G. E. BRODALE A N D W. F. GIAUQUE
1268
The Heat of Hydration of Cobalt Sulfate Hexahydrate to Heptahydrate. Their Solubilities and Heats of Solution’
by G. E. Brodale and W. F. Giauque Low Temperature Laboratory, Departments of Chemistry and Chemical Engineering, University of California, Berkeley, California (Received October 10,1964)
The heat of dilution of a saturated solution of CoS04.7H20 together with heats of solution of CoS04.7Hz0 and CoS04.6H20 have been measured to give CoS04.6H20(s) HzO(l) = CoSO4.7H2O(s), A H z ~ s . ~= ~ o-2455 K . f 10 cal./mole. The solubilities of the two hydrates have been measured from 25 to 65”. The heat of the transition reaction, CoSO4.7H2O(s)= 0.9122CoS04.6HzO(s) 0.0878CoS04.17.389H20(1), which occurs at 317.78”K., has been determined as 2848 =t15 cal./mole.
+
+
When investigating the heat capacities and heats of solution of solid hydrates, a common procedure is to work with samples which are predominantly one hydrate, but contain small amounts of a higher or lower hydrate, also under investigation. An analysis of the water content is used to evaluate the properties of the pure hydrates. However, cases have arisen where more than two water-containing phases are present and this invalidates the method. As a typical example, in the heat capacity work of Rao and GiauqueZa sample of CoSOc.7H20 was prepared and partially dehydrated to CoS04.6H20. During the low temperature heat capacity measurements on this material a small amount of heat absorption occurred at the CoS04.7H20-ice eutectic melting point showing that the sample contained small inclusions of saturated solution along with the hepta- and hexahydrates. Various delays in sampling and in the heat of solution measurements caused total water variation and possible “brine hole” niigration. In any case a serious third-law “discrepancy” developed in the hexa- to heptahydrate reaction which could not be accepted without some explanation. The fact that the isomorphous hexahydrated sulfates of Zn3 and h4g4 have inconipletely explained low temperature heat capacity irregularities helped obscure heat of solution errors as the principal source of the unpublished discrepancy In the work of Rao and Giauque.2 The present work is designed to eliminate the source of error mentioned above and it has made it possible to use the heat capacity data to conclude that none of the phases involve residual entropy due to frozen-in disorder. T h e Journal of E’hgsical ChemistrQ
The general procedure was to prepare samples under conditions such that it would be very unlikely that more than two phases would be present. Crystals of CoS04. 7H20 were left slightly wet so that the two phases were heptahydrate and saturated solution whether adhering or within “brine holes.’’ Similar dilution measurements were made on a solution with the composition CoSO4.AH2O,where A = 23.309. For a saturated solution at 25”, A = 23.26. In the case of lower hydrates, such as the hexahydrate in the present case, which must be crystallized at a higher temperature, excess solution, whether as inclusion or not, has always combined with adjacent lower hydrate when cooled into the range where the higher hydrate becomes stable. N o eutectic calorimetric heat effect has ever been observed in such cases in various experiments in this laboratory. Thus, in the present case, we interpret the total water in the (‘hexahydrate” sample as distributed exclusively between the hexa- and heptahydrates, again enabling simple correction. Preparation of the Samples. Two preparations of CoS04were made: (a) by crystallization from solution after treating CoCO3 with reagent grade H2S04,and (b) (!) This work was supported in part by the National Science Founds, tion. (2) R. V. G. Rao and W. F. Giauque, J . P h y s . Chem., 69, 1272 (1965). (3) R. E. Barieau and W. F. Giauque, J . Am. Chem. Soc., 72, 5676 (1950). (4) W. P. Cox, E. W. Hornung, and W. F. Giauque, ibid., 77, 3935 (1955).
