T H E THERMAL DECOLIPOSITION OF CARBON TETRABROMIDE B Y H. AUSTIN T A I L O R
The number of homogeneous gas reactions involving relatively simple inorganic molecules, which up to the present have been shown to occur unimolecularly is extremely small in comparison with the rapidly accumulating number involving complex organic molecules. Two alone in fact are a t present known : the decompositions of nitrogen pentoxide' and nitryl chloride.: The decomposition of carbon tetrabromide has been shown? to occur according to the following equations: zCBr4 = C2Br6 Br2 C2Brg = C2Br4 BrI
+
+
with a further possible reaction yielding hexabrom-benzene: 3C2Br, = CsBrs
+ 3Br2
There seems to be some doubt whether the latter possibility does occur: and, if a t all, it is certainly to a small extent, the main product being tetrabromethylene. The possibility presented itself that a study of the complete decomposition might yield on analysis a complex rate made up of two consecutive reactions, the first being probably bimolecular and the second, the decomposition of hexabromethane, possibly unimolecular. I n view of the work of Stewart and Edlund4 on the bromination of ethylene, a completely heterogeneous reaction, it was suspected that the decomposition here involved might be in part a t least heterogeneous but it was hoped that a measurement of the rate with known surface of contact would allow an estimation to be made of the extent of homogeneity if any were present and permit thereby an analysis of another homogeneous unimolecular reaction. The reaction however was found to be completely heterogeneous and apparently, approximately of first order. The latter is to be expected for a catalysed reaction in which the true order is the second. The method of following the reaction was essentially a colorimetric one, Measurements were made of the times taken for a known sample of the carbon tetrabromide to yield concentrations of bromine vapor a t the temperature of the experiment, sufficient to match certain color standards, the color comparison being made by use of a Lummer-Brodhun photometer head. The color standards were made up from bromine in carbon tetrachloride solution, yielding concentrations in the ratio of I : z :3 up to six in the six comparative standards used, covering the range of bromine intensity developed by an average amount of carbon tetrabromide. Two sixty-watt lamps were used as l
2
4
Daniels and Johnston: J. Am. Chem. Soc., 43, j 3 (1921). Schumacher and Sprenger: Z. Electrochemie, 35, 653 (1929). Merz and Weith: Ber., 11, 2235 (1878). See Edgar: Diss. Minnesota (1927). J. Am. Chem. SOC.,45, 1014 (1923).
1796
H. ACSTIN TAYLOR
sources of illumination, one for the reaction vessel and one for the standards, operated from the same electrical source so that any variation in line voltage would affect each to the same extent. The reaction vessel consisted of a pyrex tube of 1 2 0 ccs. capacity with flat ends and carrying a small entrance tube at the side through which the solid carbon tetrabromide could be introduced. The reaction vessel was located in the center of a long tubular furnace lying horizontally so that the light passed down the length of the reaction tube to the photometer. Suitable circular diaphragms served to cut out from the light beam entering the photometer, any light passing down the furnace between the sides of bhe reaction vessel and the sides of the furnace. A glass cell similar to those containing the color standards, but holding carbon tetrachloride alone was used to balance the light intensity passing through the empty reaction tube, the positions of the two light-sources being arranged to this end. The complete procedure was then to weigh the reaction tube empty, introduce a suitable quantity of carbon tetrabromide,' remove the air by evacuation with a hyvac pump for five minutes (longer evacuation was found only to result in removal of the bromide), seal under vacuum at a previously constricted point, and reweigh the tube together with the remainder of the side arm now removed. The difference between the two weights after due allowance is made for the air removed gives the w i g h t of bromide taken. Experience showed that by having the furnace rheostat regulated for the temperature required for the experiment but with the furnace some twenty degrees higher at the moment that the cold reaction vessel was put in place, the furnace was so rapidly cooled and the vessel \?armed to the temperature required that little further control was needed to maintain a temperature constant to within + I . ' The time at which the tube was placed in the furnace was noted as also the succeeding times at which the bromine intensity developed by the reaction balanced each of the six standards. The latter were mounted on a smooth sliding carriage so that each in turn could be moved rapidly into position. By plotting these observed times as ordinates against six equidistant points as abscissae a smooth curve was obtained which cut the time axis at a point close t o the origin. The deviation was noted and used as the time required to heat the tube to the temperature in question and vaporize the carbon tetrabromide. This time, in other words, was taken as the starting point of the reaction, the observed time intervals being reduced accordingly. The values quoted are these corrected time intervals. The results obtained at 300' C. are given in Table I and show the variation of the rate of reaction with amount of the bromide taken. To determine the approximate order of the reaction the usual test for the time of a certain percentage decomposition may be applied. The times taken for the bromine developed by reaction to reach a certain standard, correspond, The author is indebted to Prof. H. G. Lindwall for the sample used, prepared by the bromination of acetone.
