THEENTROPY OF NH3.2H20
3053
The Entropy of NH;ZH,O.
Heat Capacity from 15 to 30O0K.I
by J. P. Chan and W. F. G h q u e L o w T e m p e r a t w e Laboratory, Departments of Chemistry and Chemical Engineering, University of California, Berkeley, California (Received J u n e 30,1964)
The heat capacity of NH3.2H20 has been determined from 15 to 300OK. The melting point was found to be 176.16 f 0.05OK. and the ice-NH3,2H20 solution peritectic point is 176.09 f 0.05OK. The ?;",.2H2O-NH,OH eutectic temperature is 175.42 f 0.05"K. The heat of fusion a t the mlelting point is 1673 cal./mole. For the reaction 2H20(1) NH3(sat. 1) = KHs.2H20(sat. l), we find, from available data, AF250 = 2123 cal./mole, AH250 = 2842 cal,/mole, and A8250 = -2.413 gibbs/mole. Similarly for the reaction 2H20(l) ?JHs(g,f = 1) = NH3.2H20(sat.l), AF250 = -825 cal./mole, AHz5. = -7874 cal,/mole, and ASwo = -23.643 gibbsimole. From these results and other available data the entropy of NH3.2H20(sat.1) is found to be 55.81 gibbs/mole a t 298.15"K. The more accurate entropy value calculated from the present calorimetric data is 55.67 gibbs/ mole and the small difference is within the limits of error of the various experiments which have enableld the comparison. This result indicates that no disorder such as that involving hydrogen bonds is present a t limiting low temperatures. Tables of C,, X, ( F O - H o o ) / T and (HO - H o o ) / T are included.
+
+
Low temperature calorimetric studies of N118,2 H20,3 and their compounds (NH4)204and NH40H4 have previously been completed in this laboratory. More recently Rollet and Vuillards found another hydrate, NH,.2Hz0. Ice has disordered hydrogen bonding and since hydrogen bonds must play an important role in bonding6 "3.2H20, an investigation of this substance designed to show whether its hydrogen bonds become ordered a t low teimperatures was of interest. We may say a t once that XH3'2H20 has been found to approach zero entropy and thus attains perfect crystalline order, a t limiting low temperatures. Preparation and Purity of N H , . d H z O . XH3 was prepared from NH4C1 and KOH as described by Overstreet and Giauque.2 This was combined with distilled water, boiled t o elirninate CO2, to make the hydrake. The sample was puirposely made with an excess concentration of NH40€I to avoid the presence of ice a t temperatures below the peritectic point of the dihydrate. This is undesirable because it would not be possible to use the calorimetric measurements on the KH3.2HzO",OH eutectic for analytical purposes and also because the presence of excess ice would be certain to trigger the separation of NH3.2Hz0 into ice and
solution when the peritectic point was reached. Actually, it was found that this almost always occurred, even without excess ice. When the composition is on the NHdOH side of NH3* 2H20 the heat of fusion of the small amount of their eutectic mixture serves as a quantitative means of adjusting the composition by guiding the removal of measured amounts of the more volatile KH3 gas from the system. These were condensed in a bulb of known volume by means of liquid nitrogen and measured approximately by gas pressure a t ordinary temperatures. The amount of ammonia removed was finally measured accurately by the difference in the initial and final weights of the material in a closed container and thus as in vacuo. Apparatus and Temperature Scale. The nieasurements were made in Gold Calorimeter V as described
-
(1) This work was supported in part by the National Science Founda-
tion. (2) R. Overstreet and W. F. Giauque, J . Am. Chem. Soc., 59, 254 (1937). (3) W. F. Giauque and J. W. Stout, ibid., 58, 1144 (1936). (4) D. L. Hildenbrand and W. F. Giauque, ibid., 75, 2811 (1953). (5) A. P. Rollet and G. Vuillard, Compt. rend. Acad. Sci. P a r i s , 243, 383 (1956); G. Vuillard, Publ. Sci. Univ. Alger., B3, 80 (1957). (6) L. Pauling, J. Am. Chem. Soc., 57, 2680 (1935).