HEATO F HYDRATIOX OF COS04 *6H,O TO COSOa. 7Hz0
by dissolving 99.999% cobalt metal6 in reagent grade HzSOa(0.0004~oresidue on heating) and evaporation of excess H2S04. CoSOa.7Hz0 was prepared a t ordinary temperatures and centrifuged to remove most of the adhering solution. CoS04.6Hz0was prepared a t about 55" and centrifuged at that temperature. We have found it convenient to therniostat a centrifuge roughly for this type of preparation. In several preliminary experiments the water content was left intermediate between the hexa- and heptahydrates to check against the possibility of an intermediate hydrate; however, the heat of solution data, while inferior to those given below, are linear with water content and thus do not support this possibility. Apparatus. The heats of solution and dilution were measured in a calorimeter described by Kunzler and Giauque.6 The samples were enclosed in small glass cylinders with thin glass cover plates attached to the ends by means of paraffin. The cover plates could be broken by a glass plunger to start the solution process. The total volume of the calorimeter was about 1 1. Analysis for Water Content. The analysis for water content was by dehydration to 500" after careful preliminary heating to avoid spattering. In the case of the solubility measurements, and of the heptahydrate, soft-glass weighing bottles were used. For the hexahydrate, Pyrex containers were used and the residue was checked for decomposition to Co304 by filtering and weighing the insoluble residue. The weight losses were in the range 0.01 to 0.03% of the anhydrous CoS04, which was about the limit of error of the determinations. Solubility of Cobalt Sulfate Hydrates. The above type of correction requires a value of the solubility of CoS04. 7H20 a t the temperature of measurement, which was 25". This solubility was determined as part of a more extensive investigation of the solubility of both the hepta- and hexahydrates over the range 25 to 65". This was done in an attempt to see if any hydrates other than those expected above and below the hepta-hexa transition at 44.6302were involved in the crystal preparation. No forms other than the ordinary hepta- and hexahydrates were found. The solubility data are given in Table I. The molecular weights of CoS04 and water were taken as 154.9948 and 18.0153, respectively. In making corrections for buoyancy, the density of CoSOl was taken as 3.70, that of CoS04.7H20 as 1.942, and that of CoS04.6H207as 2.006 g./cm.a. Also, the density of CoSO4.23.309H20 was determined as d251.334 g./cm.3. The data in Table I are given in the order of the experiments. Heats of Dilution of Concentrated Aqueous Cobaltous
1269
Table I : Solubilities of CoS04.7HzO and CoS04.6H20 Temp., O C .
8. of coso4/1oo g . of s o h .
62.23 60.27 55.69 51.04 48.23 45.89 4 3 . 52b 39.98 37.74 31.04 25.22
36.462 36.064 35.164 34,252 33.729 33.332 32.907 31.597 30.855 28.830 27,069
34.06 39.18 41.19 41.70 42.37 42.98 43.56 44.53 44.92 45.66 47.14 50.76 55.00 25.00 44.63
29,771 31.346 31.983 32.149 32.367 32.556 32.764 33.066 33.184 33.327 33.589 34.244 35.032
An
Series 1 14.992 15.253 15.863 16.515 16.897 17.208 17.541 18.625 19.280 21.239 23.180 Series 2 20.295 18.843 18.297 18.158 17.978 17.823 17.655 17,416 17.323 17.212 17.011 16.517 15.955 (23.26) (17.389)
A = moles of HnO/mole of CoSO4. solution.
Solid phase
COS04 .6H20
C0S04.7Hz0
CoSO4.7HzO
~OS01.6Hzo
CoSO4.7HzO Transition Supersaturated
Sulfate near Saturation. The heats of dilution of concentrated cobalt sulfate solutions are given in Table 11. In tjhe experiments near 25 O the concentrated sample was contained in two sample tubes which were broken separately as experiments 1 and 2. Thus the final conditions of experiment 1 were the initial conditions of experiment 2. The same type of sequence was used for experiments 3 and 4. I n making the corrections for temperature in Table II we made use of the equation given by Giauque, Barieau, and KunzleP for the apparent molal heat capacity of aqueous ZnS04as a function of temperature and compo(5) Johnson, Matthey and Co., London, England. (6) J. E. Kunzler and W. F. Giauque, J. A m . Chem. SOC.,74, 3272 (1952). (7) A. Zalkin, H . Ruben, and D. H . Templeton, Acta Cryst., 15, 1219 (1962). (8) W. F. Giauque, R. E. Barieau, and J. E. Kunzler, J . A m . Chem. SOC.,7 2 , 5fB5 (1950).
