H3 Chemistry 9813 · Study focus: H3 Chemistry: Predict IR absorption count and vibrations for simple molecules
H3 Chemistry: Predict IR absorption count and vibrations for simple molecules
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Predict IR absorption count and vibrations for simple molecules
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Count frequencies, not arrows
Linear CO2 has four normal-mode coordinates: symmetric stretch, asymmetric stretch and two perpendicular bends.
The two bends are degenerate and the symmetric stretch is IR inactive, so ideal CO2 gives two fundamental IR absorption frequencies.
Normal modes and IR-active absorptions
A non-linear molecule with N atoms has 3N−6 normal modes; a linear molecule has 3N−5. The syllabus task is to identify modes for supplied simple molecules, not merely quote the formula.
A mode produces an IR absorption only if it changes dipole moment; degenerate modes share one frequency.
CO2: symmetric stretch inactive; asymmetric stretch active near 2350 cm⁻¹; doubly degenerate bend active near 667 cm⁻¹. Therefore two distinct absorptions arise from three active coordinates.
Bent SO2 is non-linear and has three fundamentals—symmetric stretch, bend and asymmetric stretch—which are all IR active and non-degenerate.
| Molecule / geometry | Motion | Degeneracy | Dipole change | IR activity | Band / cm⁻¹ |
|---|---|---|---|---|---|
| CO₂ / linear 180° | symmetric stretch | non-degenerate | no change; remains zero | inactive | — |
| CO₂ / linear 180° | bend in/out of plane | doubly degenerate; one frequency | changes | active | ≈667 |
| CO₂ / linear 180° | asymmetric stretch | non-degenerate | changes | active | ≈2350 |
| SO₂ / bent | symmetric stretch | non-degenerate | changes | active | distinct |
| SO₂ / bent | bend | non-degenerate | changes | active | distinct |
| SO₂ / bent | asymmetric stretch | non-degenerate | changes | active | distinct |
Text alternative: Table headers explicitly state geometry, motion, degeneracy, dipole change, activity and approximate wavenumber; no colour-only coding.
CO2 compared with SO2
CO2: symmetric stretch inactive; asymmetric stretch active near 2350 cm⁻¹; doubly degenerate bend active near 667 cm⁻¹. Therefore two distinct absorptions arise from three active coordinates.
Bent SO2 is non-linear and has three fundamentals—symmetric stretch, bend and asymmetric stretch—which are all IR active and non-degenerate.
Account for CO2's two bands
Do not count the two perpendicular bends twice in a frequency list; label them as a degenerate pair.
Do not count the symmetric stretch because its dipole derivative is zero in ideal CO2.
- Predict the number and origins of ideal CO2 fundamental absorptions.
Open the feedback checkpoint after attempting
- Two: one degenerate bending frequency near 667 cm⁻¹ and one asymmetric-stretch frequency near 2350 cm⁻¹.
Build the SO2 mode table
For bent SO2, enter symmetric stretch, bend and asymmetric stretch as separate rows.
Mark all three IR active because each changes the dipole of the bent polar molecule.
- Complete the SO2 table with three modes and three distinct absorptions.
Open the feedback checkpoint after attempting
- Credit three: symmetric stretch, bend and asymmetric stretch; do not import CO2's inactive symmetric-stretch result.
Transfer to H2O and HCN
Bent H2O has three fundamentals: symmetric stretch, bend and asymmetric stretch, all IR active.
Linear HCN has four coordinates, including a degenerate bend; use symmetry and dipole change rather than atom count alone.
- Predict the fundamental mode pattern for H2O, then explain why linear HCN needs a degeneracy check.
Open the feedback checkpoint after attempting
- H2O gives three active frequencies; HCN's two perpendicular bends share a frequency while its unlike stretches are distinct.
Normal-mode count is not peak count
The 3N−5 or 3N−6 result counts coordinates, not necessarily observed absorption frequencies.
IR inactivity, degeneracy and coincident bands can reduce the number of distinct observed fundamentals.
- Repair: “CO2 has four normal modes, so it must show four IR bands.”
Open the feedback checkpoint after attempting
- State that the symmetric stretch is inactive and the two bends are degenerate, leaving two fundamental frequencies.
From motion inventory to spectral evidence
Complete twelve fixed mode-count checks, then the unseen CS2 assessment and different H2S re-test.
Next objective: identify functional-group bands at /learning/h3-infrared-functional-groups-lesson.html.
- Produce a mode/activity table for CO2 and SO2.
Open the feedback checkpoint after attempting
- Every row must specify geometry, motion, degeneracy and dipole-change activity; totals are CO2 two and SO2 three.