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4.3.3Additional lessons

The Amount of Carbon Dioxide Produced When Fuels Burn — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 55 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 6Fuel and Carbon Dioxide

4.3.3 The Amount of Carbon Dioxide Produced When Fuels Burn

The carbon in fossil fuels ends up as carbon dioxide when it burns.

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
  • The balanced equation gives the ratio, for example one mole of propane gives three moles of carbon dioxide.
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 contains no carbon at all, so burning it releases no carbon dioxide.

Part 2 of 6Calculating 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.

Part 3 of 6Comparing Fuels

Comparing Fuels
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
  • Releases about 1.35 mol of CO₂ per MJ of energy, based on a combustion enthalpy of −2220 kJ mol⁻¹.
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
  • Fuels with a smaller ratio of carbon to hydrogen produce less CO₂ for a given quantity of useful energy.
FuelΔHc / kJ mol⁻¹mol CO₂ per mol fuelmol CO₂ per MJ
CH₄−89011.12
C₃H₈−222031.35
C₂H₅OH−136721.46

Part 4 of 6Energy Per Gram

Energy Per Gram
Specific energy
  • The energy released per gram is the combustion enthalpy divided by the molar mass, used to compare fuels by mass.
Propane example
  • 2220 kJ mol⁻¹ divided by 44.11 g mol⁻¹ gives about 50.3 kJ g⁻¹.
Choice of fuel
  • Transport fuels need high energy per gram or per volume, which helps explain the use of liquid hydrocarbons.
Trade-off
  • The fuel with the best energy density is not necessarily the one with the lowest carbon dioxide emissions.

Part 5 of 6Worked Example

Question

  • Calculate the mass of CO₂ produced when 100 g of propane burns completely.
  • (Mr of C₃H₈ = 44.11)

  1. C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l).
  2. Amount of propane = 100 ÷ 44.11 = 2.267 mol.
  3. Amount of CO₂ = 3 × 2.267 = 6.80 mol.
  4. Mass of CO₂ = 6.80 × 44.01 = 299 g.

Answer About 299 g of CO₂ (3 s.f.)

Part 6 of 6Exam 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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