Volume 69, Niimber /t
A p i l 1966
G. E. BRODALE A N D W. F. GIAUQUE
1270
Table I1 : Heats of Dilution of Concentrated Aqueous Cos04 Temp. of soln..
CoSOl(g)
A
25.05 25.09" 25.05
23.309 23.309 23.309
1018.4
A
7.8182 3.6979 11.5161
B B B
7.3704 7.1810 14.5514
25.02 25.07d 25.02
23,309 23.309 23.309
925.4
A
OC.
A , sample"
A , initial A , final calorimeter" calorimetera
Prepn.
m
m
A& cor. AHb obsd. t o 25.00'
1018.4 699.0 699.0
-650 -404 -571
925.4 980.2 480.2
-632 -439 -537
m
m
AHb,,
+2
-648
+2
-569
+1
-631
+I
-536
Remarks
Expt. 1 Expt. 2 Calcd. 1
+2
Expt.3 Expt. 4 Calcd. 3
+4
A H cor.
B B a
10.8947 10.0998
A
=
45.24 45.26
17.383 17.596 (17.389)"
moles of H20/mole of CoSO4.
' Average.
692.7 715.0
m m
-1678 -1693
AHb cor. to 44.63'
to A , initial = 17.380
AH cor. to A , final = 700
+23 +24
+1 -18
-2 +3
A Hu.ss
?i, final
- 1656 - 1684 ( - 1670)'
Final temperature of expt. 1. d.Final temperature of expt. 3.
Cal./mole.
= 700
e
Calculated.
Table 111: Heat of Solution of CoS04.7H20
Prepn.
A A B
Temp. of soln., OC.
CoSO&)
11,4090 11.5896 10.4260
24.99 24.99 24.99
A , sample'
A , finala soh.
7.122 7.122 7.362
686.7 686.4 694.4
Moles CoSOd in satd. s o h . AHt obsd., per mole ca\./mole CoS04.7HzO CoS01.7H20
0.0076 0.0076 0.0228
AH^^,^ cal./mole, cor. for A@ cor. to A& cor. to A , final = satd. s o h . A , final = 700 25.00° 700 A&
+4 +4 +13
3611 3608 3597
0 0 0
-3 -3 -1
Av.
B a
A
=
9.4990
44.11
moles of HrO/mole of
7.362
cOs04.
753.2
=
-218.7
+ 0.6 T + 191/A"'
For correcting the heat of solution for concentration, the present data show that in the range A = 500-700, aqueous CoS04 solutions have about the same heat of dilution as ZnS04 solutions. In t,he range A = 17-23, the relative partial heat content of the CoSOl solutions is less than that of aqueous ZnS04 by the order of 100 cal./mole, so that the curves are nearly identical. Heat of Solution of CoSO4.7H20. The heat of solution data for cobalt sulfate heptahydrate are given in Table I11 where they are also corrected for the saturated solution present in the samples. T h e Journal of Physical Chemistry
+58
3383
+12
=
A& cor. t o 44.63O
A H4r.sab A , final = 700
-4
3449
I n cal./mole.
sition, since these solutions should have very similar properties and in any case the correction is small.
C,, (apparent)
0.0357
AH25
3612 3609 3609 3610
Heat of Solution of CoS04.6Hz0. The heats of solution of samples with a water content CoS04%.078H20 are given in Table IV where they are also corrected for the amountof CoS04.7H20present. The Heat of Hydration of CoS04.6HzO to CoS04.7HzO. From the data in the several tables the AH of the following reaction may be given as
+
COSOI.~HZO(~, HzO(1) = CoSO4'7HzO(S, A H ~ W=. ~-22455 ~ i 10 cal./mole
(1)
The Heat of Transition of Cos04 7 H z 0 to C0S04 6 H 2 0 One of the principal purposes of the heat of solution measurements was the determination of the heat of transition in reaction 2, so that it
+ Saturated Solution.
