Kinetics of the GPS Phase PY~OIYS~S of 1,l-Dichloroethane
G. R. De Mar& P. Goldflnger, G. Huybrechts, J. Olbregts, and M. Toth Universit; Libre d e Bruxelles Brussels 5, Belgium
I
A physical chemistry experiment
Guillory ( I ) has pointed out that there are relatively few experiments for the elementary physical chemistry laboratory which allow the kinetic study of a gas phase reaction (2). The pyrolysis of 1,1dichloroethane (1,l-DCE) has been used for several years as an undergraduate experiment in this department and is very satisfactory. The students determine the stoichiometry, order, and rate constant of the reaction and examine the effects of added gases such as propene, nitric oxide, and air. A few experiments on the decomposition of 1,Pdichloroethane (1,SDCE) are also performed for comparison. The results obtained under different experimental conditions are pooled by the students for the purposes of proposing reaction mechanisms and calculating the activation energy and preexponential factor for the pyrolysis of 1,l-DCE. The thermal decomposition of 1,l-DCE has been studied by Hartmanu, et al. (8) and by Barton and Howlett (4). The stoichiometry of the reaction corresponds to the overall equation 1,l-C2H,Cb
-
ClHsCl
+ HC1
The thermal decomposition of 1,2-DCE also yields vinyl and hydrogen chlorides (5). In this case, however, (1) there is an induction period, (2) it is difficultto obtain reproducible results, and (3) the reaction rate is affected by the presence of propene, nitric oxide, oxygen, and other impurities (6). The simplest explanation for the wide difference in behavior shown by 1,land 1,Z-DCE towards C3H6,NO, and 0%is that the former decomposes by a molecular mechanism and the latter by a radical chain mechanism. The Experiment The experiments are performed in the apparatus represented in Figure 1. The 60-ml Pyrex reaction vessel (R), which is centered in a capper cylinder surrounded by asbestos insulation, is heated electrically. The temperature is controlled by a relay device which is actuated by a change in resistance of a platinum resistance thermometer. Pressures are measured by means of mercury manometers. The reaction vessel is conditioned by putting 100 torr of 1,l-DCE in it at pyrolysis temperature for one night. Reproducible results are thus obtained without coating the walls with potassium chloride (3) or carbon (4).
(1)
Since 2 moles of products are formed for each mole of 1,l-DCE reacted, the increase in pressure, A P , in a constant volume system is a measure of the substrate which has reacted. The final pressure, P,, will be twice the initial pressure, Po; the pressure of 1,1-DCE a t any instant t is given by (Po - AP) = (PO- (PI - Po)) = (2Po
- Pt)
(2)
where P , is the total pressure a t time t. Integration of the firsborder rate law gives logm(Co/Ct)= (k/2.303)1
(3)
From the ideal gas law we have C = n/V = P / R T so that a t constant volume and temperature eqn. (3) may be rewritten log~0(P0/(2P~ - P C ) )= (k/2.303)1
(4)
The pyrolysis of 1,l-DCE a t pressures above 20 torr obeys eqn. (4) up to a t least 80% reaction (8, 4). Further, it is insensitive to the presence of propene, nitric oxide, and oxygen. We thank a referee far pointing out that since the data can be handled equally well by plotting log (2Po - P A )against time, the calculation of the fraction P0/(2Po - P,) for each experimental point may be an unnecessary waste of the student's time. This would be particularly so if log tables me used. Students here are encouraged to use their slide rules, however, and read the log of the fraction directly. The log Po/(2Po - P C )representation permits a better comparison of experiments performed at different initial pressures.
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Journal of Chemical Education
Figure 1.
Kinetic apporotvr
After "degassing," a sample of 1,l-DCE (Th. Schuchardt, reinst, used without further purification) is transferred from the reservoir to the calibrated, 400-ml volume ( V ) . The pressure, from 30 to 200 ton, is read on manometer B. (The pressure expected in the reactor after expansion may thus be calculated.) A student opens stopcock C for a few seconds and starts the chronometer; a. second student reads the pressure on manometer A. At the end of the reaction, to avoid their passage through the diffusionandmechanical pumps, the reactionproducts are frozen in a removable trap (Fig. 1) for disposal. Typical student plots of PLversus time are shown in Figure 2. The initial pressure POmay be calculated as above, extrapolated from the P,versus time curveor deduced from the final pressurep, in those experiments where the pyrolysis is carried out to completion. The rate constant is obtained from a plot of logia Po/(2Po - P,)versus 1 (eqn. (4)); such a plot1 is given in Figure 3. The pyrolysis may be studied conveniently over the tempera, ture range 400460PC. At the higher temperatures reaction is complete in about 30 min (99%) while at the lower temperature the half-life (11/, = 0.693/k) of the re?ction is 5 3 hr. An
Figure 2. DCE.