THERMAL DECOMPOSITION OF CARBON TETRABROMIDE
'797
TABLE I Temperature 3ooOC Weight of bromide taken 0.3260 Standard
0.4488
0.6240
0.9 74%
Time Intervals in mins. 5.0
4.5
2.0
11.75
9.0
4.0 8.5 14.0 20.5
5.5 8.0
28.0
10.5
22.0
42 .o 17.0 -
16.5 29.5 45.5 65.0
These data are plotted graphically in Fig.
1.0 2
.o
3.5
I.
FIG.I
however, to different total amounts of reaction, when different initial weights of carbon tetrabromide are used. A comparison has therefore to be made between the time taken for one standard to be reached in one case, with the time taken for a different standard in another case depending on the initial amount of reactant used and its rate of reaction. Assuming the reaction to be unimolecular the rate and therefore the concentration of bromide, wrill be proportional t o the initial amount. That is, the amount of decomposition corresponding to the first standard in the first case cited above will be equal to Standard 0.4488/0.3260 = 1.38 in the second case and so on. These times read from the plotted results are in the four cases, 5.0, 6.0, 4.5 and 4.0 minutes respectively, or with standard 2 in the first case and its equivalent in the other cases, 11.7j, 13, 1 2 . 5 and 11.0 minutes. The order of magnitude of these figures is obviously the same, indicating the correctness of the assumption of first order for the rate. That the agreement is not better is no doubt due to the fact that the reaction is not a simple one and by a disadvantage
1798
H. AUSTIN TAYLOR
inherext in the method, the above times correspond to a high percentage total reaction. For measurements to be made early in the course of reaction, extremely dilute standards must be used when the error involved in a photometric balancing becomes largely increased. The fact that the reaction is not simply that given by the stoichiometric equation soon became obvious after a number of runs had been made in the same tube. The surface of the tube became somewhat blackened with a deposit which was not removed by the ether used to clean out the other products after each reaction. The deposit, probably carbon, negligible in a single reaction, became visible therefore, only after a series of runs had been made in the tube. I t was found that after thoroughly cleaning the tube with chromic acid solution, the next reaction tried was several times slower than the ones previously made. I t \vas thought a t first that this might be due to some catalytic effect of the carbon deposit. The complet'e absence of any autocatalytic trend in t'he rate curves however was significant and the cause was finally shown to be due to reaction between the bromine liberated and the alkali in the glass with the result that the time taken to develop a given visible bromine concentration was increased and the rate apparently reduced. By rinsing the reaction tube, after cleaning with chromic acid and water, with a bromine solution in carbon tetrachloride, reproducible rates could be obtained. To determine the presence of heterogeneity in the reaction, a thin layer of powdered pyrex was introduced into the reaction tube and after treating with bromine as mentioned above a rate measurement was made. The rate found was so rapid, that it was almost impossible to get a single match with any one of the six standards accurately. The extent of surface was therefore increased less drastically by the introduction of small lengths of glass tubing. The internal surface of the reaction vessel was found to be 190 sq. cms. and an additional surface area of 2 0 0 sq. cms. was int,roduced, the volume being reduced from 1 2 0 to I I O ccs. The surface-volume ratio therefore mas slightly more than doubled. Table I1 gives one example of a series of measurements made with this increased surface, the data from Table I using approximately the same initial amount of bromide are included for comparison.
TABLE I1 Temperature 3oo0C. Weight of bromide taken 0.432j (increased surface) Standard
5 6
0.4488g.
Time intervals in mins.
4.5 9.0 16.j 29.5 45.5 6j.o
THERMAL DECOMPOSITION O F CARBON TETRABROMIDE
I799
TABLE I11 Weight of carbon tetrabromide used = 0.4488g.
’-
It can readily be seen that the reaction is practically entirely heterogeneous, since a doubling of the surface has doubled the reaction rate. The effect of temperature on the reaction rate may be seen from the data given in Table I11 which are the time intervals observed a t four different temperatures using approximately the same amount of carbon tetrabromide, corrected proportionately, on the basis of the unimolecular relationship shown above, for the small difference in initial amount taken. The values given therefore are
,
;
9
:@ ,IO
I
‘1s
summary
The thermal decomposition of carbon tetrabromide is shown to be a heterogeneous reaction of apparent first order over the temperature range 300 to 330’c. with an activation energy of 5 7 , 2 0 0 calories. A-zchols C h e m c a l Laboratory, S e i c York Cniz erszty, ,Ye& Y o r h , S . Y .
