In the Laboratory
Monitoring the Rate of Solvolytic Decomposition of Benzenediazonium Tetrafluoroborate in Aqueous Media Using a pH Electrode
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Floyd L. Wiseman† Department of Chemistry and Life Science, United States Military Academy, West Point, NY 10996;
[email protected] This laboratory experiment uses pH measurements of an aqueous solution to monitor the course of a solvolytic reaction. Generally, in undergraduate laboratories reaction-rate measurements are monitored by spectral techniques. This experiment offers an approach that is seldom used for monitoring the rate of a reaction. This experiment is appropriate for a physical chemistry laboratory course. Kinetic studies of the solvolytic decomposition of the benzenediazonium ion using spectral techniques have been published (1, 2). However, the solvolytic decomposition of the benzenediazonium ion generates nitrogen gas, which creates bubbles in the reaction solution that alter the solution path length when using a spectral technique. Attempting to continuously free the solution of the bubbles as they form is a difficult task, particularly for undergraduate students. The use of a pH electrode to monitor the reaction offers a distinct advantage because bubbles do not interfere significantly with the signal from a pH electrode. This experiment is unique in that the solution pH, rather than a spectral signal, is used to track the extent of decomposition. Laboratory Equipment and Chemicals • Fast-responding pH electrode • Standard pH meter that reads at least to the hundredths decimal place • Temperature bath with a thermometer • Potassium chloride (or other non-sodium salt) • Deionized water • Benzenediazonium tetrafluoroborate • pH buffer solutions 7.00 and 4.00
Laboratory and Data Analysis Benzenediazonium tetrafluoroborate was synthesized according to the procedure given by Canning and co-workers (1). A known volume of a low ionic strength aqueous solution was placed in a reaction vessel and the vessel inserted into a constant-temperature water bath. A calibrated pH electrode was inserted into the vessel and the pH read after thermal equilibration. A small, known quantity of benzenediazonium tetrafluoroborate was added to the solution. The initial pH measurement was taken as the reaction mixture was thoroughly mixed and then the pH was measured with †
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time. The data were analyzed using a nonlinear regression program (see eq 14 in the Supplemental MaterialW) to obtain the observed rate constant. The experiment was conducted at different temperatures to obtain the observed rate constants at various temperatures. Finally, the natural logarithm of the observed rate constant was plotted as a function of the inverse absolute temperature to obtain the activation energy. The benzenediazonium tetrafluoroborate was stored in a refrigerator or freezer when not in use. Hazards There are no unusual or excessive hazards associated with this laboratory experiment. Some benzenediazonium salts are shock sensitive. However, benzenediazonium tetrafluoroborate is less sensitive than other benzenediazonium salts. Moreover, only milligram quantities are used in this experiment. Discussion This experiment allows students to gain experience in taking precise pH measurements, to use nonlinear analysis techniques for analyzing kinetic data, and to use the Arrhenius equation for determining the activation energy. This experiment also affords the students experience in testing proposed reaction mechanisms against kinetic information. Cadets enrolled in the first-semester physical chemistry course at the United States Military Academy have conducted this experiment. Each laboratory group in each of two sections conducted the experiment at different temperatures. As expected, the quality of the results for individual laboratory groups varied. The laboratory groups that conducted kinetic runs at lower temperatures generally obtained better results. Arrhenius plots of the compiled data from each section yielded an activation energy around 100 kJ兾mole, which is close to the literature value. No formal student evaluations of the experiment were conducted, although many of the students liked the experiment. W
Supplemental Material
A detailed description of the experiment including a theory section and typical data are available in this issue of JCE Online. Literature Cited 1. Canning, P. S. J.; McCrudden, K.; Maskill, H.; Sexton, B. J. Chem. Soc., Perkin Trans. 2 1999, 2735. 2. Maskill, H.; McCrudden, K. Croat. Chem. Acta. 1992, 114, 1816.
Vol. 82 No. 12 December 2005
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Journal of Chemical Education
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