1950
To further elucidate the exchange mechanism, these DMF complexes are being studied in other solvents. Also complexes of these halides with other solvents are being investigated. Acknowledgments. This research was supported in part by a grant from the National Institutes of Health.
Comparison of Hydrogen and Deuterium Solubility in Palladium-Rich Alloys. Gold-Palladium by Arnulf Maeland and Ted B. Flanagan Department of Chemistry, University of Vermont, Burlington, Vermont 06401 (Received November 7, 1966)
Recently, large differences in equilibrium isotopic solubilities were observed for hydrogen and deuterium in certain platinum-palladium alloys (25', 1 atm of Hz (or DZ)).lJ (It must be emphasized that these solubilities refer to the individual solubility of either pure hydrogen or pure deuterium in the alloy and not to their isotopic mixture^.^) These differences in isotopic solubility in certain platinum-palladium alloys result from the following empirical considerations. I n platinum-palladium alloys, large solubility of hydrogen near room temperature obtains only if the second, p, phase is formed because the hydrogen content at the maximum a-phase boundary is small.2 The heat of the reaction corresponding to absorption of hydrogen (1 atm) into the two-phase region becomes less exothermic as the platinum content of the alloy increases. The corresponding value of A S becomes somewhat less negative as the platinum content increases. At certain platinum contents, the value of AG becomes zero for the formation of the second phase and the hydrogen solubility is consequently small. The corresponding value of the heat of absorption of deuterium in the two-phase region of these same alloys is approximately 700 cal/mole of Dz less exothermic than for the hydrogen. The magnitude of A S and its changes with added platinum metal are comparable to the values obtained for hydrogen. These considerations mean that there is a range of platinum contents, e.g., -9 to 11 atomic % ' ( 2 5 O , 1 atm), where the freeenergy change is negative for the formation of the @ phase in the hydrogen system and zero or positive in the deuterium system. By contrast, in the gold-palladium system, absorption of hydrogen (1 atm) into the two-phase region is The Journal of Physical Chemistry
NOTES
characterized by an increase of exothermicity with added gold content. Since the accompanying change in entropy alters only slightly with gold content, the free energy for the formation of the second phase becomes increasingly negative with added gold. Reasoning from the arguments presented for the platinumpalladium system, a large difference in the solubilities of hydrogen and deuterium in certain gold-palladium alloys should not be expected, at least for those alloys which form two phases. The purpose of this note is to examine the absorption of deuterium by a series of goldpalladium alloys experimentally. The experimental setup was identical with that previously e m p l ~ y e d . ~Approximately 0.02 N DCl solutions were employed for the electrolyte through which deuterium gas was bubbled after having been purified by passing through a silver-palladium membrane. In some cases hydrogen-helium mixtures were employed to decrease the rate of the absorption reaction. The course of the absorption was followed by changes in the electrode potential of the specimen with respect to an Ag-AgC1 reference electrode. The activity of the solution was determined by measuring the electrode potential of the Ag-AgC1 electrode us. a Pt-Dz (1 atm) electrode in the same solution. The measured electrode potentials were corrected for the vapor pressure of DzO which tended to reduce the deuterium pressure to values slightly below 1 atm.
Results and Discussion Typical absorption data for deuterium, e.g., plots of electrode potential us. time where time can be taken as directly proportional to D/M, are similar to data obtained for hydrogen4 except that the two-phase electrode potential is considerably reduced for deuterium as compared to hydrogen. When the specimen reaches an equilibrium deuterium content (1 atm of Dz, 25') the electrode potential is zero with respect to the DZ (1 atm)-Pt electrode. The equilibrium deuterium contents as determined by vacuum degassing are shown in Table I as compared to corresponding values for hydrogen. It may be seen from Table I that there is very little difference in the equilibrium solubilities of the two isotopes even for alloys which do not form two phases, i.e., Au 2 17 atomic %. This behavior is in marked contrast to the behavior of platinum-palladium alloys. Several X-ray powder patterns were taken in the two~~
(1) (2) (3) (4)
~
T. B. Flanagan, J . Phys. Chem., 67, 203 (1963). A. Maeland and T. B. Flanagan, ibid., 6 8 , 1419 (1964). F. Botter, ibid., 69, 2487 (1965). A. Maeland and T. B. Flanagan, ibid., 69, 3575 (1965).
