1.3.3—Fossil fuels
- Syllabus
- First assessment 2025
- Objective
- 1.3.3
- Level
- SL
Comparing Fossil Fuels
Compare coal, crude oil and natural gas using carbon dioxide released per unit of useful energy, energy output, and pollutants such as CO, particulates and volatile organic compounds from incomplete combustion.
A lower carbon footprint is a specific comparison, not a blanket claim that a fuel causes no pollution. Include methane leakage when evaluating natural gas.
Compare fuels on a common service basis such as kilograms of CO₂ per megajoule of useful energy, not per mole of fuel. Natural gas can have a lower combustion CO₂ intensity than coal yet lose part of that advantage through upstream methane leakage; define the system boundary before ranking.
Greenhouse link: atmospheric CO₂ absorbs selected outgoing infrared wavelengths and re-emits energy in multiple directions, reducing net energy loss to space at those wavelengths. A sustained increase in CO₂ changes Earth's energy balance until a warmer state restores balance. This mechanism is separate from comparing absolute fuel emissions or emissions per unit useful energy.
Evaluating Fossil-Fuel Choices
Assessment in practice
Representative question
Question 1
[Maximum number: 2]
Outline why the combustion of methane has a lower environmental impact than gasoline.
methane produces less CO2 per kJ of energy
methane produces fewer pollutants/particulates/CO/VOCs
For M1 do not accept simply methane produces less CO2.
For M2 accept gasoline undergoes incomplete combustion.
Combustion and Thermodynamics Summary
Retrieve the route: identify complete or incomplete combustion products, compare fuels and biofuels, balance fuel-cell half-equations, calculate ΔS° and ΔG°, then use ΔG, Q and K to reason about spontaneity and equilibrium.
Check products before balancing, evidence before evaluation, oxidation versus reduction, kelvin and unit consistency, the sign of ΔG, and whether Q is below, equal to, or above K.