4.3.3Additional lessons
The Amount of Carbon Dioxide Produced When Fuels Burn — IB Chemistry SL Revision Notes & Topical Questions
5 min read 12 parts Lesson 55 of 98
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.
Section 1 of 12Fuel and Carbon Dioxide
The carbon in fossil fuels ends up as carbon dioxide when it burns.
Carbon content
- Carbon content Every carbon atom in a fuel that burns completely becomes one molecule of CO₂, so more carbon atoms mean more CO₂ per molecule.
Mole ratio
- Mole ratio The balanced equation gives the ratio, for example one mole of propane gives three moles of carbon dioxide.
Hydrogen
- Hydrogen The hydrogen in a fuel forms water rather than CO₂, so fuels rich in hydrogen release less CO₂ for the same energy.
Hydrogen as fuel
- Hydrogen as fuel Hydrogen contains no carbon at all, so burning it releases no carbon dioxide.
Section 2 of 12Calculating the Mass of CO₂
1
Write the balanced equation for complete combustion and identify the mole ratio of fuel to CO₂.
2
Convert the mass of fuel to moles by dividing by its molar mass.
3
Multiply by the mole ratio to obtain the moles of CO₂.
4
Multiply by the molar mass of CO₂ (44.01 g mol⁻¹) to obtain the mass of CO₂ released.
5
Compare fuels by dividing by the energy released, using the enthalpy of combustion.
Section 3 of 12Comparing Fuels
Comparing Fuels
Methane
- Methane Releases about 1.12 mol of CO₂ per MJ of energy,
- which is lower than larger hydrocarbons,
- so natural gas is a relatively low-carbon fossil fuel.
Propane
- Propane Releases about 1.35 mol of CO₂ per MJ of energy, based on a combustion enthalpy of −2220 kJ mol⁻¹.
Ethanol
- Ethanol Releases about 1.46 mol of CO₂ per MJ, based on −1367 kJ mol⁻¹, but ethanol from biomass takes up CO₂ as it grows.
Reasoning
- Reasoning Fuels with a smaller ratio of carbon to hydrogen produce less CO₂ for a given quantity of useful energy.
| Fuel | ΔHc / kJ mol⁻¹ | mol CO₂ per mol fuel | mol CO₂ per MJ |
|---|---|---|---|
| CH₄ | −890 | 1 | 1.12 |
| C₃H₈ | −2220 | 3 | 1.35 |
| C₂H₅OH | −1367 | 2 | 1.46 |
Section 4 of 12Energy Per Gram
Energy Per Gram
Specific energy
- Specific energy The energy released per gram is the combustion enthalpy divided by the molar mass, used to compare fuels by mass.
Propane example
- Propane example 2220 kJ mol⁻¹ divided by 44.11 g mol⁻¹ gives about 50.3 kJ g⁻¹.
Choice of fuel
- Choice of fuel Transport fuels need high energy per gram or per volume, which helps explain the use of liquid hydrocarbons.
Trade-off
- Trade-off The fuel with the best energy density is not necessarily the one with the lowest carbon dioxide emissions.
Section 5 of 12Worked Example
Question
- Calculate the mass of CO₂ produced when 100 g of propane burns completely.
- (Mr of C₃H₈ = 44.11)
- C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).
- Amount of propane = 100 ÷ 44.11 = 2.267 mol.
- Amount of CO₂ = 3 × 2.267 = 6.80 mol.
- Mass of CO₂ = 6.80 × 44.01 = 299 g.
Answer About 299 g of CO₂ (3 s.f.)
Section 6 of 12Exam Checklist & Tips
4.3.3 – Key Points for the Exam
- Complete combustion Burning with enough oxygen for all carbon to form CO₂ and all hydrogen to form H₂O.
- Mole ratio The ratio of amounts of two substances in a balanced equation.
- Specific energy The energy released per gram of fuel burnt.
- Fossil fuel A fuel formed over millions of years from the remains of living organisms, such as coal, oil or natural gas.
Common Mistakes
- Forgetting to multiply by the mole ratio from the equation.
- Using the molar mass of the fuel for the product instead of the molar mass of CO₂.
