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.
Study this topic on the go
Get flashcards and quizzes in ChemEasy, or plan your revision with ChemPlan IB.