1272
R. V. G. RAOAND W. F. GIAUQUE
Combinations 1 and 2 agree well a t 2852 and 2850 and the close agreement is obviously due to the fact that the measured AH for reaction 1 dominates the contribution made by the experimental data obtained a t 44.63".
In combination 3 all of the data were obtained near 44.63" and the result, 2842 cal./niole, is in very satisfactory agreement, We accept an average of 2848 f 1.5 cal./mole for the heat of transition a t 44.63".
The Heat Capacities and Entropies of Cobalt Sulfate Heptahydrate and Hexahydrate from 15 to 330OK.l
by R. V. G . Rao and W. F. Glauque Low Temperature Laboratory, Departments of Chemistry and Chemical Engineering, University of California, Berkeley, Calijornia (Received October 80,196'4)
The heat capacity of CoSOd.6Hz0 has been determined from 15 to 330°K. and that of CoS04.7H20from 15'K. to the hepta-hexa solution transition, which was found to occur a t 317.78"K. Values of So, (F" - H"o)/T, and ( H " - H o 0 ) / Thave been tabulated. The entropy change in the reaction CoSO4.7H20 = CoS04.6H20 HzO(g) has been determined from the free energy and heat of reaction a t 298.15, 305.65, 309.80, and 313.35"K. to be 35.97, 35.92, 35.89, and 35.88 gibbs/mole, respectively. The corresponding values from the third law of thermodynamics are 35.92, 35.90, 35.88, and 35.87 gibbs/mole. The agreement indicates that there is no residual entropy due to structural disorder in either hydrate. AHoo = 13,771 cal./mole for the above reaction.
+
+
The work reported here is part of a series of calorimetric and magnetic investigations on the hydrated sulfates of the elements of the first transition group. The present work measures the heat capacities of cobalt sulfate hepta- and hexahydrates. Such investigations are a desirable preliminary to detailed low temperature magnetic investigations for several reasons, including evidence that the crystalline forms stable a t ordinary temperatures do not undergo transitions before reaching the temperatures of the magnetic investigation. I t is especially important to show that no transition will occur in a single crystal which has been prepared and placed for axial measurements of its magnetothermodynamic properties. Also, evidence from the application of the third law of thermodynamics can show that there is no frozen-in disorder, such as that The Journal of Physical Chemistry
possibly due to disordered hydrogen bonds, to complicate the interpretation of the magnetic systems. At one time some inaccurate preliminary unpublished heats of solution used in computing the entropy of hydration indicated disorder in the hexahydrate, and following this, Zalkin, Ruben, and Tenipleton2 investigated the crystal structure of CoSOc.6Hz0a t ordinary temperatures and found no evidence to support any type of hydrogen bond disorder. Their similar investigation3 of the isoniorphous r\lgS04.6H20led to the same conclusion a t ordinary temperatures. I t has (1) This work ww supported in part by the National Science Foundation. (2) A. Zalkin, H. Ruben, and D. H. Templeton, Acta Cryet., 15, 1219 (1962).
(3) A. Zalkin, H. Ruben, and D. H. Templeton, ibid., 17, 235 (1964).
