H. BLOOM, J . 0.U o c ~ ~ i rN. s , E. .
2044
sociation pressure data of Na2S04.10Hz0. It has been very accurately measured b y a number of observers and the value 19.19 cm. Hg = 0.02525 atm. is probably accurate to 0.1%. Thus for hydration
means of the values used here yields a discrepancy of 1.54 cal. deg.-l mole-'. However, as noted above, their values of the heat of hydration and eiitropy vary by substantial amounts from the ones determined here. The agreement results partly from inherent cancellation of some of the errors due A F = lORT In P dissoc. to maldistribution of water in their samples and A F z ~ ~ . I ~=x -21,795 . cal. mole-' partly due to chance cancellation within about their The heat of hydration may be obtained from the estimated 0.3 cal. deg.-' mole-' error 011 their heat of solution data including the value -560 cal. value ofJOT c, d 111 T . Pitzer and Coulter placed mole-' for NazS04 to infinite dilution obtained by Pitzer and C ~ u l t e r . Their ~ value is supported by their data books at our disposal and i t was of inthe more recent work of C o ~ g h l i nwho , ~ made solu- terest to check some preliminary data obtained near tion measurements at 30". Coughlin's result cal- the eutectic region shortly after the sample had been culated to 25" is -541 =t 35 cal. mole-' in satis- placed in the calorimeter. This showed a heat factory agreement with the result of Pitzer and absorption of 127 cal. mole-' a t the eutectic. After standing for 17 days the heat absorption a t Coulter. The heat of vaporization of waterlo is taken as the eutectic had decreased to 23.5 cal. mole-' show 10,520 cal. mole-' when the ideal gas is the final ing that water was reaching the anhydrous portion state. The entropy of water'l in the ideal gas and converting i t to the decahydrate. This constate is available from spectroscopic data. S298.16 tributes evidence in line with the earlier statement above to the effect that the samples used in the heat = 45.106 cal. deg.-' mole-' on the scale 0°C. = 273.16"K. When this is corrected to the new of solution measurements of P. and C. niay have scale, 0°C. = 273.15"K., S298.150~. = 45.104 cal. had some incomplete adjustment of the n-ater content. Additional evidence may be iioted from the deg.-' molew1. The above values give for the entropy of hydra- fact that the heat capacities of P. and C. above the eutectic temperature are considerably higher than tion the present results due to a heat of s ilution effect. NaZSOd(s) 10H?O(g) = NaZSOa.lOHzO(s) Our experience with other hydrates leads us to conclude that this particular hydrate system can AH - A F -124,749 21,795 AS = distribute water a t a much greater rate than is posT 298.15 sible in most other cases many of which might re= -345.31 cal. deg.-l mole-' quire years. SS'ithin the limits of accuracy the disS deca. = S anh. l O S ~ t o ( , )- 345.31 order effect of 1.51 cal. deg.-' mole-' is equivalent = 35.73 +- 451.04 - 345.31 to I< In 2 = 1.38 cal. deg.-l mole-' which could = 141.46 result from one water molecule with a double choice TheJOT C, d In T given above for Na2S04.10H20 of position or a grouping with multiple choice of was found to be 139.05 cal. deg.-' mole-'. The dis- orientation. I t seems highly probable that most crepancy indicates that Na2S04.10H20 retains of the n-ater molecules are held in definitely ordered 141.46 - 139.95 = 1.51 cal. deg.-'mole-lof residual arrangement by the attracting ions. In general entropy. This confirms the result obtained by the orienting forces between ions and water inolePitzer and Coultera4 Pitzer and Coulter used cules are so great that in nearly all cases order will earlier values of the entropy and heat of vapori- be produced a t the temperatures where the hyzation of water. Correction of their results hy drates are formed froin solution. Cases where two or more positions of nearly equal energy exist. lead(9) J. P. Couglilin, THISJ O U R N A L , 77, 808 (1955). ing to the persistence of disorder to temperatures (10) 11, K. Papadopoulos and LT7, F. Giauque, ibid., 77, 2740 so low that an ordering mechanisni cannot operate, (1955). are likely to occur vcry rarely. (11) "Selected Values of Chemical Thermodynanic Propcrties,"
+
+
+
Sational Bureau of Standards Circular No. 500, issued 1952.
