NOTES
Nov., 1963
2491
lead ions leads to a change of approximately 15% in the value of the Seebeck coefficient for lead chloride. Acknowledgment.-We wish to express our appreciation to Dr. A. E. Potter for his valuable assistance.
THE THERMODYNAMICS OF ASSOCIATION OF THE TRISETHYLENEDIAMINEPLATINUM(IV) ION WITH VARIOUS AKIONS BY DONALD C. GIEDT AND C. J. NYMAN Department of Chemzstry, Washington State University, Pullman, Washington Received M a y .90, 1868
--t m + 2
Fig. 3.-Plot
*
of F( A@/AT) against - t M + 2 / 2 . Ssalt] -67 e.u.
-
Slope = [8*sa~t
potential in ionic crystals.* This latter equation is based on the models of Frenkel and Schottky for lattice disorder aind permits a more detailed analysis of the system. Since the structure of a fused salt corresponds to that of tht: crystal, the conduc1,ion process in the melt can be considered to be a function of ions moving through vacancies. Only those ions with sufficient kinetic energy to overcome the coulombic forces at their lattice position will participate. The negative polarity at the hot junction of the thermocell can be considered to be established by the immigration of cations to the cooler junction. Once in the lower temperature zone, they lose the energy necessary for their return. The thermopotential, as well as the conductivity of these systems, is a function of the number of mobile ions present and their ability to move through the liquid. If we assume that, a t about 25' above the melting point, the number of vacancies is much greater than the number of mobile ions, then the thermopotential is solely a function of the number of mobile species present. In this case the slope of the plot of A@ against AT (Seebeck coefficient) will vary in the same manner as the rate of formation of mobile cations. If we take the slope of the last three points in Fig. 2 for the PbCL system, the value of F ( A @ / A T ) is 9.31. This is a 15y0 change over the value of 8.09 given in Table 11. When the value of 9.31 is used to calculate QM +2**/Tin eq. 2, the value obtained is 68.6 e.u. (equal to S p b i 2 * - S P b t 2 1 . If Spb+Z* is relatively the same (-4 e.v.), this represents about a 3% change in S p b t2. Thus, a 3% change in the rate of formation of mobile (8) R. E. Howard and A. B. Lidiard, Phil. Mag., 2,1462 (1957).
The previously reported1 stability constants for the 1:1 outer-sphere ion pairs formed by trisethylenediamineplatinum(1V) with various anions were 11 for C1-, 8 for Br-, 3.3 X lo3for S04-2, ca. 0.8for r\;O3-, and 0 for C104-. These values supported King, Espensen, and Visco's2 claim that the stability constants for outer-sphere complexes reported by earlier workers were too high. Evans and IL'ancollas3plotted values of A S of formation for the ion pairs vs. A S of hydration values for the anion and found that these plots for the [CO(NHJ~]+~, [ C ~ ( e n ) ~ ]and + ~ ,Fe+a(aq.) cations were parallel lines. By means of an entropy cycle they intt:rpreted this close linear relationship to mean that t'he magnitudes of the entropies of association for ion pairs of the same cation but different anions were dependent upon the differences between the entropy of hydration of the anion and the entropy of hydration of the ion pair. The present work was undertaken to study the effect of temperature on the stability of the outer-sphere ion pairs and to establish values for the thermodynamic functions AH', AF', and AXo for the formation rea,ctions of [Pt (en)3]+4 with various anions. Experimental Solutions.-All stock solutions were prepared as previoudy reported.' It was necessary to deoxygenate the water used in the preparation of the iodide and thiocyanate solutions by sweeping it for 30 min. with 2 stream of nitrogen prior to use. The solutions prepared for the [Pt(en)3f4,C1-] ion pair study contained 1.017 X lO+M [Pt(en)l]+4and1.003 X lO-aM [H*], with a chloride concentration varying from 0.005 to 0.1 Id. The solutions prepared for the other anions studied containled 1.016 X IObs M [ P t ( e r ~ ) ~ ]1.208 + ~ , X lO-lM [H+], and 1.208 X M [C104-]. The concentrations of the complexing anions in these solutions variied from 0.001 to 0.05 M . The acidities of all final solutions were maintained a t pH 3 or less to suppress acid dissociation of the [Pt(en)r]+4. The ionic strength varied depending on the composition of the solution. Procedure.-A Cary Model 14 recording spectrophotometer was used to measure the absorbance of the solutions in matched 1cm. quartz cells. The apparent molar extinction coefficients D were 9easured a t 2600 A. for the C1- and Br- anions and a t 2700 A. for the I- and SCN- anions and at temperatures of lo', 25', and 40". A constant temperature in the cell compartment of the spectrophotometer was maintained by circulating water of (10.0 f O.Z"), (25.0 f 0.2'), or (40.0 f 0.2') as required.
Results and Discussion The spectrophotometric data were evaluated by the same method as employed by Bale, Davies, and Monk4 and later by Kyman and P1ane.l Activity coefficients (1) C. J. Nyman and R. A. Plane, J . Am. Chem. Soc., 82, 5787 (1960). (2) E. L. King, J. H. Eaipensen, and R. E. Visco, J . Phys. Chem., 63, 765 (1959). ( 3 ) M. G. Evans and G. H. Nancollas, Trans. Faradaw SOC.,49,363 (1853). (4) W.D.Bale, E. W. Davies, and C . B. Monk, ibi.d, 52,816 (1956).
