Explain why carbon dioxide is a greenhouse gas but nitrogen (N₂) is not, with reference to molecular structure and the interaction of molecules with infrared radiation.

IB DP Physics Higher Level (2023 syllabus) — B.2 Greenhouse effect · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

CO₂ is a linear triatomic molecule with a permanent dipole moment that changes during its vibrational modes, making it IR‑active. When an IR photon has a frequency that matches one of CO₂’s vibrational frequencies, the changing dipole couples to the photon’s electric field and the molecule absorbs the energy. N₂ is a homonuclear diatomic molecule; its bond dipole is zero and the symmetric stretch does not alter the dipole moment, so it cannot absorb IR radiation.

After absorbing IR radiation, CO₂ re‑emits photons in all directions. Some of this re‑emitted radiation is directed back towards the Earth’s surface, increasing the surface temperature – the greenhouse effect.

N₂, lacking an IR‑active vibrational mode, does not absorb or re‑emit IR radiation and therefore does not contribute to the greenhouse effect.

Examiner tips

  • Mention CO₂’s changing dipole moment and IR‑activity; contrast with N₂’s zero dipole change.
  • Explain resonance absorption and re‑emission back to the surface.
  • Use correct terminology: IR‑active, vibrational mode, dipole moment, resonance absorption, greenhouse effect.

Common mistakes

  • Saying CO₂ has a permanent dipole moment (it has no permanent dipole but a changing one);
  • Claiming N₂ can absorb IR via other mechanisms;
  • Forgetting to link absorption to re‑emission and back‑to‑surface radiation.

Mark scheme (4 marks)

  1. CO₂ has a dipole moment that can change during certain vibrational modes (it is IR-active), whereas N₂ is a homonuclear diatomic molecule and its bond dipole moment cannot change during vibration.
  2. The changing dipole moment in CO₂ allows it to absorb photons of infrared radiation whose frequency matches the natural vibrational frequency of the molecule (resonance absorption).
  3. N₂ cannot absorb infrared radiation because its symmetric vibration produces no change in dipole moment, so there is no mechanism for the photon's oscillating electric field to couple to the molecule.
  4. After absorbing IR radiation, CO₂ molecules re-emit infrared radiation in all directions, including back towards Earth's surface, raising the surface temperature — this is the greenhouse effect.

Key terms in this question

greenhouse gas · infrared radiation

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