Activation Energy for the Disproportionation of HBrO2 and Estimated Heats of Formation of HBrO2 and BrO2

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2006-06, Vol.110 (25), p.7867-7873
• Main Authors: Ágreda B, Jesús Alberto, Field, Richard J
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0606412

The kinetics of the reaction HBrO2 + HBrO2 → HOBr + BrO3 - + H+ is investigated in aqueous HClO4 (0.04−0.9 M) and H2SO4 (0.3−0.9 M) media and at temperatures in the range 15−38 °C. The reaction is found to be cleanly second order in [HBrO2], with the experimental rate constant having the form k exp = k + k‘[H+]. The half-life of the reaction is on the order of a few tenths of a second in the range 0.01 M < [HBrO2]0 < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for k are found to be E ‡ = 19.0 ± 0.9 kJ/mol and ΔS ‡ = −132 ± 3 J/(K mol) in HClO4, and E ‡ = 23.0 ± 0.5 kJ/mol and ΔS ‡ = −119 ± 1 J/(K mol) in H2SO4. The activation parameters for k‘ are found to be E ‡ = 25.8 ± 0.5 kJ/mol and ΔS ‡ = −106 ± 1 J/(K mol) in HClO4, and E ‡ = 18 ± 3 kJ/mol and ΔS ‡ = −130 ± 11 J/(K mol) in H2SO4. The values Δf H 29 8 0{BrO2(aq)} = 157 kJ/mol and Δf H 29 8 0{HBrO2(aq)} = −33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Δf H 29 8 0{HBrO2(aq)} lies between Δf H 29 8 0{HOBr(aq)} and Δf H 29 8 0{HBrO3(aq)} as Δf H 29 8 0{HClO2(aq)} lies between Δf H 29 8 0{HOCl(aq)} and Δf H 29 8 0{HClO3(aq)}. The estimated value of Δf H 29 8 0{BrO2(aq)} agrees well with calculated gas-phase values, but the estimated value of Δf H 29 8 0{HBrO2(aq)}, as well as the tabulated value of Δf H 29 8 0{HClO2(aq)}, is in substantial disagreement with calculated gas-phase values. Values of Δr H 0 are estimated for various reactions involving BrO2 or HBrO2.