H3 Chemistry 9813 · Study focus: H3 Chemistry: Relate polyatomic-gas IR absorption to the greenhouse effect
H3 Chemistry: Relate polyatomic-gas IR absorption to the greenhouse effect
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Relate polyatomic-gas IR absorption to the greenhouse effect
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Match terrestrial IR to molecular modes
Earth emits strongly in the infrared. CO2 absorbs near 667 and 2350 cm⁻¹ through bending and asymmetric stretching; H2O absorbs across rotational-vibrational regions.
Absorbed energy is re-emitted in all directions, reducing the net rate of outgoing IR at absorbing wavelengths.
IR-active greenhouse gas
A greenhouse gas has vibrational modes that absorb outgoing terrestrial IR by changing molecular dipole moment.
Polyatomic identity alone is insufficient: the mode frequency must overlap terrestrial radiation and the mode must be IR active.
Linear CO2 has no permanent dipole, yet its bend and asymmetric stretch create changing dipoles. N2 has only a homonuclear stretch that does not change dipole and is IR inactive.
H2O's bent polar geometry supplies multiple active modes; CHF3 has many active vibrations because its bond dipoles do not cancel through those motions.
| Species | Mode | Band / cm⁻¹ | Dipole during vibration | IR active? | Terrestrial-IR overlap |
|---|---|---|---|---|---|
| CO₂ | bend | 667 | changes | yes | yes |
| CO₂ | asymmetric stretch | 2350 | changes | yes | yes |
| CO₂ | symmetric stretch | — | zero at equilibrium and remains zero | no | none |
| H₂O | bend and stretches | multiple | changes | yes | yes |
| CHF₃ | multiple modes | multiple | changes | yes | yes |
| N₂ | homonuclear stretch | — | zero throughout | no | none |
| Ar | no molecular vibration | — | not applicable | no | none |
Text alternative: The table spells out every activity judgement and band numerically; a text flow describes absorption, vibrational excitation and re-emission without relying on arrows or colour.
Why CO2 works and N2 does not
Linear CO2 has no permanent dipole, yet its bend and asymmetric stretch create changing dipoles. N2 has only a homonuclear stretch that does not change dipole and is IR inactive.
H2O's bent polar geometry supplies multiple active modes; CHF3 has many active vibrations because its bond dipoles do not cancel through those motions.
Compare CO2 and N2
CO2's 667 cm⁻¹ bend overlaps outgoing IR and changes dipole. N2's vibration retains zero dipole.
Therefore equal atmospheric abundance would not imply equal greenhouse absorption.
- Explain why CO2 contributes vibrational IR absorption but N2 does not.
Open the feedback checkpoint after attempting
- Credit changing dipole in CO2 active modes, no dipole change for homonuclear N2, and absorption/re-emission of terrestrial IR.
Trace one photon pathway
An outgoing terrestrial IR photon matching a CO2 bend is absorbed, raising vibrational energy.
Subsequent emission is in random directions; some energy returns downward while collisions also redistribute energy.
- Write the cause-to-effect chain from matching photon to reduced net outgoing flux.
Open the feedback checkpoint after attempting
- Include frequency match, vibrational excitation, re-emission in all directions and reduced net escape at that band.
Compare H2O and Ar
H2O has angle/stretch modes that change dipole; monatomic Ar has no molecular vibration.
Do not claim all atmospheric gases absorb IR merely because they contain particles.
- Explain the different IR roles of H2O vapour and Ar.
Open the feedback checkpoint after attempting
- H2O absorbs through active vibrations; Ar lacks vibrational degrees of freedom and does not absorb by this mechanism.
Permanent dipole is not required
The selection rule concerns change in dipole during a mode, not only the equilibrium dipole.
CO2 is the counterexample: equilibrium dipole zero, bending/asymmetric-stretch dipole non-zero during motion.
- Repair: “CO2 cannot be a greenhouse gas because it is non-polar.”
Open the feedback checkpoint after attempting
- State that active CO2 modes create oscillating dipoles and absorb terrestrial IR at fixed bands.
Evaluate gases from fixed mode data
Complete twelve gas comparisons, then the unseen CH4 assessment and different O3 re-test.
Next objective: explain nuclear spin at /learning/h3-nmr-nuclear-spin-lesson.html.
- Use the fixed gas table to rank whether CO2, H2O, CHF3, N2 and Ar absorb by vibrational IR.
Open the feedback checkpoint after attempting
- Credit mode-specific dipole change and spectral overlap; reject blanket polyatomic/permanent-dipole rules.