been shown by Cox, Hornung, and Giauque4 that MgSO, 6 H 2 0 undergoes an irreversible transformation, with heat evolution over a range 100 to 140°K. and absorption over the range 235 to 265OIZ. This was believed due to deconiposition of the hexahydrate to microcrystals of the mono- and heptahydrates during cooling. Barieau and Giauque5 had reported a similar irreversible transition in the isomorphous ZnS04.6Hz0, which has heat evolution between 70 and 120’K. and absorption between 230 arid 27OoK., and in that case it is known that the hexahydrate is thermodynamically unstable with respect to the hepta- and monohydrate. S o low temperature X-ray structural investigations of the hexahydrates of cobalt, zinc, or magnesium sulfates are available. For these reasons we were concerned over the possibility that isomorphous CoS046Hz0 would be structurally altered 011 cooling. As will be discussed below, a sniall niaxiniuni rather similar to those in ZnS04.6H20 and 3IgSO4.6H,O occurs in the heat capacity curve near 2 1 5 O K . and it has so far not been possible to decide if this represents some form of transition. In any case, a rough estimate of the amount of entropy under the hump in the heat capacity curve was small enough (0.14 gibbs/mole) to encourage the belief that, despite our lack of knowledge concerning the hump, a third-law test for residual entropy was practicable, especially since no evidence supporting irreversibility could be found. This led Brodale and GiauqueO to reinvestigate the heats of solution of CoS04.6H20 and CoS04.7Hz0, and mith the assistance of these new data, agreement with the third law has been found. However, this result cannot be taken to prove that no structural changes would occur in a single crystal of CoS04.6Hz0during cooling to liquid helium temperatures for a magnetic investigation of its properties. Preparation of C o ~ 9 0 ~ . 7 Hand ~ 0CoS046H2O Used for the Heat Capacity Measurements. The starting material used in the preparation of cobalt sulfate was Mallinckrodt reagent grade cobaltous chloride hexahydrate. I t was stated to contain o.15y0 nickel, 0.2570 alkali salts, and remaining impurities, totalling only 0.088%. The nickel was removed by converting the cobalt to cobaltic hexaamniine trichloride by oxidation with ammoniacal silver chloride. The method is a standard procedure’ for the preparation of large amounts of Zuteo-cobalt chloride. This material was heated on a hot plate and then over a gas burner. The hard cake produced was extracted with water and heated until it was a light blue powder without further evolution of ammonium chloride fumes.
C.P. concentrated sulfuric acid wm added and the sulfate was heated with slow evaporation to dryness. This procedure was repeated to ensure elimination of chloride which was confirmed by test. The total heating time with sulfuric acid was about 80 hr. The cobalt sulfate was dissolved in warm distilIed water and filtered and the resulting solution was used for crystallization of the hydrates. No trace of nickel was found by the sensitive dimethylglyoxinie test,* which can detect one part of nickel in 5000 parts of cobalt. The heptahydrate was crystallized by bubbling pure dry oxygen (which happened to be available in surplus from laboratory production) through the aqueous sohtion a t room temperature. The wet crystaIs were further dried in a stream of oxygen, while they were mixed in a bottle which was slowly rotated about its horizontal symmetry axis by a motor, in an attempt to obtain uniform drying. The procedure continued until the analysis showed a water content slightly below that corresponding to heptahydrate. The crystals were small and it had been hoped to avoid any entrapped solution; however, this was not entirely successful and, as will be shown later, the heat capacity measurements showed a small heat effect a t the CoSOa.7HzO-ice eutectic. The CoSO4.6H20was prepared in a similar manner except that the crystallization was carried out a t 55”. The drying procedure to remove CoS04.7Hz0 which was formed on cooling the adhering solution to ordinary temperatures was the same as for the heptahydrate and was continued until dehydration analyses indicated only a small remaining amount of CoS04.7H20 in the CoS04.6H20. Apparatus and Samples. The low temperature calorimeter used was similar to that described by Giauque and Egang and the actual cylindrical copper calorimeter used has been described by Papadopoulos and Ciauque.lo It was 11 cm. long and 4.8 cm. in i.d. A double silk-covered gold resistance thermometerheater was wound on the outside. After the sample (4) W. P. Cox, E. W. Hornung, and W. F. Giauque, J.A m . Chem. Soc., 77, 3935 (1955).
(5) R. E. Barieau and W. F. Giauque, ibid.,72, 5676 (1950). (6) G. E. Brodale and W. F. Giauque, J . Phya. Chem., 69, 1268 (1965). (7) H. Biltz and W. Biltz, “Laboratory Methods of Inorganic Chemistry,” adapted from the German by W. T. Hall and A. A. Blanchard, 2nd Ed., 1928, John Wiley and Sons, Inc., New York, N. Y. (8) W ,W. Scott, “Standard Methods of Chemical Analysis,” Vol. 1, 5th Ed., D. Van Nostrand Co., Inc., New York, N. Y.,1939, p. 619. (9) W. F. Giauque and C. J. Egan, J . Chem. P h y s . , 5 , 45 (1937). (10) M . N. Papadopoulos and W. F. Giauque, J . A m . Chem. Soc., 77, 2740 (1955).
