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Inorganic Reactions of Iodine(III) in Acidic Solutions and Free Energy of Iodous Acid Formation

par Schmitz, Guy
Référence International journal of chemical kinetics, 40, 10, page (647-652)
Publication Publié, 2008-10

Article révisé par les pairs

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Résumé :

An analysis of the former works devoted to the reactions of I(III) in acidic non-buffered solutions gives new thermodynamic and kinetic information. At low iodide concentrations the rate law of the reaction IO3- + I- + 2 H+ = IO2H + IOH is k+B [IO3-][I-][H+]2 - k-B [IO2H][IOH] with k+B = 4.5x103 M-3s-1 and k-B = 240 M-1s-1 at 25°C and zero ionic strength. The rate law of the reaction IO2H + I- + H+ = 2 IOH is k+C [IO2H][I-][H+] - k-C [IOH]2 with k+C = 1.9x1010 M-2s-1 and k-C = 25 M-1s-1. These values lead to a Gibbs free energy of IO2H formation of - 95 kJ/mol. The pKa of iodous acid should be about 6, leading to a Gibbs free energy of IO2- formation of about - 61 kJ/mol. Estimations of the four rate constants at 50°C give respectively 1.2x104 M-3s-1, 590 M-1s-1, 2x109 M-2s-1 and 20 M-1s-1. Mechanisms of these reactions involving the protonation IO2H + H+ = IO2H2+ and an explanation of the decrease of the last two rate constants when the temperature increases, are proposed.