When comparing fuels, we need standard measures. The IB uses two quantities:
Specific Energy
Energy released per unit mass.
\( \text{Specific energy} = \frac{\text{energy (kJ)}}{\text{mass (kg)}} \)
Units: kJ kg⁻¹ or MJ kg⁻¹
Energy Density
Energy released per unit volume.
\( \text{Energy density} = \frac{\text{energy (kJ)}}{\text{volume (dm}^3\text{)}} \)
Units: kJ dm⁻³ or MJ dm⁻³
Comparing Common Fuels
Specific Energy Comparison (MJ kg⁻¹)
Key Insight
Hydrogen has the highest specific energy (by mass) but the lowest energy density (by volume) because it is a gas at room temperature. This is a major challenge for hydrogen fuel storage.
Think About It
Why does ethanol have a lower specific energy than petrol, even though both are liquid fuels?
Ethanol (C₂H₅OH) already contains one C–O bond and one O–H bond, meaning it is partially oxidised. Less energy is released when it combusts compared to a pure hydrocarbon of similar mass.