NOTES
1951
Table I : Equilibrium Solubilities (750 f 10 mm, DZ(Hd, 25')
0
Atomic % Au
H/M
0 8.70 18.80 26.48 44.76 55.77 68.84
0.69" 0.48' 0.33' 0.23' 0.104 0.035' 04
0.65" 0.4gb 0.32b 0.22b O.0fib 0.024b Ob
T. B. Flanagan, J . Phys. Chem., 65,280 (1961).
This work.
phase region of the D-Au-Pd system. The fcc lattice showed the presence of two phases and the lattice constants of a m a x and P m i n were very similar (f0.004 A) to those observed for the corresponding alloys containing hydrogen. This suggests that the compositions of amaxand Pmin are comparable for the HAu-Pd and D-Au-Pd alloys. Thermodynamic quantities corresponding to the reaction 1/2D2(latm)
+ ~D,,,:yAu:zPd
+
where x and Z' represent the number of D atoms charand &in, respectively, and y and z acteristic of emax represent the number of atoms of Au and Pd in the alloy under consideration, were determined from the value of the two-phase electrode potential and its variation with t e r n p e r a t ~ r e . ~The , ~ two-phase potentials were very reproducible and stable, Le., 0.05 mv. Results for the values of AG, AS, and AH are shown in Table 11. (These thermodynamic values refer unambiguously to the reaction given above; these
Table I1 : Thermodynamic Parameters of the D-Au-Pd and H-Au-Pd Systems in the Two-Phase Region (25O,1 atm)
- AG, Isotope H2
D2 H2 D2 H2 D2 H2 D2
Atomic % Au
cal/ mole of Dz (HI)
5.66 5.66 8.70 8.70 11.90 11.90 15.26 15.26
2464 1513 2496 1541 2565 1601 2644 1679
-AH,
-As, eu
23.6 24.2 23.9 24.7 24.4 25.5 24.1
...
cal/ mole of D1 (Hz)
9503 8744 9634 8896 9832 9214 9852
...
Ref
4 This work 4 This work
4 This work
4 This work
quantities will henceforth not be referred to as standard values since the standard state of the H (or D) containing solid phase in analogous systems has been defined differently el~ewhere.~,'These are not differential values since they represent a discontinuous change in the H (or D) content in going from the CY phase to the P phase.) It is seen from Table I1 that the free-energy and enthalpy changes corresponding to absorption in the twophase region become more negative with increase of gold content for both hydrogen and deuterium. There has been a great deal of interest recently in the absorption characteristics of hydrogen by palladium-rich alloys. From the data now available, these alloys can be grouped into two broad classes. Group I alloys (I = increase) will be designated as those in which the free energy for absorption into the two-phase region becomes more negative as the added metal content of the alloy is increased. Group I includes to date sil~er-palladium,~~~ gold-palladi~rn,~ tin-palladium, and lead-palladium.9 The other group will be designated as group D alloys (D = decrease) and these are characterized by the opposite behavior; Le., the negative free energy decreases with added metal content. Group D includes : platinum-palladium, lo rhodium-palladium, nickel-palladium, l1 e l 2 iridium-palladium, l 3 and copper-palladium. l4 It can be predicted that for Group I alloys there will be no significant difference in the isotopic solubilities of pure hydrogen and pure deuterium. The absence of a marked difference between the solubilities of deuterium and hydrogen in gold-palladium alloys which do not form two phases (atomic % Au I 17) suggests that the free energy for absorption increases with gold content for these alloys too, although in this work only thermodynamic parameters for the two-phase formation were (5) R. J. Ratchford and G. W. Castellan, J. Phye. Chem., 62, 1123 (1958). (6) E.Wicke and G. H. Nernst, 2. Elektrochem., 68, 224 (1964). (7) H.Brodowsky and E. Poeschel, 2. Phyaik. Chem. (Frankfurt), 44, 143 (1965). (8) (a) F. A. Lewis and W. H. Schurter, Naturwissenschaften, 47, 1477 (1960); (b) A. C. Makrides, J . Phys. Chem., 68, 2160 (1964); (c) Z. L. Vert and I. Tverdovski, Russ. J. Phys. Chem., 28, 317 (1954). (9) H. Brodowsky and H. Husemann, Ber. Bunsengee., 70, 626 (1966). (10) A. Carson, T. Flanagan, and F. Lewis, Trans. Faraday SOC., 56, 1311, 1324 (1960). (11) (a) J. Barton, J. Green, and F. Lewis, ibid.. 62, 960 (1966); (b) I. P. Tverdovski and A. I. Stetsenko, Dokl. Akad. .&-auk SSSR, 84, 997 (1952). (12) I. P.Tverdovski and 2.L. Vert, ibid., 88, 305 (1953). (13) M.LaPrade and T. B. Flanagan, to be published. (14) (a) R. Karpova and I. Tverdovski, Zh. Fiz. Khim., 33, 1393 (1959); (b) D.Chisdes and T. Flanagan, unpublished results.