Increose in
total pressure Pt with time in
the pymlyrir of
1.1-
Arrhenius plot of the rate constants found by the students is reproduced in Figure 4. The broken and the solid straight lines are those calculated from the rate constant expressions given by Hoslett (4) and Hartmann, el 01. (S), respectively. The r?sults are very gratifying since the best Arrhenius values should be actually those of Hartmann, et al. (8) as discussed by Setser and Hassler (7). The procedure followed in doing ctn experiment with an added gas (additive) is one of the following: (a) known quantities of 1,l-DCE and additive are mixed in the volume V and introduced together into the reactor or (b) after introducing the 1,l-DCE into the reactor, the students follow the reaction while the remaining 1,l-DCE is evacuated from the ramp. At s. known time? the additive, which was previously stored in volume E at adequate pressure, is introduced into the reactor and the change in pressure with time noted continuously. If the additive has no effect on the pyrolysis then the slopes of the log,, Pa/(2Po - P,) versus time plots=should he identical before and after introduction of the additive (see Fig. 3). The first procedure requires two experiments, one with and one without additive for comparison,' using otherwise identical experimental conditions and is the most practical at the higher temperatures where the reaction time is short. The second method is most useful s t lower temperatures where the half-life of the reaction is sufficiently large. Several results obtained with additives are shown in Figure 4. The results indicate that there is no effect of the additives on the pyrolysis in agreement with the published data (5,4). The surface to volume ratio can dsa he shown t o have no effect (5. . Li.. on the rate of reaction bv usine a reactor Dacked with glass beads. The pyrolysis of 1,2-DCE is carried out using the same procedure as for 1,l-DCE. The temperature range is also the same and the pressures which may be used me 3&60 torr (limited by the vapor pressure of 1,Z-DCE at room temperature). The reaction shows an induction period and is sensitive to traces of nitric oxide, oxygen, propene, and other impurities as has previously been observed (6,G). The stoichirynetries can be checked simply by showing that P, = 2P0. A more elaborate procedure consists of analysing the products as follows: the whale react,ion mixture is condensed a t liquid nitrogen temperature (-195'C) in bulb D (see Fig. 1) which is sealed off; this bulb is then crushed, either in a basic solution for HCl titratition or in a gas chromatograph for determination of C.H3CI and dichloraethane. The latter two wmpounds can be quantitatively estimated using, for example, a 4-m siliconegrease column (Edwards Ltd. Silicone High Vacuum Grease 15% by weight on 50-80 mesh Sil-0-Cel C-22 fire brick from Johns Manville U.S.A.) at about 50°C and with a Hs flowrate of 60 ml/min.
Pyroly4. of 1,l-DCE ot 702'K plot of logra Po/l2Po - Pd time: Po = 5 2 tom; (A): experiment in the absence d oir; (01: experiment in which 10 torr of air hove been odded ofter 5 min of readion Figure 3.
versus
-
Figure 4. Arrhenius plot of the rote constant k for the pyrolyrir of 1 ,I DCE. k in set-'; T in 'K; lo]: pure 1.1-DCE; Dl: 1.1-DCE propene (-1 10 tom); (0):1,l-DCE air (-10 torrl.
+
+
.
Here P.is theinitial pressureof DCE and P, the total pressure a t time 1 minus the pressure of the additive. 3 In this case the graphs of pressure versus time for experiments in the absence and presence of additives are compared.
Acknowledgment
The authors wish to thank Drs. L. Exsteen and G. Martens for their assistance. Literature Cited (1) GUILLORY, W. A,, J. CHEM.EDUC.,44, 514 (1967). (2) For examples see DANIELS,F., MATHEWS, J. H., WILLIAMS, J. W., BENDER,P., MURPHY,G. W., AND ALEERTY, R. A., "Experimental Physical Chemistry," (4th Ed.) McGrewHill Book Co., Inc., New York, 1949, p. 155; SHOEMAKER, D., AND GARLAND, C., "Experiments in Physical Chemistrv!' McGraw-Hill Book Co.. Inc.. New York. 1962, p. %8. H., HEYDTMANN, H., AND RINCK,G., 2.Physik (3) HARTMANN, Chem. (Frankfort), 28, 71 (1961). (4) BARTON,D. H. R., J . Chem. Soe., 148 (1949). BARTON, D. H. R., AND HOWLETT,K. E., J. Chem. Sac., 165 (1949). K. E., J. Chem. Sac., 3695 (1952). HOWLETT, (~ 5 ) -BARTON. D. H. R.. AND HOWLETP.K. E.. J. Chem. Sac., , 155 (i949). K. E., Natwe, 165, 860 (1950). (6) HOWLETP, J. C.,J. Chem. Ph~s.,45, 3246 (7) SETSER,D. W., AND HASSLER, (1966).
Volume 46, Number 10, Odober 1969
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