In the Laboratory
Graphical Interface for the Study of Gas-Phase Reaction Kinetics: Cyclopentene Vapor Pyrolysis†
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Ronald E. Marcotte* and Lenore D. Wilson Department of Chemistry, Texas A&M University–Kingsville, Kingsville, TX 78363; *
[email protected] The pyrolysis of gaseous cyclopentene as outlined by Shoemaker et al. (1) has been a traditional experiment in our physical chemistry laboratory for several reasons. It gives the students a hands-on introduction to gas-phase reaction kinetics and other potentially useful research techniques such as introducing samples into a vacuum system, handling liquid nitrogen, degassing samples by the freeze-pump-thaw method, and operating a laboratory high-vacuum system. However, the safety of the experiment is questionable owing to the potentially dangerous use of a mercury manometer in proximity to the 500 °C pyrolysis oven. In addition, the manometer’s lack of sensitivity requires longer runs with more opportunity for temperature fluctuations. Longer run times and oven adjustment times lead to the necessity of using more than one laboratory period to get a reasonably accurate measurement of the activation energy. We have made changes to the apparatus and procedure to improve safety, greatly increase speed and precision, and introduce methods that better reflect the current practice of physical chemistry. The experiment now features a modern, virtual-instrument-type computer interface to control the collection and display of experimental data. An electronic pressure gauge has replaced the mercury manometer. Students now have more data of higher quality available to them in far less time. Although the program could easily be designed to produce the final graphs needed and to calculate the rate constants, it was purposely designed to be passive, thus requiring the students to think through the problem of what to do with the data. The program is set up to easily store the data with labels in a format compatible with popular programs such as the Excel spreadsheet or the SigmaPlot graphing program. The operation of the high-vacuum system has been put in more conceptual terms rather than in cookbook style. This requires more interaction with the lab instructor, but it enriches the students by forcing them to a better understanding of the general principles involved in high vacuum work. Equipment The interface hardware is installed in a 120-MHz Pentiumbased computer operating Windows95. The analog-to-digital conversion is performed by a National Instruments AT-MIO16XE-50 multifunction A/D card featuring a 16-bit analog/ digital converter with 8 input channels. Many other similar cards would work here without any changes to the programs. Rather than connect the sensors directly to this card, a †
This paper was presented at the 215th National Meeting of the American Chemical Society, Dallas, TX, March 29, 1998, and the Second Annual Ronald E. McNair Scholars Program presentations, Texas A&M University–Kingsville, July 2, 1997.
Figure 1. Variation in the rate of pyrolysis of cyclopentene as a function of the temperature.
preamp/signal conditioner module was used on each input. We used the National Instruments Series 5B input modules, but many others could be substituted with the minor program changes discussed in the accompanying instructor’s notes.W Experience has shown that not only do these signal conditioners lower noise and increase sensitivity, but they also create a very robust interface especially suitable for student use. The modules have a much higher tolerance for overvoltage and reversed polarity and are much less expensive to replace than the A/D converter card. The user interface was created using the programming language LabVIEW 4.0. It was used to produce a graphical interface with the same look and feel as a physical data station with all its switches, digital readouts, and stripchart recorders. This data station includes scrolling displays of oven temperature and reaction pressure versus time. The advantages of a realistic-looking interface include its more intuitive operation, which requires less instruction time for new users and results in fewer errors in its use (2–4). Each data point displayed on the pressure-versus-time graph is the average of a batch of 100 individual readings made in rapid succession. Each batch value is displayed numerically and saved in memory. The standard deviation for each batch is also computed and stored for later use in error analysis by the students. The average oven temperature and its standard deviation are computed at the end of the run and saved along with the individual temperature readings. The middle line in Figure 1 had an overall average temperature and standard deviation of 508.3 ± 0.2 °C and a representative pressure taken at the midpoint of 52.88 ± 0.02 torr. All display units (Pa, torr, etc.) may be changed during a run without affecting the stored values, which are always torr. The data station also includes input windows for temperature and pressure calibration corrections as well as a variable time delay to set the sampling rate and a warning flasher/buzzer for oven temperatures that exceed the set maximum.
