NOTES
July, 1960
of a qualitative test of species present' and a quantitative test of the solvent-solute interactions and behavior.2 For quantitative evaluation of bem) havior in the dilute region (concentration < it is essential to know the heat of fusion accurately from a direct measurement. Two calorimetrically measured values for the heat of fusion of silver nitrate are known. Goodwin and Kalmus3 obtained the value 2580 cal. mole-' while Magnus and Oppenheimer4 obtained a value of 2757 cal. mole-'. In 1936, Kelleyl published a mean value of 2550 cal. mole-' based on an assessment of the available cryoscopic data at that time. Attention is also drawn by Kelleys to a value of 2755 cal. mole-' empirically derived from a formula quoted by Juptner.6 The present communication reports the results of a redetermination of the heat of fusion for silver nitrate by the method of drop calorimetry.
0
00
0 0
0 1
0
t
10
diG
20
Fig. 2.-The apparent rate constant of polymerization of ethylene as a function of the square root of the partial pressure of ethane. UH = VO
- PHs x
937
(6)
where VH and vo are the reaction rates at constant propylene partial pressure in the presence and absence of hydrogen, respectively, p ~is%the partial pressure of hydrogen and X an arbitrary constant. They assume that hydrogen causes hydrogenolysis of bonds between the catalyst and the growing polymer chains. For algebraic reasons the difference between equations of types (2) and (6) is well within the experimental error for the conditions of Natta's experiments. The results of this paper will, however, not fit n'atta's hypothesis as can be seen from the plot in Fig. 2. Admittedly the two systems are different. Still it is possible that also with hydrogen in heptane solution, hydrogen will occupy an appreciable portion of the active sites while those occupied by propylene are vacated by polymerization in preference to desorption. If this is the case, equation 2 must be expected to hold also for Natta's case. As to the polymerization of ethylene, the experimental evidence shows that this is a case where the rate-determining step is the adsorption of ethylene on the catalyst surface, the actual polymerization proceeding rather fast. As Laidler3 has pointed out, the tacit assumption in discussing surface catalysis that adsorption equilibrium is in fact established is rarely supported by evidence. In this instance, a t least, the experixental evidence is atll against this assumption.
Experimental Calorimeter and Accessories .-The calorimetric assembly designed to yield results with an accuracy of =!= 2% for heats of fusion in the range 2-4 kcal./mole, and the recording differential potentiometer for the temperature-time rneasurcments were the same as described elsewhere in detail.',* Silver Nitrate .-Finely powdered silver nitrate (reagent grade) was vacuum dried for several days under a stream of dry nitrogen with progressive elevation of the temperature to a point just below the fusion point of the salt. This maximum of temperature was maintained for 24 hr., after which the sample was cooled under a stream of dry gas and stored in a sealed container protected from the light. The appearance of the Agh'03 was important after this drying technique. If the drying is not complete the traces of water will react with the salt to give nitric acid and silver oxide a t the elevated temperatures. The silver oxide will cause discoloration of the salt and very small traces of oxide can be vislially distinguished. If thermal decomposition of the salt takes place traces of silver or silver oxide will be deposited depending on the decomposition process. These deposits v.31 also he visible in trace amounts. The appearance of the szlt thus indicated both dryness and whether or not decomposition had taken place. It was found that fusion points of the best selected samples were sharp and deviated by less than 0.1" from the fusion temperature of 209.6".
Results The procedures for calibration of the calorimeter and measurement of the heat of fusion have been described in the earlier paper^.^,^ Table I summarizes the data and results for a series of eleven determinations of the enthalpy change between 430 and 590'K. below and above the melting point. The analysis of the data is similar to that followed for previous calculations.g Since there was a scatter of experimental points, particularly above the melting point, 95% confidence limits were adopted and a least squares analysis was made. From the graph so obtained of enthalpy change us. initial temperature of the sample the heat of fusion of (1) E. Kordes. W. Bergmann and W. Vogel, 2. Elektrochem., 66, 600
HEAT ,4KD ENTROPY OF FVSIOK AND CRYOSCOPIC CONSTANT OF SILYER NITRATE BY GEORGE J. JANZ,DAVIDW. JAMESAND JEROME GOODKIN
Departmehi of Chemistry. Rensaelaer Polytechnic Institute, Troy, N . Y . Received January 81, 1960
Cryoscopy in silver nitrate solvent has been studied by a number of workers both from the aspect
(1951).
(2) y . Doucet, J. A. LeDuc and G. Pannetier, Compt. rend., 236, 1018 (1953). (3) H. hl. Goodwin and H. T.Kalmus, Phys. Rea., 28, 1 (1909). (4) A. &fagnus and F. Oppenheimer, Z . anorg. allgem. Chem., 168, 305 (1928). ( 5 ) K. K. Kelley, Bull. U.5. Bur. Mines. No. 371, Washington, 1936. (6) H. Juptner, Stahl u. Eisen, 18, 1039 (1898). (7) J. Goodkin, C. Solomons and G. J. Janz, Rev. Sci. Instr., 29, 105
(1958). (8) (9)
c. Solomona
and G. J. Janz. A n a l . Chem., 31, 623 (1959). Goodkin, H. J. Gardner and G. J. Janr, THIS 62, 248 (1958).
c. Solomona, J.
JOURNAL,
938
silver nitrate at its melting point was found to be 2960 h 60 cal./mole.
