IB Colourful Solutions in Chemistry

Syllabus reference R1.3.4

Reactivity 1.3.4 - Biofuels are produced from the biological fixation of carbon over a short period of time through photosynthesis.

Understand the difference between renewable and non-renewable energy sources.
Consider the advantages and disadvantages of biofuels.

Guidance

The reactants and products of photosynthesis should be known.

Tools and links

Renewable and Non-Renewable Energy Sources
Biofuels
How Biofuels Generate Energy

Renewable and Non-Renewable Energy Sources

Energy sources can be classified into two main categories: renewable and non-renewable.

Renewable Energy Sources

Definition
- Renewable energy sources are those that are replenished naturally and are not depleted when used.
Examples
- Solar Energy: Energy from the sun captured using solar panels.
- Wind Energy: Energy generated from wind using wind turbines.
- Hydropower: Energy produced from flowing water, typically using dams.
- Geothermal Energy: Energy harnessed from the heat within the Earth.
- Biomass: Energy from organic materials like wood, crop waste, and animal manure.
Advantages
- Sustainable and inexhaustible.
- Low environmental impact and reduced greenhouse gas emissions.
- Promotes energy independence and security.
Disadvantages
- Initial high costs for installation and technology development.
- Intermittent energy supply depending on weather and geographical conditions.
- Requires significant land or space for installation.

Non-Renewable Energy Sources

Definition
- Non-renewable energy sources are those that exist in finite quantities and are depleted with use.
Examples
- Coal: Solid fossil fuel used primarily for electricity generation.
- Oil: Liquid fossil fuel used for transportation and industrial processes.
- Natural Gas: Gaseous fossil fuel used for heating, electricity, and as a chemical feedstock.
- Nuclear Energy: Energy from the fission of uranium or plutonium in nuclear reactors.
Advantages
- High energy density and reliable energy supply.
- Established infrastructure and technology for extraction and use.
Disadvantages
- Finite resources that will eventually be depleted.
- Significant environmental impact, including pollution and greenhouse gas emissions.
- Risk of accidents and disasters, such as oil spills and nuclear meltdowns.

^ top

Biofuels

Biofuels are renewable fuels derived from biological materials that can be used as an alternative to fossil fuels. They are produced from plant materials, animal waste, and other organic matter.

Ethanol
- Produced by fermenting sugars from crops like corn, sugarcane, and wheat.
- Commonly used as a gasoline additive to reduce emissions and increase octane levels.
Biodiesel
- Made from vegetable oils, animal fats, or recycled cooking grease through a chemical process called transesterification.
- Can be used in diesel engines with little or no modifications.
Biogas
- Generated through the anaerobic digestion of organic matter such as agricultural waste, manure, and municipal solid waste.
- Consists mainly of methane and can be used for heating, electricity generation, and as a vehicle fuel.

Advantages of Biofuels

Renewable
- Biofuels are produced from renewable resources that can be replenished over a short period.
Reduced Greenhouse Gas Emissions
- Burning biofuels generally releases fewer greenhouse gases compared to fossil fuels.
Energy Security
- Biofuels can be produced locally, reducing dependence on imported fossil fuels.
Biodegradable
- Biofuels are less toxic and more biodegradable than fossil fuels, reducing environmental pollution.

Disadvantages of Biofuels

Land Use
- Growing biofuel crops can compete with food production and lead to deforestation and habitat loss.
Energy Balance
- Some biofuels require significant energy input for production, reducing their overall environmental benefits.
Water Use
- Biofuel production can be water-intensive, impacting water resources, especially in arid regions.
Food Prices
- Using food crops for biofuel production can increase food prices and affect food security.

Future of Biofuels

Second-Generation Biofuels
- Produced from non-food biomass, such as agricultural residues, waste materials, and dedicated energy crops, to address food security issues.
Algae Biofuels
- Algae can produce high yields of biofuel per unit area and can be grown in environments unsuitable for conventional agriculture.
Technological Advances
- Ongoing research aims to improve the efficiency and sustainability of biofuel production processes.

Conclusion

Biofuels offer a renewable and potentially more sustainable alternative to fossil fuels, with the potential to reduce greenhouse gas emissions and enhance energy security. However, challenges related to land use, water consumption, and food security must be addressed to realize their full benefits.

^ top

How Biofuels Generate Energy

Biofuels generate energy through processes that convert organic materials into usable fuels. These fuels can then be burned or processed to release energy in various forms.

