ABSTRACT
The rates of sublimation of ammonium chloride, bromide, iodide, and fluoride have been determined by two different experimental techniques over the temperature range of 100° to 600°C, corresponding to an increase in sublimation rate of 104. The two experimental methods employed were the isothermal rate of weight loss using a quartz spring balance, and the hot‐plate linear pyrolysis method. The low activation energy (about one‐third the heat of sublimation) found for ammonium chloride by earlier investigators has been verified and found to extend to the other halides. The frequency factor is also abnormally low for the ammonium halides, by a factor of roughly 104, with the exception of the fluoride, for which the factor is only about 500. These results are in fair agreement with the Schultz—Dekker mechanism for ammonium halide sublimation.
- 1. H. Spingler, Z. Physik. Chem. (Leipzig) B52, 90 (1942). Google Scholar
- 2. R. D. Schultz and A. O. Dekker, Fifth International Symposium on Combustion (Reinhold Publishing Company, Inc., New York, 1955), p. 260. Google Scholar
- 3. The accommodation coefficient for evaporation refers to the constant which appears in the Knudsen‐Hertz evaporation rate equation. It should be noted that the physical significance of this coefficient is questionable when applied to substances which dissociate upon evaporation. Google Scholar
- 4. M. Volmer, Kinetic der Phasenbildung (Dresden and Leipzig 1939). Google Scholar
- 5. O. Knacke, I. N. Stranski, and G. Wolff, Z. Physik. Chem. (Leipzig) 198, 157 (1951). Google ScholarCrossref
- 6. The half‐crystal site (Halbkristallage) is taken as that surface site where the coordination number is one‐half that for the crystal interior. As such, it is considered that the energy to remove a molecule from that site to the gas phase is exactly the heat of vaporization. An important feature of the half‐crystal site is that it is the edge of a surface step, and hence regenerating (i.e., the removal of a molecule from the site forms another filled half‐crystal site). Google Scholar
- 7. O. Knacke and I. N. Stranski, Metal Phys. 6, 181 (1956). Google ScholarCrossref
- 8. R. D. Schultz and A. O. Dekker, J. Phys. Chem. 60, 1095 (1956). Google ScholarCrossref
- 9. R. Littlewood and E. Rideal, Trans. Faraday Soc. 52, 1598 (1956). Google ScholarCrossref
- 10. Spingler’s1 studies were also carried out under isothermal vacuum heating conditions; however, his rate determinations were based upon an averaged time‐lapsed weight loss. Google Scholar
- 11. L. L. Bircumshaw and T. R. Phillips, J. Chem. Soc. 1957, 4741. Google Scholar
- 12. H. H. Landolt and R. Börnstein, Physikalisch‐Chemiscke Tabellen (Verlag Julius Springer, Berlin, Germany, 1935), 5th ed., 3rd suppl., p. 304. Google Scholar
- 13. M. K. Barsh, W. H. Andersen, K. W. Bills, Jr., G. Moe, and R. D. Schultz, Rev. Sci. Instr. 29, 392 (1958). Google ScholarScitation, ISI
- 14. R. F. Chaiken and D. K. Van de Mark, Rev. Sci. Instr. 30, 375 (1959). Google ScholarScitation, ISI
- 15. Reference is also made to the unpublished ammonium chloride hot‐plate data by K. W. Bills, M. Therneau, E. Mishuck, and R. D. Schultz [OSR‐TN‐55‐117 (1955)] in which and Google Scholar
- 16. See Eqs. (18), (19), and (28) of reference 8. Google Scholar
- 17. “Selected Values of Chemical Thermodynamic Properties” U.S. National Bureau of Standards, Circ. 500, 1952. Google Scholar
- 18. The authors are indebted to B. L. McFarland of Aerojet‐General Corporation for demonstrating the applicability of this approach. Google Scholar
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