Fall 09. Exam3 key

Fall 09. Exam3 key - CHEMISTRY 2101 Fall 2009 Dr. Philippe...

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CHEMISTRY 2101 Name___________________________ Fall 2009 Dr. Philippe Buhlmann Hour Exam III December 7, 2009 Page 2 (1) (30 points in total) Spectroscopy (a) (5 points) Which wavelengths are larger, the ones at which a molecule absorbs light or the ones at which a molecule fluoresces? Explain your answer. Fluorescence occurs at longer wavelengths than absorption. Absorption typically excites a molecule from the lowest vibronic level of the electronic ground state into an electronically and rotationally excited state. Relaxation into the (electronically still excited) vibronic ground state is fast and represents a loss in energy that later is missing when fluorescence occurs. Fluorescence occurs by transition from the vibronic ground state/electronically excited state into different vibronic levels of the ground state. (b) (5 points) The slit in a UV/Vis spectrophotometer: What are the advantages and disadvantages of a large slit width? Advantage: Sensitive measurement due to large amount of light (large photon flux) reaching the detector. Disadvantage: Results in low resolution. (c) (5 points) What is the solvatochromic effect? What is the reason for the solvatochromic effect? Solvatochromic effect: The absorbance spectrum depends on the solvent. The solvatochromic effects results from a different extent of solvation of the electronic ground state and the electronically excited state(s). {Stronger solvation of the electronically excited state results in a shift to larger wavelengths, and stronger solvation of the electronic ground state results in a shift to shorter wavelengths.}
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CHEMISTRY 2101 Name___________________________ Fall 2009 Dr. Philippe Buhlmann Hour Exam III December 7, 2009 Page 3 (d) (5 points) Why does a typical UV/Vis spectrophotometer have two different types of light sources? What are those two light sources? Explain in your answer the role of each light source.
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This note was uploaded on 10/02/2011 for the course CHEM 2101 taught by Professor Bulman during the Spring '11 term at Minnesota.

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Fall 09. Exam3 key - CHEMISTRY 2101 Fall 2009 Dr. Philippe...

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