Dec., 1963
2857
KO’PES
The identifica,tion of 1,3 and 1,4-~yclohexadienesuggests additional possibilities for the formation of dimer
0
C6H7.-t
---t
/,C& C,H,-C&
2Cp,II,-CeHp,.--c CCH7-C6Hj 2CJIi. 4 CEH7-CsII:
-cas
\Polymer
+ CCH7-Ce,H7
This mechanism is supported by the formation of phenylcyclohexadiene (or bicyclohexadiene) and biphenyl in the reaction of acetylperoxide and 1,4-cyclohexadiene in benzene solution.8 The formation of cyclohexadienes shows that benzene in the liquid phase reacts with hydrogen atoms by addition. This is in agreement with the reactions of alkyl’s9 and OH radicals. lo ( 5 ) G. E. Gibson, N. Blake, and >I. Kalm, J . Chem. Phys., 21, 1000
(1953). (6) R. W. Fessenden and R. H. Schuler, zbid., 38, 773 (1963). (7) E. L. Eliel, S. hleyerscn, Z. Welvalt, and 8. H. Wilen, J . A m . Chem. Soc., 82, 2936 (1960). (8) 34. K. Eberhardt, unpublished results. (9) M. Levy and M. Szwarc, J . A m . Chem. Soc,, 7 6 , 5981 (1954). (10) L. M. Dorfman, I. A. Taub, and R. E. Blhler, J . Chem. PhLs., 36, 3051 (1962).
CRYOSCOPY AND IONIC MIXING INTERACTIONS I N MOLTEN NITRATES BY G. J. JANZ AND T. R. KOZLOWSKI Departmenf of Chemistry, Rensselaer Polytechnic Institute, Trov, New York Received June 81,1968
Applied to molten salt mixtures, cryoscopy is convenient for investigations of ionization processes, reaction and interaction phenomena, in molten electrolytes. For the dilute iconcentration range (less than 0.15 m) the experimental data may be expressed by the simple relation A T = vKfm
(1) where m is the molality and v, the number of particles “foreign” to the solvent. The cryoscopic constant, Kf, is given by
RT02
(2)
Kf=aHfnl
where A H f and TOare the heat of fusion and temperature of fusion of the solvent. The value of nl is the number of molles of solvent in 1000 g. An extension of the theory to include a contribution from the energy of mixing, G m i x , follows from a derivation given by For1and.l For dilute solutions, the resulting equation is AT
=
AEm ix Ir‘fQm- __
ASf
(3)
where E n l i x will be some function of concentration. Results for a series of cryoscopic measurenients are (I) T. Forland, “On the Properties of Some Mixtures of Fused Salto,” N. T. 1%.Trykk, Trondheim, Norway, 1958.
reported in this communication to examine the significance of this parameter. Experimental The crj oscopic assembly used has been previously dwcribed in some detail by Janz and Solomons.2 Sodium nitrate (m.p. 306.8’) was used as the solvent. The solutes arelisted in Table I and correspond to various classes of ionic mixtures, z.e., AC in BC, BD in BC, XC2 in BC, and XD2 in BC. Hygroscopic nitrates, Mg(NO3)2.4Hz0, were dried by heating mixtures with NaSOs until fusion under vacuum. Anhydrous alkaline earth chlorides were prepared by heating the hydrates up to-and beyond-their respective melting temperatures in an atmosphei e of HCl. Salts, normally anhydraus, were used directly after air-drying at 130”. The cryoscopic additions of the solutes (0.1 to 0.2 9.) to approximately 100 g. of NaXOd were designed to cover the concentration range from infinite dilution to 0.120 M . All weights were determined to zkO.0001 g., temperatures were recorded to 10.01”. TABLE I CRYOSCOPY AND IONIC INTERACTIONS IN SODIUM NITRATE” Kf, Soluta
ZII,,,,
deg./mole
Y
LiN03 RblJOs
16 €4 16 32 cST\To3 16 03 TlWOZ 15 75 NaF 17 26 15 96 NaCl NaBr‘ 13 03 NaI‘ 10 88 15 14 Mg(SOa)z 15 49 Ca(N03)~ 16 31 Sr(NO& Ba(KO& 16 17 CaClz‘ 43 34 SrClzO 45 45 BaC12C 45 47 a M.p. -506.8’. bSee sign of A&,,,.