NOTES
2492
Vol. 67
TBLE I STABILITY CONSTANTS AND THERXODYNAMIC FUNCTION VALUESFOR IONPAIRSFORMED BY [Pt(ena)]
---
Stability
constant
KI~--
+
WITH
7
VARIOUS ANIOKS
AF'
A30
at 2 6 O . e.u.
-1.69
Anion
Method
10-
400
kcal./mole
a t 25', kcal./mole
c1-
Least squares Individual points
14.9 i0 . 9 14.9 f . 9
17.4 f 0 . 6 1 6 . 8 f .8
19.6 z t 0 . 9 19.6 .9
1.6 f0.5
f0.25
11.0 f2.1
Br-
Least squares Individual points
13.9 If: . 3 13.8 i: . 3
15.2f .3 15.1 f . 4
17.8 f . 3 17.7 zt . 4
1.3 f0.2
-1.61 f0.15
9.7 zt1.0
I-
Least squares Individual points
1 2 . 8 f .4 12.8 f . 4
1 4 . 0 f .5 1 3 . 4 1 .7
15.9 f . 4 1 5 . 2 f .5
1.3 f0.2
-1.56
*to. 25
9.6 f1.2
A P ,
SCN -
Least squares
0
25.
0
were calculated from the Davies form of the modified Debye-Huckel equation5 -log fi
=
AZi2(I1'"(l
+ I"/") - C I ]
(1) where A is a constant dependent upon the dielectric constant and the absolute temperature, 2, is the charge on the ion, I is the ionic strength of the solution, and C is an arbitrary constant. A value of 0.2 was used for C in this study. Values for K10were calculated by the "slope-intercept" method and by the "individual points" method. The best straight line used in the former method was determined by a least squares treatment. The K10 values obtained by each method are listed in Table I and are in close agreement. The discrepancy between the K,O values a t 25' for the [Pt(en)3+4,C1-] and [Pt(en)3+4,Br-] ion pairs reported in this paper and those reported by Nyman and Planel for the same ion pairs can be attributed to the earlier workers inclusion of data from solutions containing anion concentrations greater than 0.1 M . At these higher concentrations the activity coefficients calculated are not as reliable as might be desired. Two trends are evident from the KI0values for the halide series of anions listed in Table I. The K10values decrease with increasing size of the associating anion and they increase with increasing temperature. For the \Pt(en)sf4, SCN-] ion pair the KI0values mere found to be equal to or very close to zero for the entire temperature range studied. This tendency can be attributed to the relatively small charge and large size of the SCY- anion and also to a probable unfavorable entropy change. One would expect a loss of rotational entropy for the SCN- anion on association. The values of the thermodynamic functions calculated for the ion-pair formation are also included in Table I. These were evaluated through the usual thermodynamic relationships. The A H o values reported for the ion pairs formed between the [Pt(en)3]+4and the halides are small. It is doubtful that the reported data can be interpreted to mean anything more than that A H o is positive and in the range of 1 to 2 kcal./mole for the reactions studied. The entropies of association of the ion pairs studied were found t o be positive, which is typical for ion-pair formation.6 They are, however, of nearly the same magnitude, showing only a slight tendency to decrease with increasing anion size. These trends can be better (5) C. W. Davies, J . Chem. Sac., 2093 (1938). (6) J. Lewis and R. G . %'ilkins, "Modern Coordination Chemistry," Interscience Publishers Inc., New Pork, N. Y., 1960, p. 20.
..
0
understood by reference to an entropy cycle for the reaction
+
[Pt(en)3]+4 X- = [Pt(er1)3+~,X-1
(2)
AS3O
[Pt(en)3I4(d4- x-(g)
--+
[Pt(enh+*,X - k )
ASr'
+
[Pt(en)3]+4(aq.) X-(aq.) --+ [Pt(ei1)~+4,X-](as.) From the entropy cycle it can be shown that ASj'
=
ASio
+ A S z O + AX3'
4- AS4'
(3)
where ASl' and ASz' are the negative entropies of hydration of the [Pt(en)3]+4and X-, respectively, and AS," is the entropy of hydration of the ion pair. A S 3 O and Axso are the entropies of association in the gaseous and aqueous states, respectively. By the methods of Latimer' it can be shown that changes in AXs" are small and can be neglected. Since ASz' decreases with increasing anion size, then the entropy of hydration of the ion pair must increase with increasing size of the associating anion. This finding is consistent with the earlier results of Evans and Nancolla~.~ Acknowledgment.-The authors are indebted to the Washington State University Research Committee for financial support. This work is based on the M.S. thesis submitted to Washington State University by D. C. G., 1963. (7)ICV. H. Latimer, Chem. Rev.,18,349 (1936).
PHOTOREDUCTION OF HEXAFLUOROBIBCETYL BY HYDROCARBONS EV THE GASEOGS PHASE AND IN SOLUTION BY I. M. W'HITTEVORE
ARIDM.
SZWARC
Department of Chemzsti y, State Universzty College of FGrestry at Syracuse Universaty, SyTacuse IO, New Yorlc Reeezued May $ 1 , 1968
Photolysis of hexafluorobiacetyll (HFBA) vapor a t 25 and 1.50' produces CO and C2F6 in a 2 : l molar ratio, and if the reaction is continued, all the diketone may be decomposed. Surprisingly, in the presence of aliphatic hydrocarbons, e.g., iieopentane or isobutane, only a small amount of CO is formed. For example, at 65' a mixture of HFBA and 2,3-dimethylbutane in 1:150 molar ratio was photolyzed until 84y0 of the diketone was decomposed (determined by the change (1) Preparation desciibed by L. 0. Moore and J. W. Clark, U. S. Patent 3,065,913 (1962).