JChemEd.chem.wisc.edu • Vol. 78 No. 6 June 2001 • Journal of Chemical Education
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In the Laboratory
Temperatures are typically around 500 °C and are currently being measured by a chromel–alumel thermocouple attached to a National Instruments 5B37 thermocouple preamp. The preamp also contains a cold-junction compensation circuit, thus eliminating the traditional ice bath. Pressures are typically around 40–70 torr and are measured with an Omega Engineering model PX-212 pressure sensor. This particular model measures absolute pressures by sensing the strain applied by the gas on a stainless-steel diaphragm. It therefore reports pressures that are independent of the composition of the gasphase reaction mixture. Results and Discussion The interface has been tested with physical chemistry lab students. Some student data are shown in Figure 1. The regression lines show the excellent linearity and low scatter in the data points. All three curves were obtained in one lab period, a feat not possible with the methods used in previous semesters (1). The improvement in the quality of the data is directly attributable to the high resolution and speed of the computer interface. The middle curve in Figure 1 was taken at 508.3 °C and shows 4200 data points taken over only 3.4 minutes for a total pressure change of less than 1.8 torr. Each plotted point is the average of 100 readings taken in rapid succession and averaged to reduce random error. The rate constant at 508.3 °C was found to be 1.6 × 10᎑4 s᎑1 and is bracketed by literature values ranging from 0.52 to 3.9 in units of 10᎑4 s᎑1 (5). To obtain an activation energy, the oven temperature must be changed and restabilized several times during the lab period. This was tedious in the past, but is now made very much easier by the computer interface. Although it does not yet control the temperature, the scrolling plot of the oven temperatures gives rapid, high-resolution feedback highlighting the effect of subtle changes in the oven powerstat. In addition, the short run times that are now possible minimize the impact of residual temperature drift during a run. The lack of temperature drift can be seen in the high linearity of the plots in Figure 1, all of which have correlation coefficients (r) of about .9999. The rate constants obtained from the slopes of the curves in Figure 1 were used to construct the Arrhenius plot in Figure 2. The slope of this curve was taken to obtain the Arrhenius activation energy, E a , of 266 kJ/mol. Literature values vary from 242 to 255 kJ/mol (5) and our value of 266 kJ/mol passes the Q test for inclusion in this set at the 90% confidence level. Thus the students were able to use one lab period and do good experimental work, obtaining results that compared well with published results—a combination we rarely seemed to be able to get in the past. (Students actually take a total of two lab periods, but the additional time is spent gaining a
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Figure 2. Arrhenius activation energy plot using the rate constants derived from Figure 1.
greater appreciation of the computer interface and a more detailed exposure to high vacuum techniques.) Hazards A short lecture on safety around a high vacuum system is generally given, emphasizing the use of full face shields, because most students seem to be unfamiliar with the dangers associated with the implosion of glassware under vacuum. Cyclopentene (CAS #142-29-0) is an eye, skin, and respiratory tract irritant. It is very flammable and should be used in a well-ventilated area. WSupplemental
Material
Notes for the instructor and detailed laboratory instructions for students are available in this issue of JCE Online. Acknowledgments We wish to acknowledge the generous support from he Ronald E. McNair Scholars Program and the Robert A. Welch Foundation. Literature Cited 1. Shoemaker, D. P.; Garland, C. W.; Nibler, J. W. Experiments in Physical Chemistry, 6th ed.; McGraw-Hill: New York, 1996; experiment 24, based on the original research by Vanas, D. W.; Walters, W. D. J. Am Chem. Soc. 1948, 70, 4093. 2. Gotowski, R. J. Chem. Educ. 1996, 73, 1103. 3. Muyskens, M. A.; Glass, S. V.; Wietsma, T. W.; Gray, T. M. J. Chem. Educ. 1996, 73, 1112. 4. Ogren, P. J.; Jones, T. J. J. Chem. Educ. 1996, 73, 1115. 5. King, K. D. Int. J. Chem. Kinet. 1978, 10, 117-23.
Journal of Chemical Education • Vol. 78 No. 6 June 2001 • JChemEd.chem.wisc.edu
