THERMODYNAWIC PROPERTIES OF CALCIUM HYDRIDE
May, 1963
-* Q
Y
1061
I-'
7=0
Y
0.98
0
v
I
u
7=I 1
0
0
Q 100
300
I
I
500
700
-
900
1/2
Mn Fig. 6.-Reduced
temperature representation of intrinsic viecosities, eq. 10 and 11. The lowest line (points with cross line) indicates the use of the above data for the evaluation of the parameter K .
Since T,/e = 1, this would imply that the critical temperature T , is the arithmetic mean of 8 and the hypothetical temperature To a t which the intrinsic viscosity becomes molecular weight independent. Acknowledgment.-The support of this work hy grants from the California Research Corporation and the Socony Mohil Oil Company, Inc., is gratefully acknowledged.
We have thus examined the validity of eq. 2 and 1 in various ways. The representation in Fig. 6 cowers the whole range of molecular xeights and temperatures and provides a check on the observed molecular weight dependence of Toas well as the temperature dependence of h3. We note finally from eq. 10 that a = 0 for a reduced temperature T~ = 2 for both toluene and cyclohexane.
THERMODYNAXIC PROPERTIES OF CALCIUM HYDRIDE] BY R. W. CURTISASD PREMO CHIOTTI Institute for Atomic Research and Department of Metallurgy, Iowa State University, A m e s , Iowa Received October Si, 1962 The equilibrium hydrogen pressure over a system consisting of calcium and calcium hydride was measured in the temperature range 600-900". The data below 780' can be represented by log P(atm.) = -9610/T 7.346 and that above 780" by log P(atm.) = -8890/T 6.660. The change in slope a t 780" is to be expected on the basis of a solid state transformation existing in calcium hydride. The hydrogen dissociation pressures for the temperature range 600-780" were combined with known thermodynamic data for calcium along with known data on the calcium-hydrogen system to give the following thermodynamic relations: AF0cam = -42,278 31.52T (440-780') and AF0ca=2 = -41,410 24.787' 1.93T log T (25-440').
+
+
+
+
Introduction previous work on the calcium-hydrogen system jncludes the measurement of equilibrium hydrogen presOver the system consisting Of (1) Contribution No. 1236. Work was performed in the Amen Labor&tory of the United S t a t e @Atamia Energy Cernmisaion,
+
hydride, and hydrogen by Treadwell and Stecher2"and . ~results ~ of their investigations Johnson, et ~ 1 The were expressed as (2) (a) W. D. Treadnell and J. Steoher, Helv. Chem. Acta, 36, 1820 (1957); (b) W. C. Johnson, M. F. Stubbu, Ad E, Sidwell, and A. Peahukas, J . Am. Chem. ~ o c , ,61, 318 (1939),
R. W. CURTISAND PREMO CHIOTTI
1062
- 9840 (atm.) = -
log
T
+ 7.32
and log
PHz
(atm.) =
- 10,820 T
+ 8.612
respectively, for the temperature range 778-900'. Bronsted3 measured the dissociation pressure of calcium hydride in the temperature range 841-747'. He obtained from these data a value of -43,900 cal./mole for the room temperature heat of formation of calcium hydride. Bronsted also determined the heat of formation from calorimetric measurements on the heat of solution of calcium and the hydride. These measurements gave a roum temperature value of -45,100 cal./ mole as the enthalpy of formation. Lewis and Randall,4after re-evaluating the difference in heat capacity of the products and reactants for the calcium hydride decomposition, derived an expression for the free enery which was consistent with Bronsted's dissociation pressures and room temperature calorimetric enthalpy. Lewis and Randall point out, however, that the calculated entropy change for the dissociation, 34.6 cal./ mole-deg., is probably about 4 entropy units too large. A study of the allotropic modifications of calcium and the effect of various impurities on the transformations has been carried out by Smith and Bernstein5 and by Smith, Carlson, and Vest.6 The phase diagram for the calcium-calcium hydride system has been determined by Peterson and Fattore.' These new data and the availability of high purity calcium prompted a reinvestigation of the dissociation pressure of calcium hydride.
Materials and Experimental Procedure Materials.-The calcium employed in this study was prepared a t the Ames Laboratory.'' The analysis of the metal is given in Table I.
TABLE I E1ement
Content (p.p.m.)
250 200 -100 50 5
