H3 Chemistry 9813 · Study focus: H3 Chemistry: Predict UV/visible absorption from a chromophore
H3 Chemistry: Predict UV/visible absorption from a chromophore
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Predict UV/visible absorption from a chromophore
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Orientation and prerequisite retrieval
A C=C bond supplies a π chromophore that commonly absorbs in the UV.
A carbonyl supplies π and nonbonding electrons for UV transitions.
Definitions and exact language
Conjugated multiple bonds support lower-energy absorption than isolated analogues.
Ethene is expected to absorb at longer wavelength than ethane's σ→σ* region.
A lone pair alone does not guarantee visible absorption; an accessible accepting orbital is needed.
A visibly colourless substance may still absorb ultraviolet radiation.
Text alternative: Text alternative: Inspection should locate double bonds, aromatic rings, carbonyls and lone-pair chromophores. A defensible comparison names the differing chromophore and transition rather than using colour alone.
Detailed molecular explanation
A lone pair alone does not guarantee visible absorption; an accessible accepting orbital is needed.
A visibly colourless substance may still absorb ultraviolet radiation.
Worked leaf-specific case
Inspection should locate double bonds, aromatic rings, carbonyls and lone-pair chromophores.
Greater structural conjugation can move an absorption toward the visible region.
- A candidate writes, “A C=C bond cannot absorb ultraviolet radiation.” Which correction should replace it?
Open the feedback checkpoint after attempting
- A C=C bond supplies a π chromophore that commonly absorbs in the UV.
Guided evidence check
An isolated alkane generally requires high-energy far-UV σ→σ* excitation.
Prediction concerns whether an electronic transition is plausible, not a unique exact λmax without data.
- Which labelled diagram, spectrum or calculation would most directly disprove “Ethane must absorb at longer wavelength than ethene.”?
Open the feedback checkpoint after attempting
- Ethene is expected to absorb at longer wavelength than ethane's σ→σ* region.
Independent transfer
Breaking conjugation can increase the relevant energy gap and shorten absorption wavelength.
A defensible comparison names the differing chromophore and transition rather than using colour alone.
- Design a specific chemical check that would expose the error in “Molecular structure need not be inspected before predicting absorption.”
Open the feedback checkpoint after attempting
- Inspection should locate double bonds, aromatic rings, carbonyls and lone-pair chromophores.
Misconception repair
Misconception: Breaking conjugation always lengthens the absorption wavelength.
Repair: Breaking conjugation can increase the relevant energy gap and shorten absorption wavelength.
- Write leaf-specific feedback for the misconception “Breaking conjugation always lengthens the absorption wavelength.”
Open the feedback checkpoint after attempting
- Breaking conjugation can increase the relevant energy gap and shorten absorption wavelength.
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 Explain conjugation, energy gaps and longer-wavelength absorption at /learning/h3-uv-visible-conjugation-red-shift-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 defensible comparison names the differing chromophore and transition rather than using colour alone.