Ethanol

Production
- Ethanol is produced by fermenting sugars from crops like corn, sugarcane, and wheat.
- The fermentation process uses microorganisms (yeast) to convert sugars into ethanol and carbon dioxide.
Energy Generation
- Ethanol can be blended with gasoline and used as a fuel in internal combustion engines.
- When burned, ethanol releases energy in the form of heat, powering engines and generating electricity.
Combustion Equation
- C2H5OH + 3O2 → 2CO2 + 3H2O + energy
- The combustion of ethanol produces carbon dioxide, water, and energy.

Biodiesel

Production
- Biodiesel is made from vegetable oils, animal fats, or recycled cooking grease through a chemical process called transesterification.
- Transesterification involves reacting oils or fats with an alcohol (usually methanol) in the presence of a catalyst to produce biodiesel and glycerol.
Energy Generation
- Biodiesel can be used in diesel engines with little or no modifications.
- When burned, biodiesel releases energy in the form of heat, powering engines and generating electricity.
Combustion Equation
- Typical biodiesel molecule: C17H34O2
- C17H34O2 + 25.5O2 → 17CO2 + 17H2O + energy
- The combustion of biodiesel produces carbon dioxide, water, and energy.

Biogas

Production
- Biogas is generated through the anaerobic digestion of organic matter such as agricultural waste, manure, and municipal solid waste.
- Anaerobic digestion involves microorganisms breaking down organic material in the absence of oxygen, producing methane and carbon dioxide.
Energy Generation
- Biogas can be burned to produce heat, used in combined heat and power (CHP) systems to generate electricity and heat, or upgraded to biomethane and used as a vehicle fuel.
- When burned, biogas releases energy in the form of heat.
Combustion Equation
- CH4 + 2O2 → CO2 + 2H2O + energy
- The combustion of methane (the main component of biogas) produces carbon dioxide, water, and energy.

Conclusion

Biofuels generate energy by converting organic materials into fuels that can be burned or processed. Ethanol and biodiesel are commonly used in internal combustion engines, while biogas can be used for heating, electricity generation, and as a vehicle fuel. The combustion of these biofuels releases energy in the form of heat, along with carbon dioxide and water.

^ top

Worked examples

Q461-01 Which of the changes below occurs with the greatest increase in entropy?

Na₂O(s) + H₂O(l) → 2Na+(aq) + 2OH-(aq)
NH₃(g) + HCl(g) → NH₄Cl(s)
H₂(g) + I₂(g) → 2HI(g)
C(s) + CO₂(g) → 2CO(g)

Answer

Entropy can be considered the degree of disorder of a chemical system. It is increased by the number of particles and their temperature. In this case it is important to examine the number of moles of free particles on both sides of the equation.

It may be seen that in equation D there are more moles of gas (maximum entropy) on the right hand side than on the left hand side. Thus the entropy increases from left to right. correct response

Although there are more free ions in A this is not as important in entropy terms as an increase in the number of moles of gas.

In equation B there is a large decrease in entropy (two gases make a solid) and in equation C the number of moles of gas on both sides is equal.

Q461-02 In which of the following reactions is the entropy change ( S) closest to zero

SO₂(g) + ½O₂(g) → SO₃(g)
Br₂(l) → Br₂(g)
H₂(g) + I₂(g) → 2HI(g)
3Ca(s) + N₂ → Ca₃N₂(s)

Answer

Equation A the moles of gas decreases from reactants to products, ΔS is negative.

Equation B the moles of gas increases from 0 to 1, ΔS is positive.

Equation C the moles of gas stays the same from reactants to products, ΔS = 0. correct response

Equation D the moles of gas decreases from 1 to 0, ΔS is negative.

Q461-03 Estimate, without doing a calculation, the magnitude of the entropy change for the following reaction.

Fe₂O₃(s) + 2Al(s)

2Fe(s) + Al₂O₃(s)

Answer

Examination of the equation reveals that the compounds on both sides of the equation are in the solid state. As solids have very low entropy it is safe to estimate that the entropy difference between reactants and products is negligible. Hence ΔS = 0.

Q461-04 Consider the following reaction:

N₂(g) + 3H₂(g) → 2NH₃(g)

The absolute entropy values, S, at 300K for N₂(g), H₂(g) and NH₃(g) are 193, 131 and 192 JK^-1 mol^-1 respectively. Calculate ΔS^o for the reaction and explain the sign of S^o.