1 00 1 01
cal./mole
Exothermic Exothermic Exothermic Endothermir Exothermic Endothermic
A P , b
oal./mole
Exothermic Exothermic Exothermic Endothermic
0 99 98 1 07 0 99 Endothermic 81 Endothermic 67 94 Endothermic 96 Endothermic Exothermic 1 01 Exothermic Exothermic 0 98 Exothermic Exothermic 2 70 2 83 2 84 ref. 7-11. cision insufficient for
Discussion Van Artsdalen4 reported cryoscopic values of Kf for 22 solutes in NaNOd. These were in good agreement with a value calculated from calorimetric data given by Goodwin and Ka1mus,5 uix., approximately 15 deg./ mole. However, inspection of the original literature5 shows that the quoted heat of fusion (above) is in error (possibly typographical). An independent redetermination of the heat of fusion by phase transition calorimetry in this Laboratory6 confirms that the cryoscopic constant is 16.14 0.16 deg./mole. The Kf values determined from a least squares analysis of the cryoscopic data for the present 15 different mixtures are given in Table I. Using these and the preceding cryoscopic constant based on heat of fusion calorimetry, values for Y, the number of particles “foreign” to the solvent, as listed in column 3, Table I, are gained, The fluctuations in the values of v (column 3, Table I) for solutes theoretically introducing one particle “foreign” to the solvent warrant comment. If solid solution is ruled out and the solutions are cheinically stable, the fluctuations in Y may reflect ionic interactions in accord with the ~ , ,term , of eq. 3. Since the cryoscopic measurements are in regions of low concentration, the predicted nonlinearity in the graphical analysis
*
(2) G. J. Jan5 and C Solornons, Rev Scz. Inst, ,as, 302 (1958) (3) G. J Janz and C Solomons, Anal Chem , 31, 623 (1969) (4) E R Van Artsdalen, J . Phys Chem , 60, 172 (1956). ( 5 ) H 11 Goodwin and H T. Kalmus, Phus Reu , 88, 1 (1960), Jmz, J. I W l y , and J Pbrano, unpublished work
r
NOTES
2858 [aEmix
= f (concn.) ] is apparently masked in the experi-
mental scatter. Values for BInjy, therefore, cannot be ascertained from such measurements, but the determination of the sign of the term nevertheless is possible. Thus an experimental cryoscopic constant which is greater than calculated from phase-transition heat of fusion calorimetry corresponds to a negative value for A E m i x , i e . , exothermic mixing interactions in the binary ionic melt. Similarly, fluctuations corresponding to a cryoscopic constant less than the calorimetric value correspond to a positive energy of mixing effect (endothermic) in the ionic mixtures. The signs for S m i x thus predicted for each solute in molten NaN03 are given in column 4 of Table I. Enthalpies of mixing data have been reported elsewhere by Kleppa and his eo-workers for a large series of binary nitrate The signs for the enthalpies of mixing thus determined by direct soIution calorimthat etry are in cplumn 3, Table I. Inspection shows the prediction of the ionic interaction term, AEmix, corresponds exactly with the signs of the directly observed enthalpy of mixing with one exception, i e . , Ca(NO&. The results leave little doubt that an indication of the nature of the ionic interactions and enthalpies of mixing ( i e . , next nearest neighbor repulsion, partial covalency, packing effects7-'1) can be gained directly from the fluctuations in the v-values of cryoscopy if the freezing-point data are precise, and accurate heat of fusion data for the solvent are known. Acknowledgment.-This work was made possible, in large part, through financial support from the U. S. Air Force, Office of Scientific Research, Air Research and Development Command, Washington, D. C.
sJmix
(7) 0. J. Kleppa a n d L. S.Hersch, J . Chem. Phys., 36, 213 (1962). (8) 0. J. Kleppa and L. S. Hersch, tbzd., 36, 544 (1962). (9) 0. J. Kleppa, R. R. Clarke, and L. S. Hersch, ibtd., 35, 175 (1961). (10) 0. J. Kleppa, J . P h y s . Chem., 66, 1668 (1962). (11) 0. J. Kleppa and S. V. Meschel, ebid., 67, 669 (1963).
