NOTES
3053 Thus some other more easily measurable and sensitive thermodynamic property of the limiting region would be desirable. This note reports on such a criterion in the form of the enthalpy of transfer Z 7 z of various strong electrolytes (MX) from water to 1 rn aqueous nonelectrolyte (ne) solutions
Thermodynamics of Aqueous Mixtures of Electrolytes and Nonelectrolytes. V. Enthalpies of Transfer in the Limiting Region at 25"
MX(HzO) = MX[HzO-ne (m3 = l ) ]
by J. H. Stern, J. Lazartic, and D. Fost
and the enthalpy of transfer Z 7 3 of two nonelectrolytes from water to 1 m aqueous electrolyte solutions
Department of Chemistry, California &ate college, Long Beach, California 90801 (Received March 81, 1968)
ne(H2O) = ne [H20-MX (mz = 1)1 The limiting region of electrolyte-nonelectrolyte interaction in aqueous solution is of particular interest. Theoretical considerations as well as numerous experiments2 show that for this region the activity coefficient of the nonelectrolyte y3 in the presence of a strong 1:1 electrolyte a t molality mz may be expressed in the form2 log
y3
= lc3~mz
(1)
where k32 is a specific electrolyte-nonelectrolyte interaction constant. This equation, while generally obeyed only a t low nonelectrolyte concenirations, is usually applicable over a wider range of electrolyte concentrations.2 However, at sufficiently high electrolyte molalities, higher order terms must be added to the right side of eq 1. The analogous limiting equation for a 1:1 electrolyte is
2 log YZ = k~3rn3
( 7 ) r n a
log Yz = 2 ( c ) r n z
(3)
applied to eq 1 and eq 2 leads to the equality IC32
=
h23
(4)
The experimental method for obtaining k32 is very frequently based on the measurement of the nonelectrolyte solubility as a function of m2. I n the limiting region4 SO/S= Jc32m~
(7)
I n both cases the concentration of the transferred solute is very low. Thus Bzand Bs represent two similar processes, with the parts of the electrolyte and the nonelectrolyte interchanged. I n the limiting region the enthalpies are related to the temperature derivatives of the interaction constants.lb
-
- h H 3
2.303RT2 -
-hHz 2.303RT2
bk32
m2bT akz3 bT
= m3-
(9)
With equal temperature derivatives of the interaction constants, the limiting interaction region may be characterized by
(2)
where yz is the mean ionic activity coefficient of the electrolyte in the presence of a nonelectrolyte at molality ma, and kza is the nonelectrolyte-electrolyte constant. Both activity coefficients are referred to the hypothetical 1 m standard state in pure water. The McKay cross-diff erentiation relation3
a log y3
(6)
(5)
where S and So are the solubilities of the nonelectrolyte in the presence of the electrolyte a t r n 2 and in pure water, respectively. Application of the solubility method to the direct determination of k23 is restricted to sparingly soluble electrolytes, and this, unfortunately, precludes the investigation of the more important and highly soluble 1 :1 electrolytes. There is one published experimental verification of eq 4 for the systems waternitromethane-KC104 or CsC104.s
Enthalpies of transfer are reported for LiC1, LiBr, KC1, and KBr from water to 1 m HOAc and for KC1 to 1 m CHaN02. I n the HOAc systems, A x 2 was obtained by the difference between the measured enthalpy of dilution of the salt in 1 m HOAc and that in pure water,6 respectively, between the same initial and very low final concentrations. For KC1-CH3N0z, BZ was obtained by the difference between enthalpies of solution of KCl in 1 m CHaNOz and in water,6 respectively. Values of hHa are from results reported earlier.'bvO
Experimental Section The calorimeter' and general experimental procedurelb have been described elsewhere. Sealed glass (1) Previous three papers in this series: (a) J. H. Stern and J. Nobilione, J . Phys. Chem., 7 2 , 1064 (1968); (b) J. H. Stern, J. P. Sandstrom, and A. Hermann, ibid., 71, 3623 (1967); (c) J. H. Stern and A. Hermann, ibid., 71, 309 (1967). (2) F. A. Long and W. F. McDevit, Chem. Rev., 51, 119 (1952). (3) G. N. Lewis and M. Randall, "Thermodynamics," K. 8. Pitzer and L. Brewer, Ed., McGraw-Hill Book Co., Inc., New York, N. Y . , 1961, pp 572, 586. (4) See ref 3, p 684. (5) G. R. Haugen and H. L. Friedman, J . Phys. Chem., 60, 1363 (1956). (6) "Thermal Properties of Aqueous Uni-univalent Electrolytes," V. B. Parker, Ed., Bulletin NSRDS-NBS2, U. 5. Government Printing Office, Washington, D. C., 1966. (7) J. H. Stern and C. W. Anderson, J . Phys. Chem., 68, 2528 (1964).
