Definitions
| Term | Definition | Units |
|---|---|---|
| Specific energy | Energy released per unit mass of fuel | kJ g⁻¹ or MJ kg⁻¹ |
| Energy density | Energy released per unit volume of fuel | kJ cm⁻³ or MJ dm⁻³ |
🔑 Hydrogen Paradox
H₂ has the highest specific energy of any fuel (~142 MJ kg⁻¹) because it is so light, but an extremely low energy density because it is a gas at room temperature. Storing enough H₂ requires high-pressure tanks or cryogenic liquefaction.
The Greenhouse Effect. Molecular Mechanism
- Sun emits short-wavelength radiation (UV/visible) → passes through atmosphere
- Earth's surface absorbs it, warms up, re-emits long-wavelength infrared radiation
- Greenhouse gases (CO₂, CH₄, H₂O) absorb the IR because their bond vibrations (asymmetric stretch, bending) cause a temporary dipole change
- Absorbed energy is re-radiated in all directions. Some back to Earth → warming
⚠️ Examiner Trap
N₂ and O₂ are not greenhouse gases. They are non-polar and cannot change their dipole moment when they vibrate, so they do not absorb IR.
📘 Key Definitions
Specific energy = energy released per unit mass (kJ g-1)
Energy density = energy released per unit volume (kJ dm-3)
📐 Worked Example: Calculate the Specific Energy of Methane
Given: ΔHc = -890 kJ mol-1, Mr(CH4) = 16.05
Specific energy = |ΔHc| ÷ Mr = 890 ÷ 16.05 = 55.5 kJ g-1
Fuel Comparison
| Fuel | ΔHc (kJ mol-1) | Mr | Specific energy (kJ g-1) |
|---|---|---|---|
| Hydrogen | -286 | 2 | 143.0 |
| Methane | -890 | 16 | 55.6 |
| Octane | -5471 | 114 | 48.0 |
| Ethanol | -1367 | 46 | 29.7 |
Hydrogen has the highest specific energy, but its very low density means its energy density (kJ dm-3) is much lower, making storage a major challenge.
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