Feb., 1961
373
XOTES
thetical restriction on the reactivity of molecules
TABLE I1
just after the instant of energization) the rate conLEASTSQUARES VALUESOF A-COEFFICIENTS AS FUNCTIONS stant may pass through a maximum, and then OF TEMPERATURE AT p = 3.0 Tyw.,
C.
: ;
Ai
O
I
~
Ai
OP
At
UP
0.500 9 . 0 3 0 . 1 32.1 1 . 0 21.8 1 . 6 .lo0 Y,27 . 2 25.6 1 . 9 26.8 3.2 25 .500 13.70 .08 49.7 0.8 45.6 1 . 4 ,100 12.97 .09 44.7 0 . 9 32.4 1 . 6 35 ,500 15.92 .09 64.4 1 . 0 57.5 1 . 8 .lo0 13.74 .07 55.9 0 . 8 37.9 1 . 4 45 .500 18.3 .4 64.1 4 . 1 109.9 8 .lo0 15.8 .2 70.7 1.6 52.5 3 a The authors regard these numbers only as approximate standard deviations because the procedure used in this particular problem involved more than the usual least squares methods. Because of the modification of the chloride ion concentration together with the A-coefficients, straight-forward least squares error analysis does not apply. 0’
With the values of A 2 and i13as a function of acidity, Vanderzee and Rhodes calculated the hydrolysis constant, h, as a function of temperature. However, with the results shown in Table 11, inconsistent values of the hydrolysis constant were obtained. Accordingly, it was decided to calculate the heats for the A-coefficients since an accurate value of the hydrolysis const,ant of Sn(I1) does not appear to be available. Values of 2800, 3100 and 4770 cal./mole were obtained from the temperature dependence of A1, Az and A s at the 0.500 M acidity level. These values are in essential agreement with those report.ed7 far the corresponding Pcoefficients. T H E HIGH PRESSURE LIMIT OF UNIMOLECULAR REACTIONS BYM.C. FLOWERS AND H. M. FREY Chemistry DepaTtment, The Univcraity, Southampton, England Received J u l y 66,1960
Of the large number of homogeneous decompositions that have been studied kinetically in the gas phase, few are simple, most involve complex steps. In addition there is often a heterogeneous component of the decomposition as well as the simultaneous occurrence of side reactions. As a result few experimental data are available to test the predictions of the various detailed theories of unimolecular reactions. The data that, are available for such cases as X2O6,lcyclopropane2 and cyclobutane3 support the prediction of the theories that at sufficiently low pressures a decrease in the apparent first-order rate constants of unimolecular processes will occur. Until now no unimolecular reaction has been studied over a pressure range extending from the region where the “fall off” of the rat)e constant begins to pressures very many times greater than this. This region is of added interest since it has been suggested4 recently that (subject to a hypo(1) R. L. Mills and H. S. Johnston, J . A m . Chem. Soe., 7 3 , 938 (1951). (2) H. 0. Pritoliard, R. G. Sowden and A. F. Trotman-Diekenson, Proc. Roy. S O C .(London), g l S A , 416 (1953). (3) C. T. Qenaux, F. Kern and W. D. Walters, J . A m . Chem. Soc., 75, 6196 (1953).
diminish, with increasing pressure. We wish to report a study in this pressure range on the thermal isonierization of 1,l-dimethylcyclopropane. This has been shown to be a unimolecular reaction5 whose apparent first-order rate constant begins to decrease below 10 mm. The apparatus and analytical technique have been described elsewhere.6 Runs were carried out at 460.4’ in the pressure range 16 to nearly 1600 mm. The rate constants obtained are shown below. Pressure, mm. 10‘ k, uec.-l Pressure, mm. 104 k, sec. -1 Pressure, mm. lo4 k, see. -1
16 2 44 253 2.46 1008 2.41
39 2 43 283 2 42 1057 2.43
55 2.47 567 2 40 1447 2 44
54 2.43 294 2 44 1240 2.41
104 2.44 774 2 41 1596 2 44
217 2 43
Increasing the pressure one hundred and fiftyfold from the point where the high pressure limit is first reached has no effect on the rate constant. It must be concluded that the high pressure rate constant of this unimolecular reaction does not go through a maximum. The authors thank the Esso Petroleum Company for the award of a research studentship to A1.C.F. (4) D. J. Wilson, J . Phys. Chem., 64, 323 (1960). (5) M. C. Flowers and H. M. Frey, J . Chem. Soc., 3953 (1959). (6) M C. Flowers and H. M. Frey, Pioc Roy. Soc (London),2S78, 122 (1960).
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A STUDY OF EQUILIBRIA I N T H E SYSTEM IODINE CYANIDE-POTASSIUM IODIDEWATER-HEPTL4KE BY G. LAP ID US^
AND
G. M. HARRIS
Department of Chemistry, University of Bufalo, B U ~ U Z O 14, N . Y. Received July 26, 1960
In some earlier work in this Laboratory,2 the almost instantaneous exchange of iodine between ICN and K I in aqueous solution was explained in terms of a rapid establishment of the equilibrium ICN H + + II* + HCN (1)
+
Jr
along with the well-known iodide/iodine equilibration I2
+ I-
13-
(2)
.li search of the literature showed there to be considerable disagreement in regard to the magnitude of K1. II, D. Rcheer and R. Klein, ibid., 63, 1517 (1939),
