chloride are given in 'rable 11. Thc \-dues of entropy and free energy do not include the effects
of Mines, a s&iple of the nickelous chloride prepared in this research was have measured between 298 and 1336'K. The results been published by C ~ u g h l i nwho ~ ~ gives tables of the increments of the thermodynamic properties above 298.16'K
To extend the present results to the higher ternperatures
I
I
fi? - fib I/'
=
I€+ - I€: - sl" I'
We thank Dr. J. E. KunLler for assistance with the heat capacity measurements. BERKELLY, C ~ L I F O R ZI A
(1.5) J P Coupblin TITI\I O V R Y A I ,73 i i l I f I ( i i 1
[ C O N T R I B U T I O S FROM T H E b I I N E R A L S THERMODYSAMICS BRANCH, REGION111, B U R E A U OF DEPARTMENT O F THE I N T E R I O R ]
MINES,U N I T E D
STATES
Heats of Formation of Manganous Metasilicate (Rhodonite) and Ferrous Orthosilicate (Fayalite ) BY E. G. KING KECEIVEDFEBRVARY 23, 195%
The heats of formation of mangatlous inetasilicate and ferrous orthosilicate were obtained by hydrofluoric acid solution = -308.22 f 0.34 and A H ~ ~ s= . I o-346.0 i 1.1kcal./mole (heats of calorimetry. The results are, respectively, AH~SS.IS formation from the elements); and AH2~8.jti= -.i.92 5 0.17 and AH298.1fi = -8.2 i 1.1kcal./mole (heats of formation from the oxides).
Recent from this Laboratory reported heat of formation values at 298.16'K. for silicates of calcium, magnesium and zinc. The present paper gives the results of similar investigations of manmnous metasilicate (rhodonite) and ferrous orthosiicate (fayalite). '
tiescrjbetl by Kelley and Moores by hydrogen reduction a t 1100 . Analysis gave a purity of 99.92%. The hydrochloric acid solution, which enters into the ferrous silicate measurements, was standardized against sodium carbonate prepared from reagent grade sodium bicarbonate. It analyzed 25.85y0 hydrogen chloride, corresponding to HCl,5.806Hz0.
Method and Materials.-The hydrofluoric acid solution calorimeter and method of operation were described previously . I J Reagent-grade hydrofluoric acid, 20.1 % ' by weight, wab used as the solution medium. The calorimeter was operated a t 73.7' for the manganous silicate measurements and a t 60.0' for ferrous silicate measurements. All samples were dropped from 25" into the calorimeter a t operating temperature. The samples were enclosed in gelatin capsules, for which corrections were determined by separate experiments. In each determination, 940.1 g. of the acid was employed. The masses of all other materials conformed stoichiometrically with 0.7420 g. of quartz. The time required t o reach thermal equilibrium ranged from 10 to 30 min., depending upon the substance being dissolved. Likewise, the calorimeter temperature rises ranged from 0.15 to 0.65'. The calorimeter was calibrated several times during the course of each set of measurements, by supplying a measured amount of electrical energy and accurately determining the correspondixig temperature rise. Preparations and analyses of the manganous metasilicate (rhodonite) and ferrous orthosilicate (fayalite) riwrl i n t h k work were described by K e l l e ~ . ~ Anhydrous ferrous chloride was prepared by the method of Kelley and Moore,* except that a hydrochloric acid solution of pure electrolytic iron (99.9670) was used as a starting material instead of ferrous chloride tetrahydrate. Analysic of the product gave 44.15% iron and 55.9170 chlorine, as compared with the theoretical 44.06 and 55.94%. The quartz was the same as that employed in previous work.1,2 It was finely ground, water elutriated, and dried a t 180'. Correction in the measurements was made for 0.4% residual moisture content. Manganous oxide was prepared from the pure dioxide
Measurements and Results.-All thermal values are expressed in defined calories (1 cal. = 4.1833 int. joules). All weights were reduced to vacuum, and all molecular weights accord with the 1949 International Atomic Weights. Table I presents schematically the process used in obtaining the heat of formation of manganous metasilicate. I l E A T OF FORMATIOS OF
TABLE I IhSiOa( R H O D O N I T E ) ( C A L . / b f O L E ) Uncer-
Reaction
+
+ +
11) MnSiOn ( c , 2 5 ' ) 8H' (sol., 73.7') 6 F - (sol., 73.7O) * M n + + (sol., 73.7O) HzSiFe (sol., 73.7') 3H20 (sol.,73.7') (2) Si02 (c, 25') 6 H + (sol., 73.7') 6F(sol., 73.7') -+ HsSiFa (sol., 73.7') 3- 2H20 (sol., 73.70)
+
+
(3) M i l 0 ( c , 2 5 0 ) (sol..7 3 7') ' 1 1 LfrlO
! c . 2.5'1
+
4-211 +
+
AlI
(sol., 7 3 . 7 9 i M I H20 (sol., 7 3 . i 0 j
+ Sior (c. 2;")
?
