T H E THERMAL DECOMPOSITION OF SILVER CARBONATE*
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BY M . CENTNERSZWER AND B. B R U ~ S
In the course of our study of the thermal decomposition of carbonates*, we investigated the kinetics of their reactions. We have come to the conclusion that these reactions, which, according to Ostwald2, are classified as second order heterogeneous reactions, are usually monomolecular with temperature coefficients for ten degrees of about 2 . Like the decomposition of silver oxide, observed by (2. S . Lewis3, an induction pemod was observed in several cases. We sought to explain this phenomenon by postulating the formation of an intermediate phase. This hypothesis explained the decrease of velocity of carbonate decomposition when the formulas of successive radioactive decomposition were applied. The subject matter of this paper deals with a peculiar type of decomposition shown by amorphous silver carbonate. First it should be mentioned that crystalline silver carbonate shows a normal behaviour on heating. This carbonate is obtained by slow crystallization from a solution of silver bicarbonate which in turn was prepared by passing carbon dioxide through an aqueous suspension of amorphous silver carbonate. Crystalline silver carbonate decomposes above 2 18'c.directly into silver oxide and carbon dioxide. This decomposition may be formulated exactly by meaas of the uninzolecular reaction velocity equation, namely, I = I, e--kt where I is the velocity of decomposition a t time t, I,, the initial velocity, and k is the velocity constant. The ten degree temperature coefficient in the range observed ( 2 2 0 ' - 2 50' C.) has the value 2 , I 4, a magnitude shown by reactions the velocities of which are independent of diffusion. Amorphous silver carbonate showed an entirely different behaviour. It pias prepared by precipitation, with stirring, of a solution containing 34 g. of silver nitrate in 500 cc. of water with a solution of 20 g. of potassium bicarbonate in liter of water, and drying the thoroughly washed precipitate a t 50-6o0c.. This substance decomposes far more slowly on heating than does the crystalline modification, and the progress of its decomposition does not follow the simple unimolecular equation. For illustration we quote, from a number of experiments, the data obtained at 245°C. in Table I . The volumes of carbon dioxide evolved a t different time intervals were measured. A large scale curve was constructed from these data, from which the volumes of carbon dioxide evolved at equal time intervals, * Contribution from the laboratory
of physical chemistry of the University of Latvia. Acta Universitatis Latviensis 10, 495, 524; 11, 271, 289 (1924); 12 in press; Z. physik. Chem. 111, 79; 114, 237 (1924): * W. Ostwald: Lehrb. allgem. Chemie, 2 11, 291 (1896-1902). G. N. Lea-is: Z. physik. Chem. 5 2 , 310 ( 1 9 ~ 5 ) .
M. CENTNERSZWER AXD B. B R V k
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were obtained. These latter values are given in column 2 . Column 3 contains the specific velocities of decomposition, Le., cubic centimeters of carbon dioxide evolved per minute from one initial gram of silver carbonate. The fourth column contains the logarithms of the velocities. The data of Table I are m e a n values of two concordant determinations. Figure I presents the specific velocity (I) curves taken from the data of Table I . From this it may be seen that the velocity of decomposition of amorphous silver carbonate rapidly decreases at first, then remains constant
TABLE I Decomposition of amorphous Ag2C03a t 245OC.. Weig'ht of Ag2C03= 0.8020 g. Barometric pressure = 760.1 mm. Room temperature = 2oOC. Time in minutes
T'olume of COZ ev o1ved
Specific Velocity (1)
5
5.06
IO
10.01
20
16.38 20.79 24.70 28.67 32.64 36.55 40.32 43.87 47.02 52.07 55.87
1 . I94 1.128 0.629 0.509 0.492 0.492 0.492 0.469 0.448 0.397 0.329 0.236 0 I79
58.51
0. I22
60.38 61.86 62.87 69.9
0.092 0.076 0.061
30 40
50 60 70 80 90 IO0 I20
140 I 60 I80 2 00 220
cc
Log (I. 1 0 2 )
-
2.077 2.052
1.799 707 I ,692 I . 692 I . 692 1.671 I ,651 1.599 1'
1.517 1.373 1.253 I ,086 0.964 0.881 0.785
-
for a period of approximately forty minutes, and finally decreases in a regular manner asymptotically approaching the zero value. It is evident that the curve of Fig. I is analogous to the curves obtained by Curie and Danne for the rate of decay of radiation intensity of plates exposed for a short period of time to radium emanationl. Consequently, we have assumed that amorphous silver carbonate decomposes with the formation of an intermediate product, an oxycarbonate. For the occurrence of such a product, evidence can be found in the literature2. The first stage of the decomposition takes place relatively fast, with the reaction constant kl, the value of which may be obtained from the initial slope of the logarathmic velocity curve of fig. I . P. Curie and J. Danne: Compt. rend. 136,364 (1903); 138,683 (1904). E. Rose: Pogg. Ann. 8 5 , 314 (1852).
