4.3.3Additional lessons
The Amount of Carbon Dioxide Produced When Fuels Burn — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 81 of 172
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 5Linking Fuels to CO₂ Emissions
Comparing fuels by emissions per unit energy shows which burns more cleanly.
Carbon content
- Carbon content Every carbon atom in a fuel becomes one molecule of CO₂ in complete combustion, so more carbon means more CO₂ per mole of fuel.
Mole ratio
- Mole ratio Burning one mole of C₈H₁₈ gives eight moles of CO₂, while one mole of CH₄ gives only one.
Energy basis
- Energy basis A fair comparison relates emissions to the useful energy gained, not just to the amount of fuel burned.
Hydrogen
- Hydrogen A hydrogen-rich fuel releases a larger share of its energy from forming water, giving less CO₂ per kJ.
Section 2 of 5Method for Calculations
1
Write a balanced equation for complete combustion and note the mole ratio of fuel to CO₂.
2
Find the moles of fuel from the mass, using n = m ÷ M, if the question gives a mass.
3
Multiply by the mole ratio to obtain moles of CO₂ and by 44.01 g mol⁻¹ to get its mass.
4
To compare per unit energy, divide the mass of CO₂ by the energy released, using ΔHc⦵ for the amount burned.
5
State the result with units such as g CO₂ per kJ and compare the fuels sensibly.
Section 3 of 5Interpreting the Results
Interpreting the Results
Natural gas
- Natural gas Methane gives about 0.049 g CO₂ per kJ of energy, lower than longer-chain hydrocarbons.
Liquid fuels
- Liquid fuels Octane gives about 0.064 g CO₂ per kJ, so petrol is a larger emitter per unit energy.
Wider picture
- Wider picture Extraction, transport and refining add further emissions beyond the burning stage.
Climate link
- Climate link Lower emissions per kJ reduce but do not remove the contribution to global warming.
Section 4 of 5Worked Example
Question
- Compare the mass of CO₂ per kJ for methane (ΔHc⦵ = −890 kJ mol⁻¹) and octane (ΔHc⦵ = −5470 kJ mol⁻¹).
- M(CO₂) = 44.01 g mol⁻¹.
- CH₄ + 2O₂ → CO₂ + 2H₂O: 1 mol CO₂ per 890 kJ, so 44.01 ÷ 890 = 0.0494 g kJ⁻¹.
- C₈H₁₈ + 12.5O₂ → 8CO₂ + 9H₂O: 8 mol CO₂ per 5470 kJ, so 8 × 44.01 ÷ 5470 = 0.0644 g kJ⁻¹.
- Methane gives less CO₂ for the same energy.
Answer
- Methane: 0.0494 g CO₂ per kJ;
- octane: 0.0644 g CO₂ per kJ
Section 5 of 5Exam Checklist & Tips
4.3.3 – Key Points for the Exam
- Mole ratio The ratio of amounts of substances in a balanced equation.
- Carbon footprint The total CO₂ equivalent released by an activity or product.
- Molar mass of CO₂ 44.01 g mol⁻¹, used to convert moles of CO₂ into grams.
- Emissions per kJ Mass of CO₂ released divided by the energy obtained from burning the fuel.
Common Mistakes
- Using the molar mass of the fuel when converting moles of CO₂ to mass.
- Comparing mass of CO₂ per mole of fuel rather than per unit energy.
- Forgetting the 8:1 ratio for octane and treating it as 1:1.
- Claiming that cleaner burning means no CO₂ is released.
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