COMMUNICATIONS TO THE EDITOR
which is plotted aginst ill1/'for solutions of polystyrene in Aroclor in Figure 5; the dependence is linear in the range covered. Earlier high-frequency data in solvents of low v i ~ c o s i t y ~ are ~ ' in ~ general consistent with this type of treatment, but for such solutions it has not yet been possible to determine qm by direct measurement. More extensive measurements of the, Einstein viscosity in several viscous solvents of different solvent power are required to investigate the dependence of q, on the molecular configuration of the polymer in
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the various solvents. It would also be interesting to extend the measurements to higher frequencies to see if the onset of the relaxation of qm occurs close to the relaxation region of the solvent.
Acknowledgments. These studies were supported a t Wisconsin by a grant from the U. S. Public Health Service, No. GM-10135, and a t Glasgow by a grant from the National Engineering Laboratory of the Department of Scientific and Industrial Research.
C O M M U N I C A T I O N S T O T H E EDITOR
Kinetics of Iodination of Mercury Dimethyl in Various Solvents
Sir: I n previous calorimetric experiments, it was noted that the iodination of mercury dialkyls took place at markedly different rates in different solvents. We report here measurements on the iodination of mercury dimethyl in five solvents over a temperature range of 23-45 O . The iodination takes place rapidly in all solvents in sunlight, but it was found following rigorous exclusion of light that there exists a genuine dark-reaction. This thermal reaction was followed by mixing known concentrations of iodine and of mercury dimethyl, each in the solvent concerned, in stoppered flasks inside a light-tight temperature-controlled box. The extent of the reaction was determined by removing samples at known times and determining the unused iodine colorimetrically. Tests showed that our sampling technique did not advance the percentage reaction appreciably, but if the solutions were left in the colorimeter for any length of time the superposition of the photochemical reaction, caused by the photometer light source, upon the genuine therinal reaction led to higher rate constants. I n the concentration ranges used to lo-* mole/l. of e:tch reactant) the reaction was found to be first order in each reactant, and a summary of the results obtained is given in Table I. The rate constants vary by a factor of about 100, but to within the experimental error, the activation energy for the reaction is independent of the solvent. The iodination of mercury dimethyl in carbon tetrachloride has been studied
previously by Razuvaev and Savitskii,2 who found a slightly higher activation energy of 9.5 kcal/mole; their rate constants are consistently larger than ours (e.g. 0.116 instead of 0.073 at 28") and it is not clear from their account whether they were aware of the existence of a photochemical reaction. Table I Solvent
Dielectric A H I I O ~ I ( I ~ ) ,k at 2 8 O , 1. constant kcal/mole" mole-' min-1
Cyclohexane 2.05 Carbon tetra- 2 . 2 2 chloride Benzene 2.30 Chloroform 4.64 Ethanol 25.8
-5.8 -5.8 -4.25 -5.1 -1.65
E, kcal/mole
0.069 =t 0.003 7 . 1 & 1.0 0.073 =t 0.004 7 . 7 f 1.0 0.40 f 0.02 1.49 =t 0 . 0 5 6.95 f 0.35
8 . 5 f 1.0 8 . 0 f 1.0 7 . 4 =k 1.0
K. Hartley and H. A. Skinner, Trans. Faraday Soc., 46, 621 (1950).
There is no obvious correlation between the rate constants and any simple physical property of the solvents. However, there is a reasonable correlation between the rate constants and the quantity - [dielectric constant/AH,,1,(Iz) ] which might suggest that a higher rate of reaction is favored by a higher degree of ionic dissociation and by a higher degree of complexing between the I2 and the solvent. The experiments in ethanol were repeated in 0.125 M (1)H.0.Pritchard, Ph.D. Thesis, University of Manchester, 1951. (2) G.A.Rasuvaev and A. V. Savitskii,Dokl. Akad. Nauk SSSR,85, 575 (1953).
Volume 70,Number 6
M a y 1966
COMMUNICATIONS TO THE EDITOR
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sodium iodide-ethanol solutions., The rate constant a t 28" fell to 1.10 1. mole-' min-' with an over-all activation energy of 16.5 f 1 kcal/mole. The simplest explanation of this effect is that the normal reaction involves I+,but in the presence of some 20-fold excess of NaI, this is suppressed and that an alternative but more difficult path involves the I- ion. ~~~
(3) Centre for Research in Experimental Space Science, York University, Toronto.
ALLAN LORD^ DEPARTMEN r OF CHEMISTRY H. 0. PRITCHARD3 UNIVERSITYOF MANCHESTER MANCHESTER, ENGLAND RECEIVED JANUARY 10, 1966
The Thermodynamic Properties of the Aluminum Silicates Sir: Using oxide melt solution calorimetry at 695", the authors have succeeded in determining the enthalpies of formation of the three polymorphs of A12Si05(kyanite, andalusite, and sillimanite) and of 3A1203 2Si02 (mullite). The results are
+ SiOz = kyanite;
A1203(a)
AH968
A1203(a)
+ SiOz
=
=
-1.99
The Journal of PhysicaE Chemistry
=
*
-I-5.44 0.35 kcal/mole
These data are in serious disagreement with the values quoted by Rossini, et d.,' which indicate heats of formation for the polymorphs of the order of -40 kcal/mole. On the basis of these results, and of entropy, heat content, and volume data taken from the literature, the P-T diagram for the AlzOrSiOz system can be calculated for a wide range of temperatures and pressures. When our new enthalpy data are combined with information on the free energy of formation of mullite (-5.6 kcal/mole, at 1823°K; Rein and Chipman2) we are able to derive a new value for the standard entropy of mullite, S02g8= 64.43 cal/deg mole. The uncertainty in this value is estimated to be about 1 0 . 5 cal/deg mole. This result disagrees with the third-law value of the entropy given by Pankratz, Weller, and K e l l e ~ 60.8 ,~ f 0.8 cal/deg mole. However, this apparent discrepancy can be resolved in part by allowing for the disorder associated with the mixing of Si and A1 on the tetrahedral sites of the mullite structure. The present work will be published in greater detail elsewhere. -~
(1) F. D. Rossini, et al., National Bureau of Standards Circular 500, U. S. Government Printing Office, Washington, D. C., 1952. (2) R. H. Rein and J. Chipman, Trans. A I M E , 233, 415 (1965). (3) L. B. Pankratz, W. W. Weller, and K. K. Kelley, U. S. Bureau of Mines Report of Investigations, 6287, Mines Bureau, Pittsburgh, Pa., 1963.
kcal/mole
f 0.17
kcal/mole
INSTITUTE FOR THE STUDY O F METALS
kcal/mole
UNIVERSITY OF CHICAGO CHICAGO, ILLINOIS RECEIVED MARCH11, 1966
+- SiOz = sillimanite; AHgss= -1.51
AH968
f 0.15
andalusite; AH968 =
A1203(a)
-2.37
3A1203((u) -4- 2si02 = mullite;
10.15
J. L. HOLM 0. J. KLEPPA