Volume 71, Numbw 6 May 1967
NOTES
1952
measured. For group D alloys there may be anticipated to be large differences in the solubilities at certain added metal contents. These predictions have been verified for only two alloy systems: the platinumpalladium2 and the gold-palladium (ref 4 and this work).
Acknowledgments. This research was supported by the U. S. Atomic Energy Commission. The authors are most appreciative of this financial support. The authors are also indebted to Engelhard Industries, Inc., for the gold-palladium alloys used in this research.
Reactions of Diatomic Molecules. IV. Kinetics of Formation of Bromine Chloride by Peter R. Walton and Richard M. Noyea Department of Chemistry, university of Oregon, Eugene, Oregon 97409 (Received December 6 , 1966)
Calculations described elsewhere’ predict that all reactions of halogens with halogens will proceed by bimolecular mechanisms. For the formation of BrCl from the elements, the predicted activation energy is about 13 kcal/mole. Previous studies of this reaction are mostly qualitative. Jost2 and Brauer and VictorS found rapid reaction in gas phase, and Jost2 even estimated an activation energy of 14 kcal/mole. These studies did not demonstrate homogeneity of the reaction, and surface effects are obviously hard to eliminate. Although reactions in solution are more easily shown to be homogeneous, “inertness” of solvent can never be demonstrated unequivocally. Barratt and Stein4 observed the reaction to be “instantaneous” in ether and chloroform but to have a time lag of several seconds in carbon tetrachloride. Dennis Forbess at the University of Oregon also observed a measurable rate in this solvent, but he could not get reproducible results. Our own qualitative observations indicated that the rate in carbon tetrachloride was strongly accelerated by traces of moisture as reported by Hildebrands for the formation of IC1 in this solvent. This sensitivity to moisture suggested the use of pure sulfuric acid as a reaction medium. Visual examination indicated that the spectra of the halogens were the same in this solvent as in gas phase. Absorption spectra of bromine, chlorine, and a reacted mixture agreed well with those reported at wavelengths longer than 3100 A in carbon tetrachloride by Popov and Mannion,6 The Journal of .Physical Chemistry
and the position of equilibrium did not appear to be shifted from their observations. The rate of reaction was much slower than in organic solvents, was satisfactorily reproducible, and was not affected by the deliberate addition of small amounts of water. These facts offered enough evidence of “inertness” to justify the kinetic observations reported here.
Experimental Section Materials. Reagent grade bromine was purified by shaking with concentrated sulfuric acid followed by distillation. Chlorine was purified by passing through concentrated sulfuric acid. The solvent was 99.6% sulfuric acid prepared by adding reagent grade oleum to 98% acid. The composition was analyzed by electrical conductivity. Solutions of the halogens were made up prior to each run. Since air-saturated solutions of chlorine were found to undergo photochemical deterioration when heated, the solvent used in preparing solutions had been saturated with dry nitrogen. The solutions were analyzed by diluting with ice water containing potassium iodide and then titrating with standardized thiosulfate. Procedure. Mixtures of the desired composition were prepared directly in optical cells and thermostated to the desired temperature. Optical absorbance was then followed as a function of time with a Beckman spectrophotometer. The observations could be fitted satisfactorily to the equation for a reversible bimolecular process, and a computer program with a variable infinite time absorbance was used to get the best fit and to compute the rate constant. The calculations also required information as to the position of equilibrium. The equilibrium constant for the reaction
Brz
+ Cla
2BrC1
(1)
has been estimated to be almost independent of temperature and to be about 7 in vapor phase7 and in carbon tetrachloride? Very rough measurements at 75.6’ confirmed that the value in sulfuric acid is approximately the same or slightly less. Since uncertainties in this quantity have very little effect on (1) R. M. Noyes, J . Am. Chem. SOC.,88, 4318 (1966). (2) W. Jost, 2.Phyeik. Chem., B14, 413 (1931). (3) G. Brauer and E. Victor, Z . Elektrochm., 41, 508 (1935). (4) S. Barratt and C. P. Stein, Proc. Roy. SOC.(London), A122, 682 (1929). (5) J. H. Hildebrand, J . Am. Chem. SOC.,68, 916 (1946). (6) A. I. Popov and J. J. Mannion, ibid., 74, 222 (1952). (7) C. M. Beeson and D. M. Yost, ibid., 61, 1432 (1939).
