IB Chemistry›Reactivity 1›R1.3›R1.3.2

Energy Density & Specific Energy

Comparing fuels by mass and volume. And the molecular greenhouse mechanism.

Reactivity 1.3 SL & HL ⏱️ ~5 min revision

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
Fuel Comparison

The Hydrogen Energy Density Paradox

H₂ has the highest specific energy of any fuel (~142 MJ kg⁻¹) because of its minimal molar mass, but an extremely low energy density at ambient conditions because it is a low-density gas. Practical hydrogen storage requires heavy high-pressure tanks (700 bar) or cryogenic cooling (−253 °C).

The Greenhouse Effect. Molecular Mechanism

Mechanism

The Molecular Greenhouse Mechanism

  1. Sun emits short-wavelength radiation (UV and visible light) that passes through the atmosphere.
  2. Earth's surface absorbs solar radiation, warms up, and re-emits long-wavelength infrared (IR) radiation.
  3. Greenhouse gases (CO₂, CH₄, H₂O) absorb IR radiation because their asymmetrical vibrations and bending modes create a fluctuating dipole moment.
  4. The absorbed energy is re-radiated in all directions, including back toward Earth's surface, trapping heat in the troposphere.
Examiner Trap

Why N₂ and O₂ Are Not Greenhouse Gases

Diatomic homonuclear molecules like N₂ and O₂ are not greenhouse gases. Because both atoms share identical electronegativity, symmetric stretching produces no change in dipole moment, so they cannot absorb infrared radiation.

Core Definitions

Specific Energy vs Energy Density

  • Specific energy: Energy released per unit mass of fuel (\( ext{kJ g}^{-1}\) or \( ext{MJ kg}^{-1}\)).
  • Energy density: Energy released per unit volume of fuel (\( ext{kJ dm}^{-3}\) or \( ext{MJ m}^{-3}\)).
Worked Example

Calculating the Specific Energy of Methane

Given: ΔHc = −890 kJ mol-1, \(M_{\text{r}}(\text{CH}_4) = 16.05\text{ g mol}^{-1}\)

Specific energy:

\(\text{Specific energy} = \dfrac{|\Delta H_{\text{c}}|}{M_{\text{r}}} = \dfrac{890}{16.05} = 55.5\text{ kJ g}^{-1}\)

Fuel Comparison

Fuel ΔHc (kJ mol-1) Mr Specific energy (kJ g-1)
Hydrogen-2862143.0
Methane-8901655.6
Octane-547111448.0
Ethanol-13674629.7

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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