Volume 78, Number 8 August 1968
3054
NOTES
ampoules with weighed quantities of concentrated electrolyte solutions (4.500 m KC1 and 4.500 m LiCl and 2.500 m KBr and 2.000 m LiBr) or solid KCl were submerged and crushed in 450 g of 1 m HOAc or CHaN02, respectively. Enthalpies of dilution of the HOAc solutions due to transfer of small quantities of water from the ampoule were negligible. Variations in the quantity of electrolytes in the ampoule and, consequently, in the final concentration (over-all range 0.004-0).014m, the majority of runs being below 0.008 m) resulted in essentially constant enthalpies. A total of 27 runs, with a minimum of four per electrolyte, were carried out. Nitromethanelo and all other materialslb were described elsewhere.
Results and Discussion The enthalpies of transfer are shown in Table I. The over-all experimental error in all tabulated values of AH2 is f20 cal/mol, based on standard deviations of the means of measured enthalpies multiplied by factors necessary to give 90% confidence levels and estimated
Table I : Summary of Nonelectrolyte
HOAc
and A%a9
Z 8 ,
Electrolyto
cal/mol
osljmol
LiCl LiBr
110 30 190 -220 -150
110 60 150 190 150
-
KC1
CHdY"e
KBr KCI
-
errors for values from ref 6. The over-all experimental error for Z a on the same basis is kt10 cal/mol.*b It may be noted that in all but one case a 2 and AHa are equal, within the limits of experimental error. Thus eq 10 may be expressed in the form
Either set of enthalpies also allows prediction of the interaction-constant and activity-coefficient behavior as a function of temperature. Under limiting conditions the enthalpies are ionic properties;lb Le., they must consist of independent additive contributions from the cation and anion, respectively. Table I1 shows the average ion-contribution differences A(AH2)and A ( D 3 ) l bfor the HOAc systems. Table I1 : Ion-Contribution Differences A(aa),
Ion pnir
oal/mol
Li +-K + C1--Br -
270 50
The Journal of Phyaical Chemistru
A(aa), csl/mol
250 40
As a consequence of eq 11, values of A(B2) and A(AH3) must also be equal for a given ion-pair difference, and this additional requirement is met satisfactorily. Acknowledgment. The authors wish to thank the National Science Foundation for financial assistance.
The Thermal Decomposition
of Perfluoropropene by Richard A. Matula] Fluid Dynamics Laboratory, Department of Mechanical Engineering, The University of Michigan, Ann Arbor, Michigan (Received March 82, 1968)
The thermal decomposition of a few of the low molecular weight fluorocarbons have been studied. A number of investigatjors2-6 have studied the pyrolysis of tetrafluoroethylene in the temperature range 300-800°, It has been shown that this reaction system can be divided into three phases. At low temperatures (T < 550°), octafluorocyclobutane is the main product; a t medium temperatures (550 < T < 700"),perfluoropropene and a perffuorobutene are produced; and at high temperatures (T > 700") perfluoroethane and nonvolatiles are produced. The thermal decomposition of octafluorocyclobutane4~e-8 has been studied from 360 to 930". Atkinson and A t k i n ~ o nhave ~ ~ studied the pyrolysis of perfluoropropene and perfluoroisobutene in the temperature ranges 600-675 and 700750", respectively. The purpose of the present investigation is to study the thermal decomposition of perfluoropropene in the temperature range 550-675'.
Experimental Section The experiments were conducted in a cylindrical Vycor reactor vessel (250 mm long and a 60 mm i.d.), which was maintained a t a constant temperature by an electrically heated furnace. The reactor temperature, which was measured with the aid of four chromelalumel thermocouples, was controlled to within iO.5" (1) Department of Mechanical Engineering, Drexel Institute ob Technology, Philadelphia, Pa. (2) W. T.Miller, Jr., "Preparation, Properties, and Technology of Fluorine and Organic Fluoro Compounds," McGraw-Hill Book Co., Inc, New York, N . Y . , 1951,Chapter 32. (3) J. R. Lacher, G. W. Tompkin, and J. D. Park, J . Amer. Chem. Soc., 74, 1693 (1952). (4) (a) B. Atkinson and A. B. Trenwith, J. Chem. Soc., 2082 (1953); (b) B. Atkinson and V. A. Atkinson, ibid., 2086 (1957). (5) G. A. Drennan and R. A. Matula, Fluid Dynamics Laboratory Report No. 68-1,The University of Michigan, Ann Arbor, Mich., 1968. (6) B,F. Gray and H. 0. Prichard, J . Chem. SOC., 1002 (1956). (7) J. N. Butler, J. Amer. Chem. SOC.,84, 1393 (1962). (8) A. Lifshita, H. F. Carroll, and S. H. Bauer, J . Chem PhyS., 39* 1661 (1963).
