Relate polyatomic-gas IR absorption to the greenhouse effect

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.

GCE A-Level H3 Chemistry 9813-2027

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Learning objectives

Relate polyatomic-gas IR absorption to the greenhouse effect

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.

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.

Try this

Explain why CO₂ contributes vibrational IR absorption but N₂ does not.

Check your answer

A strong answer should include 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.

Try this

Write the cause-to-effect chain from matching photon to reduced net outgoing flux.

Check your answer

Include frequency match, vibrational excitation, re-emission in all directions and reduced net escape at that band.

Compare H₂O and Ar

Water vapour is bent H₂O; argon is monatomic Ar.

Apply the same test as for CO₂ and N₂: which vibrations exist, and does any of them change the dipole?

Try this

Explain the different IR roles of H₂O vapour and Ar.

Check your answer

H₂O has a bend and two stretches that each change its dipole, so it absorbs terrestrial IR. Ar is a single atom with no bond to vibrate, so it has no vibrational absorption; being a gas particle is not enough.

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.

Try this

Better reasoning: “CO₂ cannot be a greenhouse gas because it is non-polar.”

Check your answer

State that active CO₂ modes create oscillating dipoles and absorb terrestrial IR at fixed bands.

Evaluate gases from fixed mode data

After the check questions, explain the CH₄ case without notes. Return later for a different O₃ question.

Try this next: explain nuclear spin.

Try this

Use the fixed gas table to rank whether CO₂, H₂O, CHF3, N₂ and Ar absorb by vibrational IR.

Check your answer

A strong answer should include mode-specific dipole change and spectral overlap; reject blanket polyatomic/permanent-dipole rules.