- Comparing fuels only per mole, when per gram or per MJ is more meaningful.
- Believing that burning hydrogen produces CO₂.
Section 7 of 12Fuel and Carbon Dioxide
The carbon in fossil fuels ends up as carbon dioxide when it burns.
Carbon content
- Carbon content Every carbon atom in a fuel that burns completely becomes one molecule of CO₂, so more carbon atoms mean more CO₂ per molecule.
Mole ratio
- Mole ratio The balanced equation gives the ratio, for example one mole of propane gives three moles of carbon dioxide.
Hydrogen
- Hydrogen The hydrogen in a fuel forms water rather than CO₂, so fuels rich in hydrogen release less CO₂ for the same energy.
Hydrogen as fuel
- Hydrogen as fuel Hydrogen contains no carbon at all, so burning it releases no carbon dioxide.
Section 8 of 12Calculating the Mass of CO₂
1
Write the balanced equation for complete combustion and identify the mole ratio of fuel to CO₂.
2
Convert the mass of fuel to moles by dividing by its molar mass.
3
Multiply by the mole ratio to obtain the moles of CO₂.
4
Multiply by the molar mass of CO₂ (44.01 g mol⁻¹) to obtain the mass of CO₂ released.
5
Compare fuels by dividing by the energy released, using the enthalpy of combustion.
Section 9 of 12Comparing Fuels
Comparing Fuels
Methane
- Methane Releases about 1.12 mol of CO₂ per MJ of energy,
- which is lower than larger hydrocarbons,
- so natural gas is a relatively low-carbon fossil fuel.
Propane
- Propane Releases about 1.35 mol of CO₂ per MJ of energy, based on a combustion enthalpy of −2220 kJ mol⁻¹.
Ethanol
- Ethanol Releases about 1.46 mol of CO₂ per MJ, based on −1367 kJ mol⁻¹, but ethanol from biomass takes up CO₂ as it grows.
Reasoning
- Reasoning Fuels with a smaller ratio of carbon to hydrogen produce less CO₂ for a given quantity of useful energy.
| Fuel | ΔHc / kJ mol⁻¹ | mol CO₂ per mol fuel | mol CO₂ per MJ |
|---|---|---|---|
| CH₄ | −890 | 1 | 1.12 |
| C₃H₈ | −2220 | 3 | 1.35 |
| C₂H₅OH | −1367 | 2 | 1.46 |
Section 10 of 12Energy Per Gram
Energy Per Gram
Specific energy
- Specific energy The energy released per gram is the combustion enthalpy divided by the molar mass, used to compare fuels by mass.
Propane example
- Propane example 2220 kJ mol⁻¹ divided by 44.11 g mol⁻¹ gives about 50.3 kJ g⁻¹.
Choice of fuel
- Choice of fuel Transport fuels need high energy per gram or per volume, which helps explain the use of liquid hydrocarbons.
Trade-off
- Trade-off The fuel with the best energy density is not necessarily the one with the lowest carbon dioxide emissions.
Section 11 of 12Worked Example
Question
- Calculate the mass of CO₂ produced when 100 g of propane burns completely.
- (Mr of C₃H₈ = 44.11)
- C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).
- Amount of propane = 100 ÷ 44.11 = 2.267 mol.
- Amount of CO₂ = 3 × 2.267 = 6.80 mol.
- Mass of CO₂ = 6.80 × 44.01 = 299 g.
Answer About 299 g of CO₂ (3 s.f.)
Section 12 of 12Exam Checklist & Tips
4.3.3 – Key Points for the Exam
- Complete combustion Burning with enough oxygen for all carbon to form CO₂ and all hydrogen to form H₂O.
- Mole ratio The ratio of amounts of two substances in a balanced equation.
- Specific energy The energy released per gram of fuel burnt.
- Fossil fuel A fuel formed over millions of years from the remains of living organisms, such as coal, oil or natural gas.
Common Mistakes
- Forgetting to multiply by the mole ratio from the equation.
- Using the molar mass of the fuel for the product instead of the molar mass of CO₂.
- Comparing fuels only per mole, when per gram or per MJ is more meaningful.
- Believing that burning hydrogen produces CO₂.
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