Jay A. Rubin and Stephen V. Filseth Harvey ~ ~ College d d
The Thermal Decomposition of 2,s-Dihydrofuran Vapor
Claremont, Col~forn~a9171 1
A n experiment in gas kinetics
For the past few years we have offered a gas kinetics experiment as part of our introductory three-semester course in chemistry. It is the purpose of this experiment to illustrate manipulations with a vacuum system and the conduct of kinetic measurements. During this period, we have experimented with several systems in which one expects a doubling of pressure upon complete decomposition. Several systems have been previously suggested for undergraduate gas kinetics experiments.' Our desire was to select a compound whose decomposition had been well characterized as first order in the high pressure region and which was available as a liquid of reasonable purity and low toxicity. Preliminary work with cyclopentene was discontinued when, after multiple experiments, a viscous darkly colored liquid was observed to he formed, accompanied by a change in the kinetics to apparent second order. We have also attempted unsuccessfully to use both perfluorocyclobutane and l,4-cyclohexadiene I n the former case, the final pressure upon complete decomposition was observed to considerably exceed twice the initial pressure and in the latter case, a fairly low vapor pressure made visual manometric measurements difficult. We have recently had good results with 2,5-dihydrofuran, (DHF), which has been observed2 to decompose homogeneously in a first order process producing furan and hydrogen.
cept for the reaction flask. The flask valve is then opened and quickly closed as a timer is started. Manometer readings are recorded as a function of time for a t least one half-life of the reaction. For the reaction,
the rate constant can be written as
where p, is the initial reactant pressure, obtained by extrapolating a plot of the total pressure, p, versus time to zero time. When the rate data are plotted according to this equation the rate constant is simply the slope. Some typical data are shown in Figure 2. By compari-
Procedure and Results
A schematic diagram of the apparatus employed in this experiment is given in Figure 1. The experiment is begun by expanding DHF into the entire system ex-
TO MASS SPECTROMETER
Time Figure 2.
(minutes)
Typical kinetic data through two holf-liver
EXPANSION RESERVOIRS
MCLEOD DHF GAUGE STORAGE 'IMPS
OVEN
HO MANOMETER
Figwe 1.
Schematic illustration of experimental opparotur.
Figure 3. Ternperdure dependence of rate constmh 70 Torr DHF; closed circler: 0.1 Torr DHF.
Volume 46, Number
Open circler:
I, Junuary 1969 / 5 7
son with the data of other students obtained at different temperatures, the activation energy can be obtained. The results of such measurements are shown in Figure 3. The reaction was also investigated a t lower pressures in an attempt to observe the decrease in first-order rate constant in the pressure fall-off region. These measurements were made at approximately 0.1 Torr by sampling continuously from the reaction flask through a molecular leak into a mass spectrometer. The progress of the reaction was observed by following the disappearance of the m/e = 70 peak of the DHF and/or the appearance of the m/e = 68 peak of furan. Within experimental error as shown in Figure 3, the rate constants were the same as in the high pressure region. The rate constants measured in this experiment were found to he approximately half those reported prev i o u ~ l y . ~This was determined to be due to the temperature difference between the interior of the flask and
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Education
the outer wall where the thermocouple used to measure temperature was located. Placement of the thermocouple in a recessed well in the flask should avoid this problem. Acknowledgment
This project was supported by the National Science Foundation through its Science Curriculum Improvement Program. The authors also appreciate the loan of a Quad 160 Laboratory Mass Spectrometer from Electronic Associates, Inc. DANIELS,WILLIAMS,BENDER,ALDERTY,AND CORNWELL, "Experimental Physical Chemistry," (6th Ed.) McGraw-Hill D., A N D Book Co., Inc., New York, 1949, p. 96; SHOEMAKER, GARLAND, C., "Experiments in Physical Chemistry," (2nd Ed.) McGraw-Hill Book Co., Inc., New York, 1962, p. 228; P R ~ C E , A. H., AND BAKER,R. T.K., J. CHEM.EDUC.,42, 614 (1965); GUILWRY, W. A., J. CHEM.EDUC.,44, ,514 (1967). 'WELLINGTON, C. A., AND WALTER$W. D., J. Am. Chem. Sac., 83, 4888 (1961).