Discussion The values for the heat of fusion of silver nitrate reported from 1898 onward are summarized in Table 11, together with the value from the present investigation. In view of the markedly higher value gained from the present calorimetric measurements, an evaluation of the factors possibly contributing to the differences in the previously determined yalues is of interest. T ~ B LI E ENTHALPY O F SILVER KITRATE AT V.4RIOTJS TEMPER 4TURE3 (Tt) REL.4TIT-E TO STAXDARD TEhIPER.4TURE (3Oo'K. ) -Sample--Initial temp.,
T t , OK.
Vol. 64
NOTES
rlnal temp.,
OK.
Cal. Calorimevolved eter temp. by 1 mole change
AgNOz
Corm. t o 298.15OK.
HT HZO~.IK,
cal./niole
4.83 10945.61 41.22 11036.83 58i.2 302.3 301.3 3.43 9316.53 68.98 9385.51 547.7 301.3 :?.Xi 9245.78 68.98 9314.76 517.3 3.90 7916.64 106.79 8023.43 503.3 303.0 305.8 2.89 i913.53 168.99 8082.55 500.2 3.82 7733.45 131.25 '7864.70 491.5 304.1 300.2 1.99 5221.68 44.52 5266.40 483.2 l.i4 4479.91 60.09 4540.00 470.4 300.9 1.59 4110.81 53.41 4164.22 460.2 300.6 1.52 3939.52 80.10 4029.62 450.2 301.8 1.44 3752.74 42.29 3795.03 438.4 300.1 Wt. of AgNO; sample, g. 30.0256 16.0315 Wt. of Pt crucible, g. Heat equiv. of calorimeter 1st expt. 441.07 cal./deg.; remaining expt. 539.10cal./deg.
TABLEI1 T a r ~ r ~FOR s s HEATOF FUSION FOR AgSOs Inyestigator
H. Juptner (1898) H. Goodn-in and H. Kalmiis (1909) h. llaynus and F. Oppenheimer (1928j K. Kellry (1936) Present investigation ( 1959)
SIethod
Estimated
AHP,
where K is the modulus of elasticity and p is the density of the salt. No information relative to the temperature a t which the modulus or density were measured is given. The agreement of this estimated value with other results must be regarded as fortuitous. Further tests of this expression to evaluate its applicability would seem of interest. The value by Kelleys is based on seven sets of cryoscopic data varying from 2,300 to 2,800 cal. mole-'. Assumptions relative to the thermodynamic ideality of the solutions and the dissociation processes for the solutes are implicit in the calculations of the heats of fusion from the above cryoscopic results. For theoretical discussions of cryoscopic behavior of solutes in silver nitrate a value of the heat of fusion (and thus the cryoscopic constant) independently derived by a different experimental technique is almost essential. The values for the heat of fusion (2960 cal./mole) entropy of fusion (6.14 e.u./mole) and cryoscopic constant (26.47 deg./mole/1000 g.) derived from the present calorimetric measurements are recommended for practical and theoretical calculations. Acknowledgment.-This work was made possible in part by support received from U. S. Air Force, Air Research and Development Command, Office of Scientific Research. Active participation in the earlier phases of this study and continued interest by Dr. Cyril Soloinons is gratefulIy acknowledged.
THE PREPARATION OF SINGLE CRYSTALS OF CERTAIN TRANSITIOY METAL FLUORIDES BY H. GUGGENHEIM
eal./mole
Ref.
2755
6
Bell Telephone Laboratories,Incorporated, Murray Hall, .Vcw Jersev Reeemed January BS, 1960
Single crystal WiF2 was grown from the melt using a modification of Stockbarger's' method. NiF2 is reported2 to have 5t vapor pressure of one atmosCalorimetry 2757 4 phere a t about 1000" and a melting point over 1300". ........ 2550 5 For these reasons it is impractical to grow it from Calorimetry 2960 the melt using conventional methods, but a sealed It has been realized only recently how difficult it platinum container has been found to overcome the is to remove all traces of water from a crystalline difficulties. Dry NiFz was prepared by passing salt; in the present investigation a rigid vacuum- H F over "low cobalt" NiCl at 850" for 16 hours. elevated temperature technique wa,s employed as The resulting material was a light greenish-yellow. the most efficient drying method. Magnus and The Pt-10% Rh alloy tube was used. To facilitate Oppenheimer4 used an isothermal calorimetric pinching off of the tube, a smaller Pt tube was method in which the sample was open to the air a t welded to the larger tube after charging with NiF,. all times. The presence of water thus possible in To prevent moisture and oxygen contamination, their samples would contribute to a lower value for the tube was heated to 250" while connected to a , ~ vacuum line. The vacuum was broken with argon AHf. In the work of Goodwin and K a l m u ~ in addition t,o this factor, a heat loss owing to chimney and then the small tube was pinched off near the convection in the design of drop calorimeter used is top with barrel jam pliers. The pinched end was seen possible and would contribute to the lorn re- welded with a gas flame and the tube was placed sult'. The melting point for AgSOs is reported as inside an alumina tube attached to a clock motor 219' by Goodwin and Kalmus3 (cf. 209.6') and this by a sprocket chain. The tube was lowered may indicate tha,t all temperatures in this work were through a hot zone (1420") a t 0.075" per hour. incorrect,. After 100 hours the tube was removed a t room The yalue attributed to Juptner6 was obtained temperature and opened by cutting along the not by direct measurement but is a value calculated length with a circular diamond saw. The KiF2 \I: w e of the empirical relation boule was emerald green, weighing 56 g., and was 2 Calorimetry
2580
3
(1) D.C. Stockbarger, Rsv. Sci. Inatr., 10, 205 (19391. (2) C. Poulenc, Ann. Chem. Phys., 2, 41 (1894).
