Oct., 1961
REACTION OF SULFURTETRAFLUORIDE WITH URANIUM TRIOXIDE
represent the data better than a straight line. 2 = 1, stoichiometric NbC is found to have a heat of formation of -33.6 O.G kcal./mole. This value, however, is only of acadernic interest because stoichiometric S b C cannot be made under ordinary conditions, if a t all. The values obtained by Mah and Boylee and Kusenko and Gel’d’ are compared to this work in Fig. 1. NbzC also has a range of homogeneity but one that is very narrow. At the temperature of preparation the samples NbC0.489and NbCo.m probably bracketted this region very closely.
If this equation is extrapolated to
*
1849
*
Therefore, a value of -43.1 1.7 kcal./mole is the value of A H for Nb,C in equilibrium with Nb, and -46.6 i 1.2 kcal./mole is the value of A H for Nb2C in equilibrium with NbC. The form of the relat,ionship between these extremes is unknown. These values can only be compared with 0. H. Krikorian’s estimate3 of -40 i 8 kcal./ mole. Acknowledgments.-Valuable assistance was rendered by F. H. Ellinger, X-ray analysis; J. A. Mariner, spectrochemical analysis; and G. C. Heasley and L. Gritzo, chemical analysis. Thanks are due R. K. Zeigler for programming the data for the IBM 704 computer.
KINETICS OF THE REACTIOY OF SULFUR TETRAFLUORIDE WITH URANIUM TRIOXIDE AND URANYL FLUORIDE’ BY CARLE. JOHNSON AND JACKFISCHER Chemical Engineering lhisbn, Argonne National Laboratory, Argonne, Il&mk Received April 1, 1961
The reactions hetween sulfur tetrafluoride and uranium trioxide and sulfur tetrafluoride and uranyl fluoride have been studied between 255 and 370’ by following the change in weight of the solid phase using a therrnobalance. The rate of production of uranium hexafluoride is in agreement with the kinetics expected for reaction between a gas and a solid a t a continuously diminishing spherical interface. The rate of reaction is temperature dependent. The activation energies for reaction of sulfur tetrafluoride with uranium trioxide and uranyl fluoride are, respectively, 6.0 and 32.0 kcal./mole. The rat,e of reaction with uranyl fluoride is dependent upon the partial pressure of sulfur tetrafluoride. A 1.55 exponent was found. A minimum gas velocity was found above which the reaction rate is independent of the velocity of gaseous reactant past the solid.
Introduction The factors which govern the kinetic laws for A study of the rates of reaction between gaseous gas-solid reactions are: the chemical reactions a t fluorinating agents and uranium and its com- the phase boundary, the diffusion rates of reactants pounds provides information about the kinetics and the rates of nucleation and crystallization of and mechanisms of the reaction of gases with solid solids. A reaction taking place a t a surface may surfaces. In the majority of cases the end product in general be separated into five stepsI2 is volatile uranium hexafluoride and even though (1) Transport of the reacting gas to the surface intermediate compounds may be produced, the (2) Chemisorption of the gas (3) Chemical reaction a t the surface reaction proceeds on a reactive surface at all times. (4) Desorption of the reactant product Thus the rates are governed primarily by the proc(5) Transport of the reaction product an-ay from the esses a t the gassolid interface and do not depend surface on diffusion through a layer of condensed product, Transport of the reacting gas to the surface of as is the case in many oxidation reactions. the solid and transport of the reaction product An extensive amount of research has been done cm gassolid reactions for which the end products away from the surface are ordinary diffusion procare solids. Kinetic studies are more limited for esses. Diffusion in the gaseous state is considercd the case in which the products of reactions are to be fast involving little or no activation energy gaseous. They include work on the C-H20,2-* and is therefore not expected to limit the reaction C-C026s6 and C-027.8 systems, the UF4-F2,Q in any way. The processes of chemisorption of the gas and desorption of the reaction product are UF4-ClF31Oand Ki-C1211systems. much more likely to be the slow steps in hetero(1) Work performed under the auspicea of the U. 9. Atomic Energy geneous reactions since both may involve appreciaCommission. ble energies of activation. In practice, however, (2) G. 9. Scott, Ind. Eng. Chem., 33, 1279 (1941). (3) J. Gadsby, C. Rinshelwood and K. W. Sykes, Proc. Roy. Sor. it is not always convenient to separate the chemical (London), A189, 129 (1946). reaction a t the surface from the process involved (4) F. J. Long and .:E W. Sykes. tbrd., Al93, 377 (1948). in desorption of the reaction product, because of (5) L. J. Jolly and A . Poll,J . Inat. Fuel. 26, 33 (1953). insufficient knowledge about the desorption proc(6) J. Gadsby, F. J. Long, D. Sleightholm and K. W. Sykea. Proc. Roy. SOC.(London), A193, 357 (1948). ess. Therefore, what is generally done is to con(7) P. L. Walker, R. J. Foresti and C. C. Wright, Ind. Eng. Chem.. sider the reaction a t the surface, giving rise to 45. 1703 (1953). gaseous products, as a single step. (8) F. J. Long and K. W. Sykes, J. Chem. Phys.. 47, 361 (1950). I n this paper further work is reported on the re(9) V. Y. Labaton arid I
