H3 Chemistry 9813 · Study focus: H3 Chemistry: Construct and interpret pi-MO diagrams for benzene and polyenes
H3 Chemistry: Construct and interpret pi-MO diagrams for benzene and polyenes
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Construct and interpret pi-MO diagrams for benzene and polyenes
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Orientation and prerequisite retrieval
A conjugated π-MO diagram contains one level for each contributing p AO.
Electrons fill conjugated π levels from lowest energy upward.
Definitions and exact language
Butadiene's second π MO is its ground-state HOMO.
Butadiene's third π MO is its ground-state LUMO.
Benzene fills one lowest and a degenerate pair of bonding π MOs.
Benzene's lowest antibonding degenerate pair forms the LUMO level.
Text alternative: Text alternative: Each MO sketch must show phase at every contributing p centre. A useful diagram labels occupancy, degeneracy, HOMO, LUMO and qualitative phase pattern.
Detailed molecular explanation
Benzene fills one lowest and a degenerate pair of bonding π MOs.
Benzene's lowest antibonding degenerate pair forms the LUMO level.
Worked leaf-specific case
Each MO sketch must show phase at every contributing p centre.
Increasing node count corresponds to increasing energy in the qualitative ordering.
- A candidate writes, “A conjugated π diagram always has two levels.” Which correction should replace it?
Open the feedback checkpoint after attempting
- A conjugated π-MO diagram contains one level for each contributing p AO.
Guided evidence check
The HOMO–LUMO gap is read vertically between the frontier levels.
A π electron count is distinct from the total number of electrons in the molecule.
- Which labelled diagram, spectrum or calculation would most directly disprove “Butadiene's fourth π MO is its LUMO.”?
Open the feedback checkpoint after attempting
- Butadiene's third π MO is its ground-state LUMO.
Independent transfer
Hund's rule applies when a degenerate conjugated level is partly filled.
A useful diagram labels occupancy, degeneracy, HOMO, LUMO and qualitative phase pattern.
- Design a specific chemical check that would expose the error in “Phase need not be shown in a conjugated MO sketch.”
Open the feedback checkpoint after attempting
- Each MO sketch must show phase at every contributing p centre.
Misconception repair
Misconception: Hund's rule does not apply to degenerate π MOs.
Repair: Hund's rule applies when a degenerate conjugated level is partly filled.
- Write leaf-specific feedback for the misconception “Hund's rule does not apply to degenerate π MOs.”
Open the feedback checkpoint after attempting
- Hund's rule applies when a degenerate conjugated level is partly filled.
Practice, unseen assessment, re-test and next step
Complete the diagnostic questions, review the feedback, then attempt the final check.
After delayed re-test, continue to Relate electromagnetic radiation, photons and E = hf at /learning/h3-spectroscopy-photons-energy-lesson.html.
- Review the feedback for each diagnostic question, then attempt the final check.
Open the feedback checkpoint after attempting
- Before moving on, state this boundary: A useful diagram labels occupancy, degeneracy, HOMO, LUMO and qualitative phase pattern.