V o l u m e 68, Number 10
October, 1964
J. P.CHAXASD W. F. GIACQUE
3054
previously7 except that copper-constantan standard thermocouple No. 102, used previously, was replaced because of its accidental breakage. A new therinocouple calibrated a t the triple (13.94') and boiling (20.36') points of hydrogen, the triple (63.15') and boiling (77.34') points of nitrogen, and also a t 176'K., through comparison with standard KO. 102 before it was broken, was installed. A gold resistance thermometer-heater of about 260 ohins a t room temperature was used for high precision in obtaining temperature drifts. A value of 0°C. was taken as 273.15"K., and 1defined cal. as 4.1840 absolute joules. Heat Capacity Measurements. The heat capacity ~
~~~
Table I : Heat Capacity of NHz.1.8687Hz0 and NHa.1.9991HzO" T&Y> OK.
TW, cmeasd
",.I .8687H20sample -----Series 1-14.40 0.729 15.90 0.880 17.84 1.176 19.99 1,571 22.14 2.031 24.55 2.505 27,22 3,050 30.09 3.610 32.93 4.229 35.91 4.775 39.19 5.326 42.48 5.927 45.98 6.538 50.15 7.234 -Series 2-7 157.69 22.01 164.14 22.93 170.35 23.78 -Series 340.83 5.659 44.60 6,292 48.30 6.910 51.90 7.521 55.39 8.089 59.18 8.700 63.50 9.374 68.15 10.10 73.11 10.90 78.24 11.57 84.10 12.36 90.49 13.24
OK.
cmemessd
KH,. 1.8687H20 sample 96.94 14.06 103.63 14.93 110.32 15.85 117.24 16.85 124.34 17.80 131.49 18.70 138.35 19.56 145.32 20.45 152.52 21.36 159.58 22,30 166.66 23.24 NH3.1.9991Hz0 sample -Series 4--7 183,72 38.03 192.28 38.94 200.07 40.21 208.11 41.45 215.90 42.66 223.49 43.87 230.93 45. l o b 238.48 46.24 245.97 47.41 253,07 48.40 259.93 49.24 266.98 50.41 274.34 51.20 281.63 52.10 288.15 52.82 - - - - - S e r i e s 5-7 141.43 19.92 148.37 20.81 155.63 21.74
a 0°C. = 273.15"K.; mol. wt. S H 8 = 17.0306, Ha0 = 18.0153; 108.337 g. of NH3.1.8687€Tz0or 105.963 g. of SH3.1.9991Hz0 in the calorimeter (wt. in uacuo). 1 gibbs = 1 defined cal./defined degree. Heat capacity in gibbs/mole of S H 8 , This and SUCceeding runs of series 4 corrected for NH, vaporization.
T h e Journal of Physical Chemistry
data are given in Table I. The measurements for the solid were made on a sample which contained less (1.8687H~O)than 2 moles of water,'mole of ammonia for the reason given above. When we attempted to use the analytical procedure based on the heat of fusion of the NH3.2HzO-KH4OH eutectic in a manner similar to that described by Hildenbrand and Giauque4 for the lower hydrates, the fact that the peritectic point, 176.09", of N H 3 . 2 H z 0is only 0.67" above the eutectic temperature, which was measured at 175.12'K.. made this procedure difficult. Heat effects along the short melting line between the eutectic point and the melting point were practically inseparable from the heat of fusion and heat capacity until the amount of eutectic was reduced to a very sinall value by approaching the NH3 2H2O over-all composition. As mentioned above this was done by removing sinall amounts of animonia gas, under conditions such that the relative partial pressure of water was trivial. This was followed by extensive thermal stirring of the residual sample by heat introduction in the lower portion of the ralorinieter. When the coniposition corresponded to SH3.1.9991HzO, the heat of fusion of the eutectic and the heat capacity along the melting curve became clearly separable from the main heat of fusion. Aqueous ammonia solutions supercool readily, as had been noted by Vuillard,5 and we made use of this fact in crystallizing the dihydrate by the following procedure. The ammonia dihydrate was cooled as glass to the temperature of boiling nitrogen and was then slowly warnied while the heavy metal block surrounding the calorimeter remained cold. At approximately lj7'K., about 19' below the peritectic point, crystallization started and hydrogen gas was admitted to the space between the cold block and the calorimeter to remove some of the heat evolved during the crystallization. The heat capacity of the substance and the calorimeter were important in absorbing heat. The procedure was evidently effective, for although the evolved heat may have raised the temperature within the calorimeter to the peritectic point for a brief period, there was no evidence of composition dislocation, and subsequent cooling completed the crystallization. I n the case of XH3. 1.8687Hz0, the excess ammonia was crystallized as "4OH. The heat capacity of the solid was measured on this material and the result corrected for the known heat capacity of the amount of SHdOH present. As a check several heat capacity measurements were made on solid YH3.1.9991H~0,and agreement with the values based on the corrected mixture (7) J. B. Ott and W F Ciaugue, J Am.