BERKELEY, CALIFORIIIA
[CoSTRIBUTIOA' FROM THE DEPARTMENT O F CHEMISTRY,
UNIVERSITY
O F ~'ENNSYL\'ANIA]
Vapor Pressure and Heat of Vaporization of Some Simple Molten Electrolytes BY H. BLOOXI, J. O'M. BOCKRIS,N. E. RICHARDS AND K. G. TAYLOR R E C E I V E D hTOVEhlBER
18, 1957
The vapor pressures of the molten electrolytes NaBr, AgCI, AgBr, ZnC1 p , ZiiUr2, CdBrz, CdIz and PbBrt have been ineastired over a pressure range of 2 to 600 mm. b y a boiling point metiiod. Heat ani1 entropy of vaporization, and normal boiling puiiit have been calculated for each elwtrolyte.
Introduction Investigation vapor pressure of lnolten electrolytes is beset with considerable experilnelltal difficulty which has led to the publicatioii of
Ituiiieruus, widely varying results for iliaay i l l dividual substances. Kelleyl has coordinated and assessed the best available inlorination 011 vapor ( 1 ) I< I;
t ; c ~ i c y , 1:
s.
i i L , r c L ~,,I II,,,,-., I3uiletin
s CdL; and ZnClz > ZnBrz. This agrees with Biltz and Klemm,23Mulcahy and HeymannZ4 and others who arrived a t similar conclusions from a study of electrical conductance. Entropy of Vaporization.-Lennard- Jones and D e v o n ~ h i r e ,Mayer,26 ~~ and Eyring and HirschfelderZ7have shown that Trouton's rule can be derived from well founded equations of state for liquids. For the present group of molten salts, the entropy of vaporization varies between 24.6 and 30.5 e.u. Although this range is higher than for norinal unassociated room temperature liquids, these values do not necessarily indicate association in the molten electrolyte. Molten KCl and NaCl, commonly regarded as ionic liquids, have entropies of vaporization of 22.9 and 23.4 e.u., respectively.8 The relatively high values of the entropy of vaporization for molten salts have been explained by Eyring and HirschfelderZ7on the basis of their high boiling points and small molar volumes. These melts may therefore be regarded as normal, unassociated liquids. Acknowledgments.-The authors wish to acknowledge with thanks the financial support of this work by the office of Ordnance Research under Contract No. DA-436-034-ORD-765. They wish also to thank Professor J. G. Miller, University of Pennsyl-iania, for the loan of a cathetometer.
( 0 ) Strong, "Procedures in Experimental Physics," Prentice IIall Book Co., New York, X. Y . , 1951, p. 99. (10) Knudsen, "The Kinetic Theory of Gases," John Wiley and Sons, I n c , pi-ew York, pi-. Y . , 1950, p. 37. (11) 0. Ruff and S. Mugdan, Z . aiiorg. Chein., 117, 147 (1923). (I?) 13. Von Wartenburg and P. Albrecht, Z. Eleklrochem.. 27, 162
(19) B. Greiner and K. Jellinek, 2. physik. Chent., A166, 97 (1933). (20) 0. H. Weber, Z . anorg. Chent., 21, 305 (1899). (21) G . C. Schmidt and R. Walter, A n n . Physik, 72, 565 (1923). ( 2 2 ) F.Valmer, Pliys. Z.,30, 590 (1929). (33) W Biltz a n d W . Iilemm, Z . un07.e. Chem.. 162, 267 (1928). (24) hr. F.K. hfulcahy and E. Heymann, J . Pkys. Cham., 47, 485 (1943). ( 2 5 ) J. E. Lennard-Jones a n d A . F. Devonshire, Proc. Roy. Soc ( T m z d o n ) ,165A, 1 (1938). (?U) J. E. Mayer. J . Chem. Phrs., 6. G7 (1937); J. E, >.layer and P. G. Ackermann. i b i d . , 5 , 74 (1937). ( 2 7 ) H. Eyring and J. 0. Hirscbfelder, 2 . P/iys. Chein., 41, 249 (1937).
(1921).
(13) H Von Wartenburg and 0 . Bosse, i b i d . , 28, 384 (1922). (14) K. Jellinek and -4. Rutiat, Z. p h y s i k . Chem., A143, 55 (1929). ( I n ) 1;.Jellinek and R. Koop, ibid,, A145, 305 (1929). ( 1 6 ) D. N. Tarasenkov and G . V. Skulkova, J . Gen. CheJn..L:.S.S.K , 7, 1721 (1937). (17) D. pi. Tarasenkor and A. V. Babaeva, i b i d . , 6 , : i l l (1936). (18) M. S. Desai, Proc. .Vul. A c a d . S L ~I n. d i a , 2, 119 (1933).
PHILADELPHIA, PESNA.
