Chemical Education Today
NSF Highlights Projects Supported by the NSF Division of Undergraduate Education
Introduction of Laser Photolysis—Transient Spectroscopy in an Undergraduate Physical Chemistry Laboratory: Kinetics of Ozone Formation by Lev N. Krasnoperov * and Victor Stepanov
A new undergraduate physical chemistry laboratory was developed as part of an ongoing effort to modernize the physical chemistry curriculum in the Department of Chemical Engineering, Chemistry, and Environmental Science at the New Jersey Institute of Technology. The main goal of this new laboratory experiment is to introduce a modern experimental technique in studying fast, elementary chemical reactions. The technique combines pulsed-laser photolysis (to generate highly reactive intermediates such as atoms and free radicals) and transient UV–Vis absorption spectroscopy (to monitor the kinetics of reactant depletion and product formation). Currently, there is a growing interest in the use of lasers in the educational process, both in chemical and physical undergraduate laboratories (1–10 ). Examples include experiments in spectroscopy (4 –7 ), light beam properties (8), and other related topics (1, 9–10). The range of lasers used spans from He–Ne lasers (4 ) to nitrogen and tunable dye lasers (6, 9). Pulsed-UV-laser photolysis, described below, is currently used in chemical kinetics research for selective production of transient species, such as free radicals and excited states. The new physical chemistry lab experiment developed here introduces the technique of pulsed-laser photolysis for kinetic measurements and exemplifies the approach through the study of the reaction of ozone formation in the gas phase. An extension of the experiment to reactions in solutions is planned for the future. Traditional experiments in chemical kinetics usually
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employ relatively slow (and often complex) chemical reactions and are mainly designed for the determination of the apparent kinetic parameters, such as reaction order and activation energy. A low reaction rate is required to allow convenient reactant mixing and monitoring. However, many chemical reactions of practical interest (e.g., combustion, atmospheric chemistry, and chemical vapor deposition, etc.) consist of a great number of fast, elementary reactions. Quite different experimental techniques are used to study such fast reactions. These techniques are not adequately represented in the traditional physical chemistry laboratory. The main reasons for choosing the ozone formation reaction are: the reaction of oxygen atoms with oxygen molecules is the major source of stratospheric ozone (and therefore has been extensively studied); and the reactants required (oxygen–nitrogen mixtures) are safe and inexpensive. A simplified version of the experiment can be performed without a reactor, using ambient air as the reaction mixture. Oxygen atoms are produced via molecular oxygen photolysis by an Ar–F excimer laser at 193 nm in an atmosphericpressure flow reactor: O2 + hν (193 nm) → 2 O
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Oxygen atoms formed during the laser pulse (ca. 10 nsec) undergo fast reactions with molecular oxygen to form ozone: O + O2 (+ M) → O3 (+ M)
Journal of Chemical Education • Vol. 76 No. 9 September 1999 • JChemEd.chem.wisc.edu
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Chemical Education Today edited by Susan H. Hixson National Science Foundation Arlington, VA 22230
Richard F. Jones Sinclair Community College Dayton, OH 45402-1460
as the photochemical activation via photodissociation of molecules with subsequent formation of reactive species (e.g., atoms and free radicals). The application of spectroscopic techniques for sensitive monitoring of transient species in low concentration is demonstrated. Beyond this, the students acquire experience in handling compressed gases and gas mixtures, gas flows, optics, spectroscopy, digital data acquisition, signal/noise improvement and signal averaging, and data processing using nonlinear curve-fitting. The course evaluations and the feedback sessions revealed the students’ very positive acceptance of the experiment. They were enthusiastic about learning a modern technique and getting experience with modern instrumentation. Acknowledgment This work was supported by a grant from the National Science Foundation Division of Undergraduate Education: grant DUE-97 51686. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Hixson, S. H.; Sears, C. T., Jr. J. Chem. Educ. 1995, 72, A214. Parks, J. F.; Feigerle, C. S.; Wiest, J. J. Laser Applic. 1994, 6, 115. Charschan, S. J. Laser Applic. 1996, 8, 5. Muenter, J. S. J. Chem. Educ. 1996, 73, 576. O’Brien, L. C.; Kubicek, R. L. J. Chem. Educ. 1996, 73, 86. Muenter, J. S.; Deutch, J. L. J. Chem. Educ. 1996, 73, 580. DeGraff, B. A.; Horner, D. A. J. Chem. Educ. 1996, 73, 279. Young, P. A. Phys. Educ. 1989, 24, 169. Hair, S. J. Chem. Educ. 1996, 73, A7. Williams, G. J. Laser Applic. 1995, 7, 229.
Lev N. Krasnoperov is in the Department of Chemical Engineering, Chemistry, and Environmental Science, New Jersey Institute of Technology, University Heights, Newark, NJ 07102. Victor Stepanov is an undergraduate student.
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The reaction time at 298 K in air is approximately 15 µsec. The ozone buildup is monitored via its transient absorption at 253.7 nm (mercury line, approximately the maximum of ozone absorption band in UV). The absorption cross section of ozone at this wavelength is 1.15x10217 cm2. Mixtures of oxygen and nitrogen with three different compositions (5, 10, and 21% oxygen) are used to record the kinetics of ozone formation and to determine the rate constant of reaction 2. A very small fraction of oxygen molecules (approximately 1.531026) dissociates after a laser pulse, which ensures the pseudo-first-order conditions. The concentration of ozone formed is small, ca. 0.3 ppm for a mixture of 10% oxygen using 10 mJ of pulse energy, resulting in rather weak absorption of the monitoring light (0.05–0.2% for a 10–20-cm reactor). Therefore, the signal is quite small and requires signal accumulation. The accumulation (digital acquisition and summation) of about 500–2000 individual kinetic curves is used to achieve acceptable signal/noise ratios. At a given concentration of molecular oxygen, the pseudo-first-order rate constant, k9, of reaction 2 is determined from the (exponential) temporal profile of ozone formation. The experiment is performed with several oxygen– nitrogen mixtures of different compositions. From a linear plot of k9 vs. [O2], the rate constant of reaction 2 is determined. From this physical chemistry lab experiment, students become familiar with an experimental technique of modern chemical kinetics—laser photolysis combined with transient spectroscopy. Students learn the basic principles of chemical kinetics—such as reaction rate, reaction order, reaction rate constant, isolation of elementary reactions and pseudo-firstorder conditions, and the integrated law. The experiment illustrates the main route of ozone formation in the stratosphere via photolysis of oxygen by far-UV radiation as well
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JChemEd.chem.wisc.edu • Vol. 76 No. 9 September 1999 • Journal of Chemical Education
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