Volume 69, Kumber 4
April 1966
R. V. G. RAOAXD W. F. GIAUQUE
1274
had been added, the calorimeter was cooled to reduce the partial pressure of water vapor, evacuated, and 1 atni. of helium was added to assist thermal conduction. There were also eight radial copper vanes to assist heat distribution. Laboratory standard copper-constantan therniocouple Xo. 105 was attached to the calorimeter throughout the measurenients. During the experiment it was compared with several fixed points and showed exact agreement a t the boiling points of normal hydrogen (20.36”K.) and nitrogen (77.34’K.). It read 0.03’ low a t the triple point of nitrogen (63.15’K.). The weight of CoSO4.6.968H20 in the calorimeter was 181.2521 g. and the weight of CoS04.6.0033Hz0 was 159.942 g., each in vacuo. One defined calorie was taken as 4.1810 absolute joules. When third-law comparisons were originally made, discrepancies developed which cast doubt on the sampling as representative of the material used in the calorimeter. Unfortunately, the analytical samples of the stock material had not been taken at the t h e the calorimeter was filled and changes of water content could have occurred. In order to solve this problem Brodale and Giauque6 made accurate measurements of the heat effect in the hepta-hexa transition reaction (eq. 1) which was found to occur at 317.78’K. The CoS04.7HzO(,)
=
( A - 7)/(A - 6)CoS04.6HzO(,)
+
1/(A - 6)CoS04.AHzOsoln (1) synibol A represents the moles of water per mole of CoS04 in the saturated solution at the transition temperature. Three determinations of the transition temperature were made: 317.78, 317.79, and 317.78’K. Carpenter and Jettell reported 318.32’K. and Rohnier12 reported 316.45’K. However, both of these values were determined by the intersection of solubility or vapor pressure curves, methods which are difficult to use accurately. The value of A = 17.389 at 317.78’K. was determined from the solubility measurements of Brodale and Giauque.‘j The quantitative heat effect due to the transition of the small amount of heptahydrate in the hexahydrate sample was observed accurately during the heat capacity observations and this permitted the evaluation of the total waier qontent of the material in the calorimeter. In the case of the heptahydrate the evaluation was slightly more complicated since the heat capacity measuremenis showed the presence of 0.0248 mole of excess water in “brine holes” (entrapped solution) per mole of cobalt sulfate heptahydrate. Otherwise the The Journal of Physical Chemistry
evaluation was straightforward. In the hexahydrate sample no entrapped solution was present since no heat effect was observed at the CoS04.7HzO-ice eutectic. The “heptahydrate” saniple consisted of 0.943 mole of CoS04.7Hz0,0.057 mole of CoS04.6Hz0,and 0.025 mole of additional water per niole of cobalt sulfate. In estimating the excess water in the “brine holes” we made use of Rohnier’sI2 solubility data to calculate A = 37.16 for the CoS04.7H20-ice eutectic at -2.7’. This was combined with the heat of fusion of ice and an estimate of the heat of saturation to obtain a value for the heat of fusion of the eutectic, A H = 46,800 cal./niole of CoS04.7H20which dissolved. The determination of the composition of the “heptahydrate” sample is less accurate than that of the hexahydrate because of the major dependence on accuracy of the transition heat.6 However, since the specific heats of the hepta- and hexahydrates are not greatly different, the accuracy of the correction is not critically dependent on the composition of the heptahydrate, which may have an entropy error of about 0.03 gibbs/ niole due to composition uncertainty. Below the CoS04.7H20-ice eutectic temperature the “additional” water was present as ice and above the eutectic temperature it was present as saturated solution. Fortunately the aniount of entrapped solution was so small that the variation of the heat effect due to the change of solubility and the related heat effect with temperature could be ignored in comparison with the major heat absorption a t the eutectic. In the case of the heat capacity measurements on CoS04~6.0033Hz0, six separate series of runs were made in an attempt to understand the origin of the hump in the C, curve near 215’K. Series 1 . The sample was cooled to 230°K. and measurements were made over the range 230-280’K. These measurements were primarily designed to check on the presence of any entrapped solution which would be present as liquid above the CoS04.7HzO-ice eutectic temperature. Yo eutectic heat was observed. It may be mentioned that no eutectic heat has ever been observed in numerous other experiments in this laboratory when the primary hydrate present was lower than the hydrate in equilibrium a t the eutectic temperature, Series 2. The calorimeter was cooled from room temperature to 169’K. in 2 hr., held a t this temperature for 12 hr., raised to 220’K. in 40 niin., and then cooled rapidly to l5’K. in 3.5 hr., followed by heat capacity measurements to 31OoI
