ChemistryChapter 12: Analytical Chemistry
UV-Visible Spectroscopy
UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs.
Learning objectives
- Explain uv-visible spectroscopy in clear and easy chemistry language.
- Use the important definitions, equations, examples, and conditions correctly.
- Apply the idea in an FBISE-style question and avoid the common exam mistake.
Definition
UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs. Electron types: σ, π, and n (nonbonding). Near-UV/Vis mainly shows n→π and π→π (σ→σ* needs vacuum UV <200 nm). Colour of transition-metal complexes and dyes links to incomplete d levels / conjugated systems absorbing part of white light; transmitted/reflected light is the colour we see (e.g. methylene blue). UV-Vis helps decide if a molecule absorbs in the region and to study complex colour.
Formula
colour = wavelengths not absorbed.
Important terms
- UV-VISIBLE SPECTROSCOPY
- UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs.
- ELECTRONIC JUMPS
- Energy order ideas: n→π* and π→π* dominate ordinary instruments.
- READING COLOUR
- Substance looks the complementary colour of light absorbed.
Relationship
- colour = light not absorbed
Overview
Transitions, then colour:
Electronic Jumps
Energy order ideas: n→π and π→π dominate ordinary instruments. Conjugation lowers gaps into visible.
Examples Carbonyl n→π* · Alkene π→π* · Vacuum UV for σ→σ* · Complex d–d colour
Reading Colour
Substance looks the complementary colour of light absorbed. If all visible absorbed → black; all reflected → white.
Examples 600–620 nm colour links · Spectrum plot absorbance vs nm · TM complex colour task · Why solution is blue
Core idea — UV-VISIBLE SPECTROSCOPY
UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs. Electron types: σ, π, and n (nonbonding). Near-UV/Vis mainly shows n→π and π→π (σ→σ* needs vacuum UV <200 nm). Colour of transition-metal complexes and dyes links to incomplete d levels / conjugated systems absorbing part of white light; transmitted/reflected light is the colour we see (e.g. methylene blue). UV-Vis helps decide if a molecule absorbs in the region and to study complex colour.
1. Electronic Jumps
Energy order ideas: n→π and π→π dominate ordinary instruments. Conjugation lowers gaps into visible. Examples: Carbonyl n→π; Alkene π→π; Vacuum UV for σ→σ*; Complex d–d colour.
2. Reading Colour
Substance looks the complementary colour of light absorbed. If all visible absorbed → black; all reflected → white. Examples: 600–620 nm colour links; Spectrum plot absorbance vs nm; TM complex colour task; Why solution is blue.
Worked method and exam use
UV-Vis: electronic transitions (mainly n→π, π→π). Seen colour = wavelengths not absorbed.
Explanation
UV-Vis: electronic transitions (mainly n→π, π→π). Seen colour = wavelengths not absorbed.
In simple words
State transition type when asked "why does this absorb UV/Vis?".
Key points
- electronic transitions
- n→π and π→π
- colour = light not absorbed
- HOMO → LUMO idea
- conjugated systems
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.UV-Visible Spectroscopy
Sticky Notes · Quick Revision
Core idea
- UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs. Electron types: σ, π, and n (nonbonding). Near-UV/Vis mainly shows n→π and π→π (σ→σ* needs vacuum UV <200 nm). Colour of transition-metal complexes and dyes links to incomplete d levels / conjugated systems absorbing part of white light; transmitted/reflected light is the colour we see (e.g. methylene blue). UV-Vis helps decide if a molecule absorbs in the region and to study complex colour.
Electronic Jumps
- Energy order ideas: n→π and π→π dominate ordinary instruments. Conjugation lowers gaps into visible.
Reading Colour
- Substance looks the complementary colour of light absorbed. If all visible absorbed → black; all reflected → white.
Worked method
- UV-Vis: electronic transitions (mainly n→π, π→π).
- Seen colour = wavelengths not absorbed.
Common misconception
- State transition type when asked "why does this absorb UV/Vis?".
FBISE exam check
- In an exam answer, include these key points: electronic transitions; n→π and π→π.
- Check: complementary colour.
60-second revision
- UV (200–400 nm) and visible (400–750 nm) absorption promotes loosely held electrons from occupied MOs to higher unoccupied MOs.
- Electronic Jumps: Energy order ideas: n→π and π→π dominate ordinary instruments.
- Reading Colour: Substance looks the complementary colour of light absorbed.
- Exam reminder: State transition type when asked "why does this absorb UV/Vis?".
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