Temperature and pressure effects on the kinetics of the Bromate ion-iodide ion-L-ascorbic acid clock reaction |
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Authors: | Istvn Fbin Rudi Van Eldik |
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Institution: | István Fábián,Rudi Van Eldik |
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Abstract: | The kinetics of the bromate ion-iodide ion-L-ascorbic acid clock reaction was investigated as a function of temperature and pressure using stopped-flow techniques. Kinetic results were obtained for the uncatalyzed as well as for the Mo(VI) and V(V) catalyzed reactions. While molybdenum catalyzes the BrO -I? reaction, vanadium catalyzes the direct oxidation of ascorbic acid by bromate ion. The corresponding rate laws and kinetic parameters are as follows. Uncatalyzed reaction: r2 = k2BrO ] I?]H+]2, k2 = 38.6 ± 2.0 dm9 mol?3 s?1, ΔH? = 41.3 ± 4.2 kJmol?1, ΔS? = ?75.9 ± 11.4 Jmol?1 K?1, ΔV? = ?14.2 ± 2.9 cm3 mol?1. Molybdenum-catalyzed reaction: r′2 = k2BrO ] I?] H+]2 + kMoBrO ] I?] H+]2M0(VI)], kMo = (2.9 ± 0.3)106 dm12 mol?4 s?1, ΔH? = 27.2 ± 2.5 kJmol?1, ΔS? = ?30.1 ± 4.5 Jmol?1K?1, ΔV? = 14.2 ± 2.1 cm3 mol?1. Vanadium-catalyzed reaction: r′1 = kVBrO ] V(V)], kV = 9.1 ± 0.6 dm3 mol?1 s?1, ΔH? = 61.4 ± 5.4 kJmol?1, ΔS? = ?20.7 ± 3.1 Jmol?1K?1, ΔV? = 5.2 ± 1.5 cm3 mol?1. On the basis of the results, mechanistic details of the BrO -I? reaction and the catalytic oxidation of ascorbic acid by BrO are elaborated. © 1995 John Wiley & Sons, Inc. |
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