9
HEATOF HYDRATION OF CoSO4.6H20TO CoS04.7H20
1271
Table IV : Heat of Solution of CoSO4.6Hz0
Prepn.
cOsok(g)
soh., O C .
A , sample"
A , final'
Moles CoSOk.7HzO per mole CoSO4.6HnO
B B B
13.4833 12.4599 13.4627
24.99 25.00 25.00
6,078 6.078 6.078
583.8 585.1 584.2
0.0846 0.0846 0.0846
Temp. of
A H t obsd.,
cal./mole AH^ cor. for A+ cor. to A , CoSOk.6HzO CoS04.7HzO final = 700
to25.00
- 305 - 305 - 305
0 0 0
1479 1498 1489
AH^
- 28 - 28 - 28
AH^^,^ A ,
cor.
final =
Av. Moles Cos04 in satd. s o h . per mole CoSO44JHzO
B A
10.1517 =
46.59
6.350
712.4
0.0325
A+ cor. for satd. soh.
+58
733
A+
001.
to 44.63'
1146 1165 1156 1155
AH,..^^,^ A , final = 700
+33
f 3
700
827
In cal./mole.
moles of HzO/mole of CoSOa.
By addition
+
CoS04.7HzO(,) = 0.9122CoSO4*6HzO(s) 0.0878CoS04.17.389Hz0,AH44.63
=
2852 cal./ mole
could be used to determine the over-all composition in the mixed hydrates used in the heat capacity work of Rao and Giauque.2 Introducing the numerical value of A = 17.389, the moles of HzO/mole of COS04 in the saturated solution at the transition temperature, 44.63", the transition reaction becomes
Combination 2
+
0.9122[C0S04*7H20(~) = COSOI.GH~O(~) H20(1)],AH44.63 = 0.9122 X 2631 = 2400
+ 693H2O
0.0878 [CoS04.7HzO(s)
CoS04*700HzO],A H 4 4 . 8 3
+
C O S O ~ . ~ H ~=O0.9122CoS04.6HzO(s) (~) O.0878C0SO4.17.389H20(~)(3)
0.0878 [CoS04.7WH20 682.611H201,
There are three independent methods of combining the calorimetric observations to give the heat of reaction 3. Two of the methods make use of the AH of reaction 1, calculated to the transition temperature by means of the heat capacity data on the hydrates2 together with the well-known heat capacity of liquid water to calculate AH a t 44-63".
s,,
44.83
AH44.83 = AH26 4-
AC,dT
=
2455
(3)
-
=
=
0.0878 X 3449 = 303
=
Cos04 17.389H20(1) 9
AH44.83
+
= 0.0878 X 1670 = 147
By addition CoSO4.7HzO(.) = 0.9122CoS04.6H20(,)
+
0.0878CoS04~17.389H~0~~~, AH44.83 = 2850 cal./ mole
(3)
Combination 3
+
+ 176 =
C O S O ~ . ~ H ~ O 693H20 (~) = CoS04.700H20, AH44.63 = 3449
2631 cal./mole
Combination 1
+
0.9122[CoS04.700H20 = C O S O ~ + ~ H ~ O ( ~ ) 694H201, AH44.63 = 0.9122 X (-827) = -754
+ H20(1),
+
CoS04.7HzOcs) = CoS04.6HzO(s)
0.0878 [CoS04.700HzO(.) = CoS04*17.389H20(1)
AH44.63
= 2631
+
0.0878 [COSO~.GH~O(,)694H20(1) = CoS04.700H201, AH44.83 = 0.0878 X 827
=
74
+
0.0878 [CoS04.700HzO = CoSO4.17.389H20 682.611H201, 111144.88 = 0.0878 X 1670
=
682.611H20], AH44.83 By addition
=
0.0878 X 1670 = 147
+
CoS04*7HzO,,) = 0.9122CoS04*6HzO(,) O.0878CoSO4.17.389H20(1),AH44.83 = 2842 cal./ mole
147 Volume 69,Number 4
(3)
April 1966
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 at 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).
