Isotope Exchange Rates. V. The Homogeneous Reaction between H2S and D2

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Abstract
Single-pulse shock-tube data were obtained for the homogeneous isotopic exchange reaction between deuterium and hydrogen sulfide, highly diluted in argon, over the temperature range 1260–1590°K. Three sets of experiments were performed independently in two laboratories. H2S/D2 ratios were varied over the range 4/1–1/4, at total reaction densities from 6 × 10−6 to 5 × 10−5 moles/cm3. The agreement among the three sets of data is generally good. An average activation energy of 52 ± 3 kcal/mole was deduced from them. All three sets of data may be fitted by a second-order power rate expression which is first order in D2 and roughly 12 order with respect to H2S and to Ar. These data support a mechanism, suggested previously for similar isotopic exchange reactions, in which metathesis reactions during collisions between H2S and D2 occur with significant probability only when the deuterium molecule contains at least a critical amount of vibrational energy. The rate-limiting process is the maintainance of an adequate population in the 4th and 5th vibrational levels of deuterium. The relative efficiencies for excitation of deuterium by collisions with the various species present, as determined from the kinetics data, are in reasonable agreement with values determined directly from vibrational relaxation studies.