THERMAL DE COMPOSITION O F SILTER CARBOSATE
kl
2.303 (log
I, -log I)
-
2.303
(2.21
-
73 5
1.62)
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= 0 . 0 4 j (min.)+. t 30 The product of decomposition of silver carbonate, namely, the silver oxycarh a t e , in turn decomposes a t a slower rate. For a certain period of time, the reactions counterbalance one another and the rate of evolution of carbon dioxide remains seemingly constant. This behavior is exactly analogous to the temporary radio-active equilibria of the Curie-Danne experiments. After the lapse of 90 minutes from the beginning of the reaction, the silver carbonate is practically entirely decomposed. The further course of the reaction consists in the decomposition of the silver oxycarbonate into silver oxide =
FIG.I Decrease of velocity of decomposition of amorphous silver carbonate with time.
and carbon dioxide, which may be formulated by the equation for a unimolecular reaction. I, = I, e-kzt. This stage of the reaction is characterized by the linear course of the logarithmic velocity curve. (Fig. I ) The value of the velocity constant kzmay likewise be calculated from the slope of this curve. 2.303 (log I1 -log Iz) - 2.303 ( I . j Z - 0.84) - 0.016 (min)-I, t z - tl IO0 We further considered the rate of decomposition of the half-decomposed silver carbonate. This was prepared by heating, a t constant temperature, amorphous silver carbonate until half of its carbon dioxide had been evolved. The preparation was then cooled and allowed to stand over night. We expected to find that the ultimate decomposition would follow the normal course of a unimolecular reaction, since the silver carbonate had been entirely converted into silver oxycarbonate and silver oxide. We performed several such experiments a t different temperatures. Contrary to our expectation, the course of the decomposition of the latter prepara-
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M , CENTNERSZWER A S D B. B R U k
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tion was analogous to the decomposition of the freshly prepared amorphous silver carbonate. A rapid decrease of the velocity in the beginning was followed by a period of apparent constancy and finally by a slow decrease. The only difference was manifested in a more rapid initial decrease of velocity. This behavior of the half-decomposed silver carbonate forces us to conclude that the intermediate product, i.e. silver oxycarbonate, is unstable, and decomposes into silver carbonate and silver oxide when the reaction is interrupted. The slight difference in the curves of the freshly prepared and the half decomposed
FIG. 2
Graphic presentation of the data of Table 11
preparations is caused by the incomplete decomposition of the intermediate product. Interesting results were obtained when we compared the decomposition of amorphous silver carbonate a t different temperatures. For this purpose, we plotted the percentage decomposition against time for different temperatures as recommended by Zawidzkil. However, it was found that these curves intersected one another. From these curves we read the intervals of time necessary for the same percentage decomposition at different temperatures; and we plotted the results as time - temperature curves. These data2are given in Table I1 and Fig. 2 . I J. v. Zawidzki, Eull. de l’Acad6mie des sciences de Cracovie, 1916A, 349; Roczniki chemii, 3, 18 (1923). For detailed data see Vol. 1 2 , Acta Universitatis Latvicnsis (Riga).
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THERMAL DECOMPOSITIOK O F SILVER CARBONATE
TABLE I1 Time required for the decomposition of equal fractions of amorphous silver carbonate at different temperatures. Temperature
Percentage Decomposition 3c70 40% 50% 47min. 94min. 139 178 222
10%
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227OC.
233 245 253 2 60 268.5
200%
26
53
7 4
I5
I3
3
8
2
I2
79 30 26 32 56
I20
I35
47 49 I 08 124
65 82
185 180
60% 270
70% 328
-
-
85 131 249 235
109 I93 302 281
It is evident from the diagram that the time intervals required for the decomposition of equal fractions of amorphous silver carbonate decrease first with temperature and reach a minimum value of 245OC. In other words a t 24jOC. the velocity of decomposition i s at a m a x i m u m . From 245 to 260°C. the velocity of decomposition decreases with the temperature. Such examples of negatice temperature coeficzents are seldom observed. At temperatures higher than 26ooc., the velocity of decomposition increases again. According to the hypothesis previously developed, the decomposition of a,morphous silver carbonate ta,kes place in two successive stages. In the first stage of the reaction, an intermediate oxycarbonate is formed, which, in turn, decomposes. This plays the role of a catalyst1. We have seen further that this intermediate product is unstable, a,nd decomposes spontaneously into silver oxide and silver carbonate. It is quite probable that the formation of this intermediate product is restricted to certain temperature ranges. If the upper limit of the'formation of the intermediate product is at 245OC. then, above this temperature, the silver carbonate must decompose directly into carbon dioxide and silver oxide. This direct decomposition, according to the experimental data, takes place more slowly than does the stagewise reaction, as has been observed in similar cases. Thus, it is possible to reconcile the negative temperature coefficient with the hypothesis of the formation of an intermediate compound. Summary In investigating the decomposition of amorphous silver carbonate, we found that; I , at constant temperature, the velocity of decomposition decreases at first, then remains constant for a certain period, and finally decreases exponentially with time; and 2 , in a certain interval of temperature (245' - 26ooC.), the velocity of decomposition decreases with rise of temperature. These facts are explained by means of the hypothesis of the formation of unstable silver oxycarbonate as an intermediate product. Rzga, Lalcia. Pranceton, N . J . 1 Similar ideas for autocatalytic reactions have been deduced mathematically by J. v. Zawidzki: Roczniki chemii, 3, 18 (1923).
