A researcher observes that a molecule's UV–Vis absorption band becomes more intense (higher peak absorbance) after adding a reagent, but remains unchanged. Path length and concentration of the absorbing species are confirmed constant. Which interpretation is most consistent with molecular absorption?
- The transition probability (molar absorptivity) increased without changing the energy gap. (correct answer)
- The energy gap increased because absorbance increased at the same wavelength.
- The sample must be emitting light at , artificially raising absorbance.
- An unchanged proves no molecular interaction occurred.
Explanation: This question tests understanding of spectroscopy and molecular absorption (4D) in the MCAT Chemical & Physical Foundations of Biological Systems section. Molecular absorption involves the uptake of light energy at specific wavelengths, with intensity related to transition probability (molar absorptivity). In this scenario, the absorption band intensifies without shifting λ_max, indicating unchanged energy gap but altered absorptivity. Choice A is correct because it accurately reflects increased transition probability without changing the energy gap. Choice B is incorrect because it misinterprets intensity increase as energy gap change, leading to a common error when λ_max is constant. When evaluating absorption spectra, consider factors affecting peak height separately from position and ensure interpretations distinguish energy from probability effects.