NOTES
2229
Acknowledgment. I would like to thank Dr. A. Carrington for suggesting this investigation, and the Shell Co. of Australia under the tenure of whose Postgraduate Scholarship the research was carried out Enthalpy of Transfer of Sodium Chloride from Water to Aqueous Hydrogen Peroxide at 25"
by J. H. Stern* and W. R. Bottenberg, Jr. California State College at Long Beach, Long Beach, California 90801 (Received February I , 1971) Publicatwn costs assisted by the National Science Foundation
Figure 1. Esr spectra (upper) of copper acetate in acetic acid solutions, 2% and 25% by volume water, compared with fitted curves of superposed lorentzians. The three small peaks are peroxylamine disulfonate marker (external). High field is to the right. The spectra were taken on a Varian V-4500-1OA spectrometer.
so obtained. The parameters for the computed spectra6are
w ( M ~ ,= ~ ~33) x 107{ [i
+ (B/A)& +
+
( C / A ) M ~ ~ I ( T ~ / (TO/T~)] T ~ ) sec-' Solution
g
aJG
rafm
BJA
CIA
273HzO 25%HzO
2.17 2.185
45 45
3.33 2.50
0.38 0.37
0.033 0.033
The agreement in shape would indicate that the spectra can indeed be accounted for by conventional relaxation mechanisms, and that in fact even the spin-rotational contribution ( E )is of minor significance. The variation in g value and the earlier observations are consistent with the following sequence of reactions. Addition of water to the glacial acetic acid converts the (triplet) dimer to doublet monomer
+ 2Hz0 +2[(H20)&u(ac)2]
H20"2u(ac)4Cu.H20
This complex with relatively low g value would presumably be quasitetragonal. It in turn takes on more water in the competing reaction (HzO)&u(ac)z
+ 2Hz0
+Cu(HzO)&)z
with the acetates now singly coordinating in the axial positions. The basis for postulating this as the final entity is that even in pure water the spectrum does show hyperfine shoulders, the peak-to-peak separation is slightly greater than that of the hexaaquo ion, and the g value is lower (2.185 vs. 2.20). As for the dimermonomer exchange reaction, identification of the nuclear hyperfine splitting in the initial doublet shows that the lower limit on the monomer lifetime is sec.
A previous paper' reported on the calorimetric enthalpy of transfer LEz of NaCl from pure water to aqueous urea, a t five mixed solvent (ms) compositions ranging from dilute urea to urea mole fraction 0.18. The present contribution describes an analogous study of SI2of NaCl from water to aqueous HzOz (up to H20z mole fraction X 3 = 0.225). I n contrast to organic nonelectrolytes (ne) (for example, urea,l acetic acid, and ethylene glycol2), little is known about the physicochemical effects of this very important inorganic ne on water and its structure. It may be noted that aqueous HzOz has an unusual broad maximum in dielectric constant (E) vs. X 3 isotherms between -40 and 30°.3 At 25" and over the composition range of the present study the transfer occurs under nearly iso-e conditions. This a t least minimizes bulk coulombic Born effects4 compared with other aqueous organic ms, where large changes often in E occur.
Experimental Section The enthalpies of transfer were obtained from the difference of the enthalpies of solution of crystalline NaCl in the ms and pure water, AH,, and AH2', respectively; h H z = AH, - AHz'. The calorimeter and general calorimetric procedures were described e l ~ e w h e r e . ' ~ ~ Sodium chloride was AR grade. Hydrogen peroxide solutions were freshly prepared from 30 or 50% stock solutions (Mallinckrodt and Fisher, respectively) and distilled, deionized water. Both were AR grade stabilized with trace phosphates (ca. 0.003-0.03%)) and the rns were analyzed by adding excess KI and titrating the released Is- with thiosulfate.6 (1) J. H.Stern and J. D. Kulluk, J. Phys. Chem., 73, 2725 (1969). (2) J. H.Stern and J. M. Nobilione, ibid., 72, 1064, 3937 (1968). (3) P.M.Gross, Jr., and R. C. Taylor, J. Amer. Chem. SOC.,7 2 , 2075 (1960). (4) R. G. Bates in "Hydrogen-Bonded Solvent Systems," A. K. Covington and P. Jones, Ed., Taylor and Francis, London, 1968, p 49. (5) J. H. Stern and C. W. Anderson, J. Phys. Chem., 68, 2528 (1964). (6) I. M. Kolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis," 3rd ed, Macmillan, New York, N. Y.,1952, p 592. The Journal of Physical Chemistry, Vol. 76, No. 1 4 , 1971
2230
NOTES
Results and Discussion
Table I: Enthalpies of Solution and Transfer of NaCl NaC1, Xa
g
O.OOO(AHzO)
0.1674 0.4274 0.3328 0.3164 0.3158
0,0244
0.3204 0.3084 0.3142
0,0639
0.3112 0.3108 0.3110 0.3150
0.113
0.3048 0.2990 0.3181
0.143
0.3191 0.3041 0.3101
0,225
0.2993 0.2951 0.3167
AH 2 Solvent, and AH%’, g
cal/mol
AT2,
oal/mol
408 997 417 998 409 1006 416 973 1002 412 Mean 990 i 15 (975 i 5)a 417 799 413 770 423 783 Mean 785 i 25 -205 507 413 413 513 427 526 498 414 Mean 510 f 15 -480 306 416 413 286 27 1 415 Mean 290 i 30 -700 419 102 414 127 416 121 Mean 115 3~ 20 -875 416 -187 415 -160 415 -177 Mean -175 f 25 -1165
i 30b
f 20
f 35
f 25
i 30
a Calculated from data tabulated by V. B. Parker in “Thermal Properties of Aqueous Uni-univalent Electrolytes,” NSRDSNBS2, National Bureau of Standards, U. S. Government Printing Office, Washington, D. C., 1965. b Over-all uncertainty of T H z is [ (eAH20)a ea^,)^] ‘I2 where eAHp and eAH2 are tabulated uncertainties of AH2’ and AH2, respectively.
+
The Journal
of
PhVsical Chemistry, Vol. 76, N o . 14, 19YI
All enthalpies of solution of NaCl are shown in Table I together with values of Elz a t all experimental compositions. The final NaCl concentration m2 in the majority.of runs was ca. 0.01 m. The data are reported within 90% confidence limits. It is assumed that El2 values are approximately equal to those a t infinite dilution of NaC1.l The negative enthalpies of transfer are lower in magnitude than those in the urea system. For example, a t ne mole fraction Xs = 0.113 the values of a H 2 differ by ca. 200 cal/mol in the two systems. I n the absence of data leading to the free energies of transfer, it may be assumed that aH2 is approximately equal to T a t , where is the entropy of transfer. This may be justified on the basis of the well established compensatory effect of the enthalpy and entropy in minimizing the free energy of transfer in a variety of aqueous ms systems, including urea. This assumption is also supported by the low values of the specific interaction coefficient of H 2 0 2in aqueous NaCl.7-9 As with urea, the negative enthalpy or entropy of transfer indicates that the nonplanar hydrogen peroxide has a strong specific effect on water, resulting in net water structurebreaking.
Acknowledgment. The authors are grateful for financial support by the National Science Foundation. (7) M. H. Gorin, J . Amer. Chem. SOC.,57, 1975 (1935). (8) R. Livingston, ibid., 50, 3204 (1928). (9) J. H. Stern, J. Lazartic, and D. Fost, J . Phys. Chem., 7 2 , 3053 (1968).
