R. R. HAMMER AND N. W. GREGORY
3 14
such as the polymerization method or the consumption of dissolved DPPH.* In some cases, the G(R) values reported to date differ considerably. However, the predicted dependence off on G(R) is readily recognized from Table 11. The fraction of particle energy which is used to produce &electrons of kinetic energies between the upper value W,,, = (4m/M) E and a lower value W1can easily be calculated by using the classical approximation of the energy transfer in a heavy particleelectron collision' 1 In W,,, 2 In W,,,
9s = -
- In W1 - In I
(6)
Here M and E are the mass and kinetic energy of the
heavy particle, m the electron mass, and I the average ionization potential of the stopping atoms (about 50 e.v. in organic materials). Since the initial kinetic energy of the He-Li particles amounts to about 1 Mev., a value for E of 0.5 Mev. may be taken as representative. By substituting qs in eq. 6 by the f-factor, W , can be calculated. This energy then may be interpreted as the minimum kinetic energy of a &electron to participate in the initiation of long-chain polymerization. Table I1 shows the value of W1 calculated from the observed f-factor for several monomers. These numbers, of course, have only a formal meaning in terms of schematicized description of the tracks of a heavy particle. At least, they indicate the order of magnitude of the energy of secondary electrons necessary to initiate polymerization.
An Effusion Study of the Decomposition of Copper(I1) Bromide
by R. R. Hammer and N. W. Gregory Department of Chemistry, University of Washington, Seattle 6 , Washington
(Receiaed August $6,1069)
+
A torsion effusion study of the reaction 2CuBr2(s) = 2CuBr(s) Brz(g) leads to values of AH" = 23.4 kcal. and AS" = 43 e.u. a t 298°K. Results are compared with earlier investigations a t higher temperatures. The condensation coefficient for the process appears to be of the order of 0.1 a t 50' and to decrease as the temperature increases. Apparent values for the activation enthalpies and entropies for the vaporizafion and condensation processes are calculated.
The decomposition of copper(I1) bromide has been 2CuBr2(s) = 2CuBr(s)
+ Brz(g)
(1)
studied by a number of investigators. Equilibrium pressures of bromine reach 1 atm. at ca. 280'. At this and lower temperatures, neither of the solid compounds has a vapor pressure which contributes significantly to the total equilibrium pressure in the system. No evidence to suggest that the two solids are significantly soluble in each other has been found. Equilibrium data have been reported by Jackson,' Shchukarev and The Journal of Phy8icaE Chemistry
Oranskaya, and Barret and Guenebaut-Thevenot, all of whom used a diaphragm gage technique in the temperature interval between 130 and 316'. The results are not in good agreement. Studies of the rate of decomposition have been made by Barret and coworkers.. C. G. Jackson, J . Chem. SOC.,99, 1066 (1911). (2) S. M.Shchukarev and M. A. Oranskaya, Zh. Obshch. Khim., 24, 1926 (1954). (3) P. Barret and N . Guenebaut-Thevenot, Bull. BOC. chim. France, 409 (1957). (1)
315
EFFCSION STCDY OF DECOMPOSITIOX OF COPPER(II)BROMIDE
Extrapolation of existing data suggests that equilibrium bromine pressures for (1) should be in the effusion range around 70'. We have made an effusion study of the reaction t o test further the applicability of the effusion method to tliis type of vaporization process5,6and to extend the temperature range over which measurements have been made to increase the reliability of the thermodynamic constants indicated for the reaction. From the reported rate studies, the condensat'on coefficient is expected to be small; varjatiori of efrusion steady-state pressures with cell orifice dimensions may provide further information concerning the kinetics of the reaction.