Chem Soc , 8 2 ,
1308 (1960).
3055
THEENTROPY OF N H 3 .2H20
was found to be within the limit of error of the measurements. The heat capacities were measured continuously and the temperature increments may be estimated by the separation of the measurements. T h e Melting and l'eritectic Points. The SH3.2H10ice solution peritectic point was observed by measuring the temperature as a function of the fraction of the heat of melting. The values are given in Table 11, along with the value of tlhe true peritectic temperature obtained by means of a plot of T us. l l f , where f = fraction melted. This plot was extrapolated as usual to the hypothetical value l/f = 0. The slope of the curve indicated a liquid soluble-solid insoluble jmpurity of 8 parts in 100,000 on a molal basis. The relative HzO : NH, proportions will not alter the peritectic temperature. This small amount of impurity was based on the high precision slope of the resistance thermomeLer data given in Table .II. The impurity probably consisted of 3 parts of ("4)&(>3 in 100,000 of hydrated ammonia. The sample was handled in a closed system or under an atmosphere of carbon dioxide-free nitrogen to avoid this possibili&. However, it was cooled in a convenient COZ bath to reduce the vapor pressure (about 1 atm. a t 2 5 " ) before it was transferred to the calorimeter and probably absorbed the 0.7 cc. KTP of COz/mole of ammonia dihydrate needed to explain the observed impurity. I n any case 0.003%1 carbonate should have a trivia,l effect on the results.
Table I1 : Peritectic Temperature of NH, .2Hz0-Ice Solution" Fraction melted
T ,OK.. by resistance therm.
T,OK., by thermocouple
0.096 0.302 0.588 Extrap.
176.060 176.076 176.085 176.088
176.08 176.09 176.10
Accepted peritectic point
=
176.09 f 0.05'K.
--
The melting point appears to be 0.07 f 0.01" above the peritectic point. This value is based on a single observation made with a preliminary sample which wm believed to be nearly of dihydrate coinposition and, after remaining isothl3rmal for a few minutes following partial melting, suddenly dropped in temperature. The drop of 0.07" is presumed to correspond to the appearance of the ice phase which would reduce the temperature to that of the peritectic. While the above
observation is somewhat uncertain, it seems quite consistent with the extrapolation of the melting point diagram of Rollet and V ~ i l l a r d . ~Thus, the thermodynamically unstable melting point is taken as 176.16 f 0.05"K. T h e Heat of Fusaon of N H 3 . 2 H z 0 . The heat of fusion of ammonia dihydrate was measured as usual by starting heat input somewhat below the peritectic point and continuing until the calorimeter was well above the ice melting curve immediately above the melting point of S H , . 2 H z 0 . Measuring the heat of fusion of a substance which temporarily forms a peritectic presents a problem caused by the fact that the first equilibrium liquid produced is of a somewhat different composition than that of the original compound. I n this case the equilibrium liquid composition is near that of S H s . 2 H z 0 , and there is no reason to believe that much composition dislocation occurred during the process of complete melting to form liquid dihydrate. Vie have assumed that the finely divided ice, which may have existed for a short period during the melting, stayed on location in the somewhat viscous liquid until melting was complete. However, in order to eliminate any cumulative effect from possible composition dislocation the sample was always thermally stirred by heating it nearly to ordinary teniperatures several times before recrystallization for another heat of fusion or other measurements. This precaution was especially indicated after the lengthy melting needed for a study of the peritectic temperature. The he'at of fusion data are given in Table I11 where they ,are corrected for theJC,dT above and below the melting point. The phase diagram of Rollet and Vuillard5 indicates that the eutectic composition is about 7 3 mole % NH3.2Hz0 and 27 mole % S H 4 0 H . In a series of rihort rung itrwas found that the heat capacity returned to a normal value after the eutectic region had been passed. The heat of fusion of the eutectic in the sample used for the heat of fusion a t the melting point was 10.1 (two values, 10.3 and 9.9) cal./inole of S H , in the sample. The fact that the eutectic point and the peritectic point were only 0.67" apart made it very difficult to determine the heat of fusion per inole of eutectic, but it cannot be far from 1600 cal./mole, which was assumed. Thus we conclude that the sample actually rnelted in the calorimeter had an over-all composition of S H , . 1.9991H20 and represented 1.9959 moles of :\TH~.~H and z ~0.0017 mole of SH40H. The eutectic melting heat of 10.1 cal./mole corresponded to the melting of 0.0046 mole of N H 3 . 2 H z 0 0.0017 mole of PTH,OH, leaving 1.9913 moles of SH3.2HzO to be melted above the "3. 2HpO-NH40H eutectic teni-
+
V o l u m e 68, N u m b e r 10
October, 1.964
~
Table 111: Hertt of Fusion of NH3.2H20"
Expt. no.