PHOTOCHEMICAL h S D RADIATION CHEMICAL REDUCTIOX OF CERIC 10s I N AQUEOUS SULFURIC ACID SOLUTIOKS. EFFECT OF FOR,MIC ACID' BY THOMAS J SWORSKI Chemistry Division, Oak Ridge National Laboratory,2 Oak Ridge, Tennessee Received Julu S, 1968
The OH radical was postulated3 as an intermediate in the photochemical reduction of ceric ion in sulfuric acid solutions. A comparative study of the radiolysis4 and photolysis5 of ceric ion-cerous ion-thallous ion mixtures yielded kinetic evidence in support of this postulate. Although both thallous ion4,6 and formic enhance the radiolytic reduction of ceric ion in 0.4 AI sulfuric acid solutions, there is one striking difference. (1) Preeented a t the 145th National Meeting o f the American Chemical Society, New Pork, N. Y . , Sept., 1963. (2) Operated for the Atomic Energy Commission by Union Carbide Nuclear Company. (3) T. J. Sworski, J . A m . Chem. Soc., 77, 1074 (1955). (4) T. J. Sworski, Radiation Res., 4, 483 (19.56). (5) T. J. Sworski, J . A m . Chem. Soc., 79, 3655 (1957). 16) T. J. Sworski, ibid., 77, 4689 (1955). (7) H. E. Spencer and G. K. Rollefson, ibid.,77, 1938 (1955). ( 8 ) T. J. Sworski, ibid., 7 8 , 1768 (1956). (9) T. J . Sworski, Radialion Res., 6 , G45 (1967).
Vol. 67
While enhancement by thallous ion is essentially dependent only on [Tl+J/[Ce+3], the enhancement by formic acid is dependent not only on [HCOOH]/ [Cef3]but also markedly on the total concentration of cerous ion and formic acid at any particular [HCOOH]/ ~ , reaction ~ [Ce f 3 ] . This difference was a t t r i b ~ t e dto of OH radical with sulfuric acid. The effect of formic acid on the photochemical reduction of ceric ion in sulfuric acid solution was investigated to obtain further evidence for the OH radical as an intermediate and for reaction of OH radical with sulfuric acid. Experimental The experimental procedures employed were previously described in more detail.4~z~Q The source of ultraviolet light was a General Electric 4-watt germicidal lamp which emits almost entirely light of 253.7 m*.lo The flux incident upon the solutions was measured with an equal volume of actinometer solution .I1 The source of ?-radiation was a nominal 300-c. Coo0 source.12 Radiation dosimetry in 0.4 M sulfuric acid solutions was based on a yield of 15.6 ferric ions per 100 e.v. for the ferrous sulfate dosimeter.1a A molar extinction coefficient of 2210 a t 25" was used for ferric ion at 305 mp.4 Formic acid solutions were prepared by addition of Baker and Adamson reagent grade sodium formate. The concentration of undissociated formic acid was assumed to be equal to the concentration of added sodium formate. Ceric ion and ferric ion concentrations were determined spectrophotometrically with a Cary hfodel 11 recording spectrophotometer. A molar extinction coefficient of 5580 at 320 mp was used for ceric ion in 0.4 M sulfuric acid solutions13 and relative values of 5380 and 5710 were determined for 0.16 and 0.8 M sulfuric acid solutions, respectively. Formic acid has no measurable effect on these molar extinction coefficients a t the concentrations employed.
Results The notation employed is that previously used4 for radiation chemical studies: subscript notation for quantum yields and 100 e.v. yields of assumed intermediates such as @OH and GOH, respectively, and parenthetical notation for quantum yields and 100 e.v. yields of observed products such as @(Clef3) and G(Ce +9, respectively. Photochemical Studies.-Initial @(Ce+3) values, a t constant light intensity, are markedly dependent on formic acid concentration as shown in Fig. 1 for 0.4 Af sulfuric acid solutions. A similar dependence was observed for 0.16 and 0.8 $1 sulfuric acid solutions. Changes in ceric ion concentration were followed by use of intermittent irradiations and the data shown in Fig. 1 for any particular formic acid concentration were obtained from a single solution. @(Ce+3)decreases with increasing time of photolysis in all solutions due to the internal filter action of cerous ion and to reaction of cerous ion with OH radical. In the photolysis of ceric ion-thallous ion mixtures in 0.4 M sulfuric acid solutions, initial @(Ce+3)was shown4 to be equal to 2+0H. Initial @(Ce+3)values of 0.208, 0.204, and 0.195 were determined for ceric ion-thallous ion mixtures in 0.16,0.4, and 0.8 M sulfuric acid solutions, respectively. Iio significance is attached to the variation in @(Ce+3)values and the assumption is made that @OH is independent of sulfuric acid concentrations employed in this investigation. Radiation Chemical Studies.-Initial G(Ce +3) values are also markedly dependent on formic acid concentration as shown in Fig. 2 for 0.4M sulfuric acid soh(10) (11) (12) (13)
J. P. H u n t and H. Taube, J . A m . Chem. Soc., 74, 5999 (1962). W. G. Leighton and G. S. Forbes, ibid., 6% 3139 (1930). J. .4. Ghormley and C. J. Hochanadel, Rev. Sei.lnstr., 2 2 , 473 (1951). C. J. Hochanadel and J. A. Ghormley, J . Ciieni. Phgs., 21, 8S0 (19.53).