Experimental The torsion effusion apparatus has been described previously.' Either a 1- or 2-mil tungsten wire, depending on orifice diameters, was used as a torsion fiber. The torsion constants were determined by calibration of the apparatus by measurement of the vapor pressures of zinc and mercury.8 CuBrz was prepared by reaction of Baker's reagent grade bromine and Raker's analyzed CuBr. The reactants were sealed in a thick-walled Pyrex tube; liquid bromine was held a t a temperature which.gave ea. 10 atni. bromine pressure and the copper bromides a t the other end of the tube were held a t 330' (for about 3 days). The bromine, previously dried over Pz05, was distilled into the initially evacuated reactor; in the distillation approximately the first and last 20% of the bromine was discarded. The CuBr was baked out a t 400' under high vacuum for 8 hr. prior t o the reaction. The product was a black crystalline material of relatively small particle size (ea. a few tenths of a millimeter average dimensions), This material was transferred to the effusion cells in a drybox. Cell orifices were temporarily sealed with Apiezon Q to keep moisture from the sample while mounting the cell on the torsion fiber. CuBrs samples were also prepared by evaporation of alcohol or of a water solution of Merck CuBrz. This material was found to require long periods of drying before results similar to those from samples prepared under anhydrous conditions were obtained. Most of th'e measurements were made with the latter. Preliminary studies showed that for relatively small samples, ca. 0.5 g. or less, steady-state effusion pressures rose as the cell temperature was increased, but after a constant t,emperature had been established, measured pressures dropped steadily with time. For larger samples, from 1 to 3 g., the steady-state pressures leveled off and remained constant and rqproducible, in general until about 10% of the sample ha3 decomposed, after which they began t o decrease slowly. It is con-
cluded that these larger samples provided an effective surface area for the reaction which remained virtually constant in the initial stages of the decomposition. Steady-state pressures (which fell off with time) for the smaller samples were less than the time-independent level established with the larger samples. After appreciable amounts of CuBrzhave decomposed, a number of complicating features arise which could account for the gradual fall-off, e.g., recession of the reacting surfaces into crevices and pores, changing condensation and/ or vaporization coefficients, slow diffusion of bromine through layers of the reaction product, etc. The CuBrz prepared from solution, after being thoroughly dried, was found to maintain steady-state pressures (the same as those above the material prepared by direct bromination) for a somewhat longer time, until samples were about 15% decomposed. All results quoted below represent steady-state effusion pressures from large samples in the timeindependent region of a pressure us. .time curve. These pressures were found to be the same for diffkrent samples and to be reproduced when a given temperature was approached from either higher or lower values. Effusion cells were cylindrical, ca. 4 cm. long, with cross-sectional areas and orifice areas indicated below. Orifices (two of nearly identical area) were placed about 1.5 em. from the point of suspension a t the center and were located on opposite sides near opposite ends of the cell. Orifice Clawing factors (included in the calibration) were virtually unity. Cells were suspended in a horizontal position with the solid sample distributed along the bottom. Table I : Cell and Torsion Fiber Characteristics
Cell
S u m of areas of both orifirrs, A , X lO'rm.2
1 (Quartz) 2 (Pyrex) 3 (Pyrex) 4 (Pyrex)
375 71 5 83 6 20 9
*a
x
lo'"
A,
78 9 8 11 5 2 4
Torsion ronstant k, inn1 radian-' ( P = k9)
0 0 0 0
0232 0634
0616 0114
a A , is cell cross-section area A 2-mil tungsten fiber, length 60 cm , was used for cells 1, 2, and 3; 1-mil fiber for cell 4.
(4)
(5) (6) (7)
(8)
P. Barret and R. Perret, Bull. soc. cliim. France, 1459 (1957); F. Barret and L. Bonnetain, ibid., 576 (1961); P. Barret and R. Perret. Compt. rend., 248, 97 (1959). E. Kay and N. W. Gregory, J . Phys. Chem., 6 2 , 1079 (1958). R. R . Hammer and N. W. Gregory, ibid., 66, 1705 (1962). R. J. S h e and N. W. Gregory. i b i d . , 64, 86 (1960). Data for zinc were taken from results of R. F. Barrow, et al., Trans. Faraday SOC.,51, 1354 (1955); K. K. Kellev, U. S. Bureau of Mines BulJetin 385, 1935. Data for mercury from R. H. Bwey and W. F. Giauque. J . A m . Chem. Soc., 75, 806 (1953).
Volume 68, Number 2
February, 1964
R. R. HAMMER AND N. W. GREGORY
316
&,
