Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms

Details Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms Kinetics of isotope exchange reactions involving intra- and intermolecular reactions: I. Rate law for a system with two chemical compounds and three exchangeable atoms

Jul 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1953c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 55, Issue 7, pp.1953-1961

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Scientific paper

Intramolecular isotope fractionation within a molecule containing two exchangeable atoms in structurally non-equivalent positions (E.G., the two sulfur atoms in thiosulfate) leads to the complexity of the isotopic exchange between the molecule and another compound (E.G., H 2 S). We have derived a rate law for the isotopic exchange reactions in a homogeneous (gaseous or aqueous) system of two-compound ( X and AB ) system with three exchangeable components ( X , A , and B ) where one overall intramolecular exchange ( A B ) and two overall intermolecular exchange ( X A and X B ) reactions exist. The isotopic value of each component ( X , A , and B ) at any time can be expressed as a function of the initial values of the three components, the concentrations of the two compounds, and the overall rate constants for the intra- and intermolecular isotope exchange reactions. Differing from the isotopic exchange behavior in a simple two-compound system described by previous investigators (E.G., Ohmoto and Lasaga, 1982), the changes with time of the values of X , A , and B may not be monotonie, because the isotopic exchange rates may not be identical for the three overall isotope exchange reactions ( A B , X A , and X B ). The In (1 - F ) vs. time curve for the overall reaction between the two compounds ( X AB ) is generally non-linear. The rate law derived in this study was applied to a set of experimental data obtained by Uyama et al. (1985) on sulfur isotope exchange reactions between H 2 S and thiosulfate (Run 120-A), where the 34 S values of H 2 S( X ) and sulfane sulfur ( A ) and sulfonate sulfur ( B ) of thiosulfate were measured during reaction for various run times at 120°C and at near-neutral pH. From their experimental data, we were able to deduce the following sets of rate constants and equilibrium fractionation factors: log k H 2 S SH of thiosulfate = -1.4, log k H 2 S SO 3 H of thiosulfate = -2.6, log k SH 2 SO 3 H of thiosulfate = -8, 1000 In SH of thiosulfate - H 2 S = -4.6, 1000 In SO 3 H of thiosulfate - H 2 S = 29.2 and 1000 In SO 3 H - SH of thiosulfate = 33.7.

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