Answer

On the left hand side there is one mole of nitrogen and three moles of hydrogen. Their entropy = 193 + (3 x 131) = 586 JK^-1

On the right hand side there are two moles of ammonia. Entropy = (2 x 192) = 384 JK^-1

The entropy change, ΔS^o, is 384 - 586 = -202 JK^-1

The negative sign indicates that the entropy has decreased from reactants to products.

Q461-05 Which reaction has the greatest positive entropy change?

CH₄(g) + 1½O₂(g) → CO(g) + 2H₂O(g)
CH₄(g) + 1½O₂(g) → CO(g) + 2H₂O(l)
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Answer

A positive entropy change means that the products have more entropy than the reactants. Gases have the largest entropy values, therefore we are looking for the reaction that produces the greatest positive change in moles of gas.

reaction 1 2½ moles gas → 3 moles of gas. An increase by ½ mole gas correct response

reaction 2 2½ moles gas → 1 mole of gas. A decrease of 1½ mole gas

reaction 3 3 moles gas → 3 moles of gas. No change in moles

reaction 4 3 moles gas → 1 mole of gas. A decrease of 2 moles of gas

Q461-06 Which reaction occurs with the largest increase in entropy?

Pb(NO₃)₂(s) + 2KI(s) → PbI₂(s) + 2KNO₃(s)
CaCO₃(s) → CaO(s) + CO₂(g)
3H₂(g) + N₂(g) → 2NH₃(g)
H₂(g) + I₂(g) → 2HI(g)

Answer

An increase in entropy change means that the products have more entropy than the reactants. Gases have the largest entropy values, therefore we are looking for the reaction that produces the greatest positive change in moles of gas.

reaction 1 0 moles gas → 0 moles of gas. No change in moles of gas

reaction 2 0 moles gas → 1 mole of gas. A increase of 1 mole of gas correct response

reaction 3 4 moles gas → 2 moles of gas. A decrease of 2 moles of gas

reaction 4 2 moles gas → 2 mole of gas. No change in moles of gas

Q461-07 Some chlorine gas is placed in a flask of fixed volume at room temperature. What change will cause a decrease in entropy?

Adding a small amount of hydrogen
Adding a small amount of chlorine
Cooling the flask
Exposing the flask to sunlight

Answer

Anything that increases the disorder, such as mixing two gases, or increasing the temperature, increases the entropy. The reverse is also tru. Hence decreasing the temperature decreases the entropy, e.g. Cooling the flask

Q461-08 Which reaction has the largest positive value of ΔS^o?

CO₂(g) + 3H₂(g) → CH₃OH(g) + H₂O(g)
2Al(s) + 3S(s) → Al₂S₃(s)
CH₄(g) + H₂O(g) → 3H₂(g) + CO(g)
2S(s) + 3O₂(g) → 2SO₃(g)

Answer

reaction 1 4 moles gas → 2 moles of gas. A decrease of 2 moles of gas, ΔS^o = negative

reaction 2 0 moles gas → 0 mole of gas. No change in moles of gas, ΔS^o = 0 (approx)

reaction 3 2 moles gas → 4 moles of gas. An increase of 2 moles of gas, ΔS^o = positive correct response

reaction 4 3 moles gas → 2 mole of gas. A decrease of 1 mole of gas, ΔS^o = negative

Q461-09 Which equation represents a change with a negative value for ΔS?

2H₂(g) + O₂(g) → 2H₂O(g)
H₂O(s) → H₂O(g)
H₂(g) + Cl₂(g) → 2HCl(g)
2NH₃(g) → N₂(g) + 3H₂(g)

Answer

A negative value for ΔS means that the products have less entropy than the reactants. There are fewer moles of gas in the products than in the reactants.

reaction 1 3 moles gas → 2 moles of gas. A decrease of 1 moles of gas, ΔS = negative correct response

reaction 2 0 moles gas → 1 mole of gas. An increase by 1 mole of gas, ΔS = positive

reaction 3 2 moles gas → 2 moles of gas. No change in moles of gas , ΔS = 0 (approx)

reaction 4 2 moles gas → 4 mole of gas. An increase by 2 moles of gas, ΔS = positive

Q461-10 Which change does not lead to an increase in entropy?

Mixing nitrogen and oxygen gases at room temperature
Cooling steam so that it condenses to water
Heating hexane to its boiling point
Dissolving sugar in water

Answer

Entropy is increased by:

Temperature increase
Increased number of free particles
Mixing

From the choices given, only cooling steam reduces the entropy of the system

^ top