THE RADIOCHEMICAL DECOMPOSITION OF DEUTEROAMMONIA J. C. JUNGERS School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received July 18, 1955 INTRODUCTION
L
The study of the mercury photosensitized decomposition of the ammonias at room temperature (2) showed that under comparable conditions ammonia decomposes eleven times faster than the ammonia-da. Part of this ratio can be ascribed to the lower quenching power of deuteroammonia; this fact taken into account brings the decomposition ratio to 5. This corresponds to a difference in activation energy of 950 calories (1). It has been shown that the rates of decomposition on a hot tungsten wire yield the ratio 1.6:1, corresponding to a difference in activation energy of 900 calories (3). It was considered of interest to find out how this difference in activation energy would affect the radiochemical reaction. EXPERIMENTAL DETAILS
For this study the method of central irradiation was chosen. This allows both numerous runs and comparable conditions. The a-ray bulb was mounted in the center of a 100-cc. sphere in which the gases were submitted to irradiation. The gases produced by decomposition were pumped off, after the undecomposed ammonia had been frozen out, and measured in a Ramsay gauge. The ammonia could then be vaporized and a new run started, the same sample being used throughout the series of runs. The first run was systematically discarded. Because the decrease in pressure due to the removal of the products of decomposition is very small (about 1/1000 of total pressure), the experiments can be considered as carried out a t the same pressure. It was not judged necessary to measure the emanation, because only relative values were wanted. The activity of all the bulbs used wafl of the order of 50 millicuries. For the runs at 20°C. the temperature was controlled by keeping the reaction vessel in a large Dewar flask,filled with water. For the runs at 1
C. R. B. Fellow from the University of Louvain to the Univergity of Minnesota. 155
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J. C. J U N G E R S
100°C. and 184°C. the vessel was kept in steam and aniline vapor, respectively. The gases used in these experiments were prepared by passing H20and DzO over magnesium nitride, obtained by interaction of purified nitrogen and magnesium a t 700°C. The deuterium content of the hydrogen in the two samples of heavy ammonia submitted to reaction was 68 per cent and 98 per cent. The last sample analyzed spectrographically before and after a long irradiation did not reveal a difference in deuterium concentration. DATA
InJEuence of temperature The data on the decomposition of ammonia and deutero-ammonia are given in table 1. The first column gives the nature and the pressure a t 20°C. of the gas submitted to reaction. The temperature at which the TABLE 1 Decomposition of ammonia and of deutero-ammonia
52.5
20 20 20.3 22 100 100 100 184
52 6
100 20 7 100 22 1 100 20 5 184
~
~
1
64558 61648 57814 53239 48284 45442 42797
2446 3373 3914 4418 2182 2129 1904
36381 33647 30333 28437 25337 23283 20603
2378 2830 1694 2810 1810 2535 2601
5.33 7.42 8.76 10.12 9.11 8.85 7.91
6 4 4 5 5 4 9
67 92 86 03 18 49 21
218 222 224 229 418 416 415 580
'
' ~
1
281 174 286 179 281 177 354
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RADIOCHEMICAL DECOMPOSITION OF DEUTERO-AMMONIA
our results at 20°C. and 100°C. are in perfect agreement. These data yield the following values for the ratios of the rates of decomposition. T in "C......................................... RNH$/RND .................................... ~.
.20 . I . 27
100 1.47
184 1.64
Injluence of the concentration of DZ A sample of ammonia containing 68 per cent deuterium compared with the ordinary ammonia yielded the data contained in table 2. The colunins
TABLE 2 Decomposition of ammonia containing 68 per cent deuterium
1
1
1
1
11.98 8.07 11.90
182 179 187
R .................................................................
181
54.9
20.2 20.0 20.4
92600 84926 78782
6582 4490 6574
4.92
152 152 154
R , .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
54.75
20
62462
3232
have the same significance as in table 1. These data in connection with those of table 1 give the variation of the rate of reaction with concentration in Dz. Concentration in per c e n t . . .....................
R N H J R N (XH D ) I . . .............................. Y
.O
68
98
.I
0.84
0.79
These data show a practically linear relationship between the rate and the deuterium concentration. DISCUSSION
The rate of decomposition A P / A E is a function of the stopping power, the specific ionization, and the ion yield,
C is a constant independent of the nature of the gas. All these data are well known for ordinary ammonia. A very careful determination of the
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J . C. JUNGERS
ion yield has been made by A. Luyckx ( 5 ) . In the case of the deuteroammonias S can be determined by Glasson’s law, giving the stopping = &“D$. No power as a function of the atomic number: this gives SNH, determination of k N D a has yet been made, but as the specific ionization varies for simple compounds between rather restricted limits one can consider the specific ionization of proto-ammonia and deutero-ammonia as being practically equal. The fact that the rate of polymerization is the same for proto-acetylene and acetylene-dz (4)seems to justify this assumption. It is thus safe to admit that the ion yields are very closely in the same ratio as the rate of reaction. The fact that this ratio, close to one at low temperat,ure, rises with increasing temperature is in striking contrast with the react,ions where the rate of reaction is determined by the activation energy. It has been shown that the photochemical reaction of ammonia is a component of decomposition and partial recombination. This is most probably also the case in the radiochemical reaction. The lower ion yield, and especially its slower rise with temperature, cannot be ascribed to a more efficient recombination in the case of ammonia-dz, its activation of formation being higher than for ordinary ammonia. The lower ion yield has thus to be ascribed to the decomposition process itself. No satisfactory explanation is a t hand, but the increase in ion yield with temperature for both ammonias points towards a chain-like mechanism, which is best explained by admitting the clusters formed by the molecules around an ion as reaction center. It is likely that it is the difference in growth in this chain, determined by the activation energy of the decomposition, which controls the ratio of the reaction rates. SUMMARY
The rate of decomposition of ammonia-ds was compared with the rate of decomposition of ordinary ammonia and found to be inferior, to rise more slowly wit,h increasing temperature, and, a t constant temperature, to increase with decreasing deuterium content.
I wish to express my heartiest thanks to Professor S. C. Lind, Director of the School of Chemistry of the University of Minnesota, for his advice and for placing his laboratory and the necessary amount of radon a t my disposal. My thanks go also to Professor Hugh 8.Taylor of Princeton University for kindly supplying me with the necessary quantity of deuterium oxide. REFERENCES
(1) EVAXS,&I. G., . ~ N DTAYLOR, H. 8.: J. Chem. Physics 2, 732 (1934). (2) JTJNGERS) J. C., AND TAYLOR, H. S. : J. Chem. Physics 2,373 (1934). (3) JUNGERS, J. C., A X D TAYLOR, H, 8.:J. .4m. Chem. SOC.67,679 (1935). (4) LIND, s. C., JUNGERS, J. c., AND SCHIFLETT, C. H.: J. Am. Chem. S O C . 67, 1032 (1935). (5) LTJYCKX, A , : Bull. soc. chirn. Belg. 43, 117 (193).