1 2 a
*
T,OK. (initial)
T , OK.
175.805 175.869
180.663 182.569
(final)
Total oal./mole
Table IV : Thermodynamic Properties of Ammonia Dihydrate (gibb~/mole)a
JT,CPdT
4H~,cal./mole
1845.0 175.3 1919.8 245.2 Accepted value
1670 1675 1673
There were 1.9976 moles in calorimet,er; 1.9913 moles melted
at m.p., 176.16"K.
(Fo
T
15 20 25 30 35 40 45 50 55 60 70 80 90 100 110 120 130 140 150 160 170 176 176 180 190 200 210 220 230 240 250 260 270 273 280 290 298 300
OK
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 l6(s) l6(l) 0 0 0 0 0 0 0 0 0 0 15 0 0 15 0
CP
0 1 2 3 4
784 583 612 638 607 5 498 6 364 7 205 8 028 8 833 10 399 11 845 13 175 14 459 15 824 17 232 18 516 19 761 21 033 22 354 23 725 24 569 37 029 37 496 38 762 40 192 41 742 43 329 44 940 46 495 47 975 49 356 50 666 51 058 51 892 53 029 53 871 54 048
- Hoe)/
S
T
0 270 0 592 1 057 1 623 2 259 2 931 3 629 4 344 5 069 5 802 7 282 8 767 10 239 11 694 13 135 14 5 i 3 16 004 17 422 18 828 20 228 21 624 22 483 31 980 32 784 34 845 36 868 38 866 40 845 42 806 44 752 46 681 48 589 50 477 51 062 52 342 54 183 55 665 55 977
0 073 0 159 0 290 0 464 0 674 0 914 1 177 1 458 1 753 2 060 2 700 3 366 4 048 4 740 5 438 6 139 6 843 7 548 8 253 8 958 9 662 10 095 10 095 10 571 11 794 12 997 14 182 15 349 16 500 17 637 18 760 19 871 20 969 21 311 22 056 23 133 24 002 24 203
(HO
- H'a)/T
0 197 0 432 0 767 1 159 1 584 2 017 2 452 2 886 3 316 3 742 4 582 5 401 6 192 6 954 7 698 8 434 9 161 9 874 10 575 11 270 11 962 12 388 21 885 22 213 23 050 23 871 24 685 25 496 26 306 27 115 27 921 28 719 29 508 29 754 30 285 31 050 31 663 31 797
perature. The data are placed on a molal basis in Table 111. The Thermodynamic PropeTties of N H , . 2H20. The heat capacity of solid ammonia dihydrate was obtained by correcting the observations on NH,.1.868'iHz0 for the presence of 0.1313 mole of X€I1OH/mole of total KH,, using the data of Hildenbrand and G i a ~ q u e . ~ The measurements on the liquid were made after the determination of the heat of fusion and following adjustment to an over-all composition of NH,. 1.9991HzO. There were 1.9976 moles of animonia in the solution of the above composition in the calorimeter during the measurements on the liquid. It was assumed that the heat capacity gram was the same for the dihydrate as for SH3.1.9991Hz0in obtaining the molal values of iSH3.2HzO. AIinor corrections were applied to the heat capacity measurements on the liquid for the heat effect due to vaporization into the small gas volume above the liquid sample. The heat per mole of KH, was estimated with sufficient accuracy for this purpose by means of the approximation AH,,, = R T 2 d In P dT, where P is thevapor pressureover NH,.2HzO as given by the tables of Scatchard, et aL8 Smoothed values of the heat capacity and related thermodynamic properties are given in Table IV. The extrapolation over the region 0-15'K. was made a Molecular weight = 53.0612; 1 gibbs = 1 defined cal./defined deg. by means of a plot of C,'T us. T 2and also by extrapolating Debye equation 8 values. The two methods agreed closely in giving the entropy a t 15°K. as 0.27 gibbs/ From the tables of Scatchard, et aL18we find, by mole. The Entropy of NH,.2Hz0. The entropy and free graphical interpolation, the following data for the reenergy values given in Table 1V for NH8.2H20 were action calculated on the assumption that this substance at2Hz01 NH3(sat. 1) = YH3.2HzO(sat.1) tained a perfectly ordered structure in approaching 0°K. and thus had no residual entropy. To test AFZ= ~ ~-2123 cal./mole the truth of this assumption we may use the data of AHz6. = -2842 cal./mole (1) Scatchard, Epstein, Warburton, and Cody' on the thermodynaniic properties of aqueous ammonia, in = -2.413 gibbs/mole combination with the well known entropies of liquid water and ammonia gas taken as 16.71 and 16.03 gibbs/ (8) G . Scatchard, L. F. Epstein, J. Warburton, and P.J. Cody. Kefrig. Eng., 53, 413 (1947). mole, respectively.
