creator

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

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