E. G. Prout and P. J. Herlev
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Rhodes University
A Constant Temperature Reaction Vessel for the Thermal Decomposition of Solids
A
study of the thermal decomposition of crystals of potassium permanganate in high vacuum has been suggested by Galwey' as a practical exercise for students. Potassium permanganate is highly suitable for this purpose since the decomposition is largely free from the irreproducibility of results which occur in the thermal decompositions of many solids. However, the experimental procedure which is described is open to criticism since the temperature control was only
H
plots than those reported. The results shown would not demonstrate the validity of the Arrhenius relationship. The decomposition chamber described by Galwey has the advantage of being easily dismantled and reassembled by students. The decompositiou apparatus originally used in the study of potassium permanganate2 does not have this property and is consequently not suitable despite the high degree of temperature control. However, we have designed and used in these laboratories an improved version of the original apparatus which is eminently suitable not only for precision research work but also for student use. The apparatus has been used by honors students for the past three years without mishap. The Apparatus
Figure 1.
The apparatus.
f1°C. At 225°C this represents a variation of approximately +7% in the rate constants, which probably accounts for the spatter of points in the activation energy plots of log kl and log kl against l/(T°K). The constants kl and k2 are the values of k in the Prout-Tompkins equation, log P/(PI - P) = kt
+c
over the acceleratory and decay periods respectively. In view of the high degree of reproducibility which is attainable with efficient temperature contr01,~a kinetic study of the isothermal decompositiou of potassium ~ermanganateshould yield better activation energy 1 GALWEY, A. K., J. CHEM.EDUC., 37,98(1960). PROUT, E. G., A N D TOMPKINS, F.C., Trans. Farada?~Soc., 40, 488 (1944).
The apparatus is shown diagrammatically in Figure 1 and consists, essentially, of an inner decomposition tube C, which is heated by the vapor of the liquid boiling in B, which in turn is heated by the vapor of a liquid with a higher boiling point in A. The liquid in A is boiled by J which consists of a ceramic tube (porous pot of a Leclanch6 cell) onto which is wound an electric furnace. A is connected to the air-condenser D. B is connected, via a ground glass joint, to a condenser E which is connected to a narrow bore threeway stopcock. This stopcock may be operated to connect with either the rotary oil pump or the atmosphere. By this means i t is possible to adjust the boiling point of the liquid in B and thus the decomposition temperature. For fine adjustment we have connected a 10-1 flask between E' and the stopcock. The apparatus is assembled with the joint K waxed in and needs no dismantling for long periods even though the organic liquid in B may undergo slow decomposition. Thus, once the apparatus is set up for the study of potassium permanganate, there is no need for the students to dismantle it between runs. The specimen is contained in the bucket I which may be platinum, silica, or Pyrex glass. It is raised and lowered by rotating G. The tempe1,atureis recorded on a calibrated thermometer placed in C and held in position a t its upper end by vacuum wax. The decomposition chamber is connected to the pressure measuring system through the stopcock H. The procedure for removing or inserting I is simple and consists of closing H, breaking the vacuum by slightly opening the joint L, and then removing F as a unit. We have found that the apparatus operates satisfactorily with mercury as the liquid in A. When this is done the air condenser, D, is connected to a glass tube bent over so that one end dips into a beaker Volume 37, Number 12, December 1960
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643
containing mercury covered with a layer of oil. The level in A should be below the bottom of B and that in B below the bottom of C. Liquids which have been used, and the range of decomposition temperatures attainable, are as follows: butyl phthalate, 320280°C; bntyl salicylate, 280-230°C; ethyl salicylate, 230-180°C; methyl salicylate, 220-170°C; salicylaldehyde, 190-140°C; and cyclohexanone, 155--95'C. The temperature control over a period of four hours, with no manual control of the pressure, was *0.03°C a t 220°C. The reproducibility of results for the Figure 3.
Activation energy plots for whole srystols of KMnO,.
A, log
k,; B, log k,; against 1/TIoK).
Figure 2. Curve A: prewre versus time plot for decomporition of a single crystal of KMnO* ot 230% Curve 8: plot for Prout-Tompkins equation.
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Journal of Chemicol Education
decomposition of potassium permanganate was satisfactory since the values of k, for three consecutive runs were 8.48, 8.58, and 8.42 X 10-2 min.?; and for k2 were 1.26, 1.24, and 1.21 X lo-' min.-'. The Prout-Tompkins plot, with the origin transferred to the end of the induction period (see Herley and Prout3), is shown in Figure 2. The linearity of the plot is indicative of the excellent temperature control. The linearity of the plots log kl and log ka against l/T("K) shown in Figure 3 are also satisfactory. 8 HERLEY, P.J., AND PROIJT, E. G., J. Phus. Chern., 64, 675 (1960).
