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.
In this paper we apply the statistical-mechanical method which has only recently been developed for reaction rates to three new groups of reactions. These are the reactions of I2, Cl and Br with the various kinds of hydrogen molecules. In the first type (I2+H2→2HI) symmetry properties enable one to divide the six internal modes of vibrations for the four coplanar atoms into three sets with one, two and three modes of vibration in each so that in obtaining the frequency nothing greater than a cubic is solved. This treatment of the frequencies will apply to a very broad class of reactions. The potential surfaces probably yield too high a zero-point energy for the activated state. In spite of this result they give the right relative rates for the various isotopic reactions because mass enters very strongly into what in the past has been thought of as the collision factor. The study of isotopic reactions in conjunction with the new theory of rates gives a detailed knowledge of the potential in the activated state and can be carried much farther than has been done in this preliminary treatment. The treatment has been carried far enough however to show its general applicability to these types of processes.
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