H3 Chemistry 9813 · Study focus: H3 Chemistry: Describe stretching vibrations
H3 Chemistry: Describe stretching vibrations
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Describe stretching vibrations
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
See bond length change
Compare three snapshots of linear CO2: both C=O bonds 116 pm, then both 119 pm, then both 113 pm.
The nuclei move along the bond axes; the O=C=O angle remains 180°, so this is stretching rather than bending.
Symmetric and asymmetric stretch
A stretching vibration periodically changes bond length. In a symmetric stretch equivalent bonds lengthen or shorten together.
In an asymmetric stretch one C=O bond lengthens while the other shortens; the changing dipole makes the CO2 asymmetric stretch IR active.
A vibration has quantised energy levels. Absorption occurs when the IR photon wavenumber matches the gap and the normal mode changes molecular dipole moment.
For ideal CO2 the symmetric stretch leaves zero net dipole and is IR inactive, whereas the asymmetric stretch near 2350 cm⁻¹ changes dipole and absorbs.
Text alternative: Alt text states each arrow direction, the changing bond lengths, the fixed 180° angle and which mode changes dipole.
Why an IR photon is absorbed
A vibration has quantised energy levels. Absorption occurs when the IR photon wavenumber matches the gap and the normal mode changes molecular dipole moment.
For ideal CO2 the symmetric stretch leaves zero net dipole and is IR inactive, whereas the asymmetric stretch near 2350 cm⁻¹ changes dipole and absorbs.
Classify two CO2 motions
Motion A changes both C=O lengths from 116 pm to 119 pm together. Motion B changes them to 119 pm and 113 pm.
A is symmetric stretching; B is asymmetric stretching and is the IR-active one.
- Name each motion and state which gives the strong CO2 band near 2350 cm⁻¹.
Open the feedback checkpoint after attempting
- Award symmetric stretch for A, asymmetric stretch for B, and the 2350 cm⁻¹ absorption to B because its dipole changes.
Track nuclei, length and angle
For H2O, draw arrows along both O–H bonds moving outwards together; label both bond lengths increasing.
Then draw one O–H increasing as the other decreases and keep the H–O–H angle approximately fixed.
- Classify the two arrow diagrams as symmetric or asymmetric stretching.
Open the feedback checkpoint after attempting
- The together motion is symmetric; the opposite bond-length changes are asymmetric. Neither diagram should be labelled bending because the angle is fixed.
Transfer to SO2
Bent SO2 has two S–O bonds and a permanent dipole; both symmetric and asymmetric stretches can change its dipole.
Use arrows and words, not peak count alone, to distinguish the two modes.
- Draw the two SO2 stretching modes and predict whether each can be IR active.
Open the feedback checkpoint after attempting
- Both modes are IR active for bent SO2; arrows must show together versus opposite S–O length changes.
A moving atom is not automatically a stretch
A vibration is classified by the internal coordinate that changes, not by whether atoms move.
Bond-axis motion changing length is stretching; transverse motion changing angle is bending.
- Repair: “Any vibration in which oxygen atoms move is a stretching vibration.”
Open the feedback checkpoint after attempting
- Reject it; name bond-length change as the criterion and contrast it with bond-angle change.
Check stretching before mode counting
Complete the twelve fixed stretching checks, then attempt the unseen HCN case and later the different N2O re-test.
Next objective: describe bending vibrations at /learning/h3-infrared-bending-lesson.html.
- Explain CO2 symmetric and asymmetric stretching using arrows, dipole change and one fixed band.
Open the feedback checkpoint after attempting
- A complete response identifies bond-length motion, distinguishes phase, and links the asymmetric stretch—not the symmetric stretch—to about 2350 cm⁻¹.