+
T h e Journal of Physical Chemistry
KOTES
I n converting the units in the tables of Scatchard, et al., we have taken 1 b.t.u. = 252.196 defined cal. We believe these tables to be most accurate near ordiLnary temperature but they evidently contain serious errors a t lower temperatures. For example, they a p pear to have been prepared on the assumption that the heat capacity was 1 cal./g. at all lower temperatures and all compositions. The heat capacities of (SHJ20, “,OH, and SHs.4!Hz0 are now available and it has become evident that the heat capacities are very much less than the values assumed’ by Scatchard, et ai. for extrapolation purposes. From the calculations of Hildenbrand and Ciauque4 for the reaction NH3(g,j
=
1)
=
KH3(sat. 1)
=
4-1298 cal./mole
AH260 =
--SO32 cal./mole
ASz6.
-21.230 gibbs/mole
AFzb0
=
(2)
Combining the data for reactions 1 and 2 2H20(1)
3057
-
+ NH3(g,f
=
1)
=
NH3.2HzO(sat.1) (3)
AF260
=
-825 cal./mole
AH260
=
-7874 cal./mole
AS26o
=
-23.643 gibbs/mole
S , NH3.2HzO(sat.1)
+
=
28, HzO(1) X, NHB(g, j = 1) - 23.643
=
2 X 16.71
=
55.81 gibbs/mole at 298.15OK.
+ 46.03 - 23.643
and this value may be compared with the more accurate result
soT
C, d In T
=
55.67 gibbs/niole obtained in the
present research. The agreement is within the liniits of the experimental error of the present data and the experiments on which the tables of Scatchard, et a1.,8are based, and indicates that the hydrogen bonding in NH, . 2 H z 0attains perfect order a t limiting low temperatures.
Acknowledgment. We thank P. R. Siemens and G. V. Calder for assistance with the measurements.
NOTES
Evidence for the Existence of the Crystalline Phase ECeS04.HzO
by D. R. Petersen, H. W. R i m , and 5.T. Sutton Chemical Physics Research Laboratory, T h e Daw Chemical C o m p a n y , M i d l a n d , Michigan (Received A p r i l 20. 1.964)
The extensive investigations, summarized by Pascal,’ of the crystalline hydrates of beryllium sulfate have been marked by disagreement over the existence of various postulated phases. For example, while the dehydration sequence of BeSO4< 4Hz0-the form found under conventional temperature and pressure conditions-is coininonly reported to be tetrahydrate to dihydrate to monohydrate to anhydrate, Campbell2 and his colleagues, in refuting some conclusions of
Rohmer3 on the nature of the dihydrate, were led to reject the monohydrate as a distinct species. The present note describes definite evidence for the existence of the disputed phase BeSO4*HzOthrough the joint application of the methods of differential thermal analysis (hereafter DTA), mass spectrometric thermal analysis (UTA), and X-ray diffraction (XRD). The study has verified that the normal dehydration sequence of the tetrahydrate is through the dihydrate and the monohydrate to the anhydrous form. Each of the members of this series was found to be crystalline and sufficiently stable to allow the photographic recording of its powder diffraction pattern. (1) P. Pascal, Ed., “Nouveau Trait6 de Chimie MinBrale,” Vol. 4 , Masson et Cie., Paris, 1958, gp. 75-80, ( 2 ) A. Tu’. Campbell, A. J. Sukava, and J. Koop, J . Am. Chem. Sac., 73, 2831 (1951). (3) R. Rohmer, B u l l . 8oc. chim., [51 10, 468 (1943).
V o l u m e 68, Number 10
October, 1961
