488
VOl. 57
NOTES The Kinetics of the Thermal Decomposition of Gaseous Methyl Iodide BY E. W. R . STEACIE AND R. D. MCDONALD
In a recent paper' Ogg has reported an investigation of the reactions of gaseous alkyl iodides with hydrogen iodide. From the results obtained he concludes that a part of the process can be attributed to a preliminary unimolecular decomposition of the alkyl iodides. In this way he arrives indirectly at the expression K , = 3.93 X
1 0 1 4 - 4 3 W W R T =,-.-I
for the velocity constant of the methyl iodide decomposition. Actually, however, methyl iodide is more stable than would be expected on this basis, and he therefore concludes that the reverse reaction is also taking place, viz.
if reaction (4) represents the main course of the decomposition a t high temperatures. It appears, therefore, that qualitative results on the direct decomposition of methyl iodide are in complete accord with Ogg's inferences. PFWSICAL CHEMISTRY LABORATORY MCGILLUNIVERSITY RECEIVED DECEMBER 29, 1934 MONTREAL, CANADA
Solution of the Conductance Equation BY RAYMOND M. Fuoss
The conductance of incompletely dissociated binary electrolytes as a function of concentration obeys the equation1D2 A = Y(AO
- a 6)
(1)
exactly up to concentrations a t which specific ionic interactions of higher order than pairwise CHsI +CHsI ( 1) become appreciable. (This concentration is in CHsI ----f CHJ (2) order of magnitude 3.2 X lo-' D 3 at 25', where the other possible reactions of the methyl radicals D is the dielectric constant of the solvent.a) At 2CHs ----f CsHe (3) present, the solution of (l), given a, c, A and 2CHs +CHzC& (4) Ao, for y, the fraction of salt existing as free ions, 2CH2 +CzH, involves a very tedious series of successive CHI CHaI +CIH~ I (5) .approximations. being unimportant compared with (2). If'we introduce a new variable z, defined as Shortly before the appearance of Ogg's paper follows we had submitted a short note on a direct in2 = do-'/r GA = ~ A ~ - v v'i%G * (2) vestigation of the methyl iodide decomposition.2 This report was qualitative only since side re- where CY = the Onsager coefficient, A. = limiting actions, etc., were too complex to make a detailed conductance, t = concentration in equivalents kinetic investigation worth while. It is, however, per liter, A = equivalent conductance and K possible to make a rough comparison of the rate = specific conductance, equation (1) may be with that inferred by Ogg. written From our results TSOis of the order of magniY = A/AOF(Z) (3) tude of 100 seconds a t 510'. This is a lower limit where F ( 2 ) is the continued fraction since some heterogeneous reaction is also present. F(z) = 1 - z ( l - z ( l - z ( l - ...)- V2)-'/*)-1/: (4) Ogg's equation indicates a value of about 0.2 Values of this function for round values of a sec. a t this temperature. Hence the rate a t 510' from z = 0.000 to z = 0.209 are given in Table I, is '/m or less of the rate calculated. In view of the at intervals Az = 0.001. For example, F(0.1044) low strength of the carbon-iodine linkage (about = 0.88928. 40,000 cal.), the stability of methyl iodide seems In order to solve (1) for y, a value of z is combest explained on the basis of Ogg's scheme puted, the corresponding value of F(z) is inter(reactions (1) and (2)). The actual decomposition polated from the table, and y is simply obtained a t high temperatures cannot follow reaction (3) by division of the conductance ratio A/&' by F. or ( 5 ) , since these would lead to the production In order to obtain A0 and K from conductance of ethane, without condensation or carbon depodata, a free-hand extrapolation of a A-
