Activation Energy for the Disproportionation of HBrO₂and Estimated Heats of Formation of HBrO₂and BrO₂

Abstract

The kinetics of the reaction HBrO(2) + HBrO(2) --> HOBr + BrO(3)(-) + H(+) is investigated in aqueous HClO(4) (0.04-0.9 M) and H(2)SO(4) (0.3-0.9 M) media and at temperatures in the range 15-38 degrees C. The reaction is found to be cleanly second order in [HBrO(2)], 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 < [HBrO(2)](0) < 0.02 M. The detailed mechanism of this reaction is discussed. The activation parameters for kare found to be E(double dagger) = 19.0 +/- 0.9 kJ/mol and DeltaS(double dagger) = -132 +/- 3 J/(K mol) in HClO(4), and E(double dagger) = 23.0 +/- 0.5 kJ/mol and DeltaS(double dagger) = -119 +/- 1 J/(K mol) in H(2)SO(4). The activation parameters for k' are found to be E(double dagger) = 25.8 +/- 0.5 kJ/mol and DeltaS(double dagger) = -106 +/- 1 J/(K mol) in HClO(4), and E(double dagger) = 18 +/- 3 kJ/mol and DeltaS(double dagger) = -130 +/- 11 J/(K mol) in H(2)SO(4). The values Delta(f)H(29)(8)(0)[BrO(2)(aq)] = 157 kJ/mol and Delta(f)H(29)(8)(0)[HBrO(2)(aq)] = -33 kJ/mol are estimated using a trend analysis (bond strengths) based on the assumption Delta(f)H(29)(8)(0)[HBrO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOBr(aq)] and Delta(f)H(29)(8)(0)[HBrO(3)(aq)] as Delta(f)H(29)(8)(0)[HClO(2)(aq)] lies between Delta(f)H(29)(8)(0)[HOCl(aq)] and Delta(f)H(29)(8)(0)[HClO(3)(aq)]. The estimated value of Delta(f)H(29)(8)(0)[BrO(2)(aq)] agrees well with calculated gas-phase values, but the estimated value of Delta(f)H(29)(8)(0)[HBrO(2)(aq)], as well as the tabulated value of Delta(f)H(29)(8)(0)[HClO(2)(aq)], is in substantial disagreement with calculated gas-phase values. Values of Delta(r)H(0) are estimated for various reactions involving BrO(2) or HBrO(2).

Measurements and Simulations of the Acidity Dependence of the Kinetics of the Iron-Catalyzed Belousov–Zhabotinsky Reaction: Proton-Catalysis in the Electron Transfer Reaction Involving the [Fe(phen)3]3+ Species

“…A value of 1.7 × 10 5 M –1 s –1 was assigned to the rate constant of R4b, and the adjustable parameter of 0.02 M –1 was used as the equilibrium constant for the protonation reaction HBrO 2 + H + ⇄ H 2 BrO 2 + . Agreda and Field showed the overall rate of HBrO 2 disproportionation to be consistent with both reactions R4 and R4b occurring in parallel, and they refined the rate constant for R4b to the value 1013 ± 16 M –1 s –1 .…”

“…This FKN mechanism involves (1) bromide consumption, (2) autocatalytic formation of an oxidizing intermediate (BrO 2 • ) and oxidation of the redox catalyst, and (3) reduction of the redox catalyst by oxidation of the organic substrate with concomitant releases of bromide back into the system. The classical FKN reactions are shown in Table along with a few newer reactions and the best rate constants for each reaction. − …”

Low frequency temperature forcing of chemical oscillations

“…where n designates the number of the reaction step, E n is the associated activation energy and R is the gas constant = 8.314 J K À1 mol À1 . The rate constants and activation energies (Table 1) were taken from earlier estimates, 23,24 except for E 3 which was adjusted to match the experimental results (see discussion).…”

Why the Acidity of Bromic Acid Really Matters for Kinetic Models of Belousov-Zhabotinsky Oscillating Chemical Reactions

“…The cerium-catalyzed Belousov-Zhabotinsky oscillating reaction [1][2][3][4] and its variants (Fe [5,6], Ru [7,8], Mn [9][10][11]) are inherently connected to the chemistry of bromine oxoacids HBrO x . The growing interest in nonlinear chemical systems [12][13][14] has provided strong incentives to study bromine chemistry, and especially the disproportionation of bromous acid has been studied in great detail [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. The bromine chemistry of the Belousov-Zhabotinsky reaction (BZR) involves hypobromous acid HOBr (1), bromous acid HOBrO (2) and bromic acid HBrO 3 (3), and the disproportionation reaction of bromous acid (R4) is a key step.…”

“…It is generally agreed that hypobromous acid (pK a (HOBr)=8.59 [30,31]) and bromous acid (pK a (HOBrO)=3.43 [29]) are not dissociated at pH values typical for the BZR. In contrast, it has been assumed that bromic acid is dissociated under BZR conditions even though the acidity of bromic acid has not been well established.…”