Biofuels (DP IB Chemistry): Revision Note
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

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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