lab_handout_Experiment 01

lab_handout_Experiment 01 - Revised May 2009 Chemistry 461...

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Revised May 2009 Chemistry 461 EXPERIMENT 1—SPECTROPHOTOMETRY AND CHEMICAL KINETICS Prepared by: Prof. T. Engel and Prof. C. T. Campbell I. HAZARDS Persons who are allergic to penicillin may observe an allergic reaction to the penicillin solutions; such persons may wish to wear gloves to prevent skin contact. II. WASTE DISPOSAL 1. Penicillin solutions—Wash down the drain. 2. All reaction products and excess 1 M HCl—Adjust pH to between 5.5 and 12 using sodium bicarbonate (check final pH with pH paper), then wash down the drain. III. THEORY Spectrophotometers. In this experiment, you will use an ultraviolet/visible spectrophotometer to study the kinetics of the hydrolysis of penicillin. The basis for the use of spectrophotometry in kinetic studies is that different molecules absorb radiation at different characteristic frequencies. If in a chemical reaction, these characteristic frequencies for the reactant, intermediates and products are sufficiently different, the evolution in time of the concentration of each of these species can be measured independently. Using this information, the kinetic rate constants and the reaction pathways can be determined. A wide range of spectrophotometers is used in chemical research, where the properties that being measured determine the appropriate one for the experiment. Figure 1 shows the electromagnetic spectrum and the types of transitions which result from absorption of radiation [1]. For many chemical reactions the outer electrons absorb in the ultraviolet/visible region of the electromagnetic spectrum. This is a convenient range of frequencies to work with and is the range in which your experiment will be carried out. You should be aware that since chemical reactions lead to structural and electronic configuration changes, nearly all portions of the electromagnetic spectrum can be used to monitor chemical changes.
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2 Figure 1. The Electromagnetic Spectrum Conventional ultraviolet/visible spectrophotometers disperse the light from a wide band source using prisms ( Figure 2a ) or gratings ( Figure 2b ) [1]. An exit slit restricts the range of wavelengths incident on the sample. By rotating the grating or prism relative to the exit slit, this wavelength can be changed. After the light passes through the sample, it falls on the detector which converts the light into an electrical signal. Figure 2a . Prism Dispersion Spectrophotometer
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3 Figure 2b . Grating Dispersion Spectrophotometer The spectrophotometer which you will use in this experiment is a more advanced type that utilizes a photodiode array in which a range of wavelengths can be detected simultaneously rather than individually as with conventional spectrophotometers. Its schematic layout is shown in Figure 3 [1]. Figure 3.
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lab_handout_Experiment 01 - Revised May 2009 Chemistry 461...

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