Volume 4 @BULLET Issue 1 @BULLET 1000e115 J Thermodyn Catal ISSN: 2157-7544 JTC, an open access journal Editorial Open Access Why the Acidity of Bromic Acid Really Matters for Kinetic Models of Belousov-Zhabotinsky Oscillating Chemical Reactions (original) (raw)
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The Journal of Physical Chemistry A, 2005
The title reaction was studied with various techniques in 1 M sulfuric acid, a usual medium for the oscillatory Belousov-Zhabotinsky (BZ) reaction. It was found to be a more complex process than the bromomalonic acid (BrMA)-BrO 3reaction studied previously in the first part of this work. Malonic acid (MA) can react with acidic bromate by two parallel mechanisms. The main aim of the present research was to determine the mechanisms, the rate laws, and the rate constants for these parallel channels. In one reaction channel the first molecular products are glyoxalic acid (GOA) and CO 2 while in the other channel mesoxalic acid (MOA) is the first molecular intermediate, that is, no CO 2 is formed in this step. To prove these two independent routes specific colorimetric techniques were developed to determine GOA and MOA selectively. The rate of the GOA channel was determined by following the rate of the carbon dioxide evolution characteristic for this reaction route. In this step, regarding it as an overall process, one MA is oxidized to GOA and CO 2 and one BrO 3is reduced to HOBr, which forms BrMA with another MA. The initial rate of the GOA channel is a bilinear function of the initial MA and BrO 3concentrations with a second-order rate constant k GOA) 2.4 × 10-7 M-1 s-1. The rate of the other channel was calculated from the rate of the BrO 3consumption measured in separate experiments, assuming that the measured depletion is a sum of two separate terms reflecting the consumptions due to the two independent channels. In the MOA channel one MA is oxidized to MOA and one BrO 3is consumed while another MA is brominated as in the GOA channel. It was found that the initial rate of the MOA channel is also a bilinear function of the MA and BrO 3concentrations with a second-order rate constant k MOA) 2.46 × 10-6 M-1 s-1. Separate chemical mechanisms are suggested for both channels. In all of the various bromate-substrate reactions of these mechanisms oxygen atom transfer from the bromate to the substrate occurs generating bromous acid intermediate. This can be of high importance in BZ systems as bromous acid is the autocatalytic intermediate there. GOA and MOA also can be oxidized by acidic bromate but a study of these reactions will be published later.
The Journal of Physical Chemistry, 1986
The rate constant of the disproportionation of HBr02, 2HBr02-HBr03 + HOBr, an important step of the Belousov-Zhabotinskii oscillating system, was measured spectrophotometrically at 240 nm by using stopped-flow techniques. Its value at [HzS04] = 0.5 M, T = 24 OC was found to be k4 = 2.2 X lo3 M-l. R ough measurements with bromide-selective electrodes led to comparable results. This value is 4 orders of magnitude smaller than the one given by Field, Koros, and Noyes. Consequently, many other rate constants, whose values are known only relative to k4, will have to be changed as well.
The Journal of Physical Chemistry A, 2012
The results are reported of an ab initio study of the thermochemistry and of the kinetics of the HOBrO disproportionation reaction 2HOBrO (2) ⇄ HOBr (1) + HBrO 3 (3), reaction (R4′), in gas phase (MP2(full)/6-311G*) and aqueous solution (SMD(MP2(full)/6-311G*)). The reaction energy of bromous acid disproportionation is discussed in the context of the coupled reaction system R2−R4 of the FKN mechanism of the Belousov−Zhabotinsky reaction and considering the acidities of HBr and HOBrO 2. The structures were determined of ten dimeric aggregates 4 of bromous acid, (HOBrO) 2 , of eight mixed aggregates 5 formed between the products of disproportionation, (HOBr)(HOBrO 2), and of four transition states structures 6 for disproportionation by direct O-transfer. It was found that the condensation of two HOBrO molecules provides facile access to bromous acid anhydride 7, O(BrO) 2. A discussion of the potential energy surface of Br 2 O 3 shows that O(BrO) 2 is prone to isomerization to the mixed anhydride 8, BrO−BrO 2 , and to dissociation to 9, BrO, and 10, BrO 2 , and their radical pair 11. Hence, three possible paths from O(BrO) 2 to the products of disproportionation, HOBr and HOBrO 2 , are discussed: (1) hydrolysis of O(BrO) 2 along a path that differs from its formation, (2) isomerization of O(BrO) 2 to BrO−BrO 2 followed by hydrolysis, and (3) O(BrO) 2 dissociation to BrO and BrO 2 and their reactions with water. The results of the potential energy surface analysis show that the rate-limiting step in the disproportionation of HOBrO consists of the formation of the hydrate 12a of bromous acid anhydride 7 via transition state structure 14a. The computed activation free enthalpy ΔG act (SMD) = 13.6 kcal/ mol for the process 2•2a → [14a] ‡ → 12a corresponds to the reaction rate constant k 4 = 667.5 M −1 s −1 and is in very good agreement with experimental measurements. The potential energy surface analysis further shows that anhydride 7 is kinetically and thermodynamically unstable with regard to hydrolysis to HOBr and HOBrO 2 via transition state structure 14b. The transition state structure 14b is much more stable than 14a, and, hence, the formation of the "symmetrical anhydride" from bromous acid becomes an irreversible reaction for all practical purposes because 7 will instead be hydrolyzed as a "mixed anhydride" to afford HOBr and HOBrO 2. The mixed anhydride 8, BrO−BrO 2 , does not play a significant role in bromous acid disproportionation.
Journal of Physical Chemistry A, 2012
The results are reported of an ab initio study of the thermochemistry and of the kinetics of the HOBrO disproportionation reaction 2HOBrO (2) ⇄ HOBr (1) + HBrO 3 (3), reaction (R4′), in gas phase (MP2(full)/6-311G*) and aqueous solution (SMD(MP2(full)/6-311G*)). The reaction energy of bromous acid disproportionation is discussed in the context of the coupled reaction system R2−R4 of the FKN mechanism of the Belousov−Zhabotinsky reaction and considering the acidities of HBr and HOBrO 2. The structures were determined of ten dimeric aggregates 4 of bromous acid, (HOBrO) 2 , of eight mixed aggregates 5 formed between the products of disproportionation, (HOBr)(HOBrO 2), and of four transition states structures 6 for disproportionation by direct O-transfer. It was found that the condensation of two HOBrO molecules provides facile access to bromous acid anhydride 7, O(BrO) 2. A discussion of the potential energy surface of Br 2 O 3 shows that O(BrO) 2 is prone to isomerization to the mixed anhydride 8, BrO−BrO 2 , and to dissociation to 9, BrO, and 10, BrO 2 , and their radical pair 11. Hence, three possible paths from O(BrO) 2 to the products of disproportionation, HOBr and HOBrO 2 , are discussed: (1) hydrolysis of O(BrO) 2 along a path that differs from its formation, (2) isomerization of O(BrO) 2 to BrO−BrO 2 followed by hydrolysis, and (3) O(BrO) 2 dissociation to BrO and BrO 2 and their reactions with water. The results of the potential energy surface analysis show that the rate-limiting step in the disproportionation of HOBrO consists of the formation of the hydrate 12a of bromous acid anhydride 7 via transition state structure 14a. The computed activation free enthalpy ΔG act (SMD) = 13.6 kcal/ mol for the process 2•2a → [14a] ‡ → 12a corresponds to the reaction rate constant k 4 = 667.5 M −1 s −1 and is in very good agreement with experimental measurements. The potential energy surface analysis further shows that anhydride 7 is kinetically and thermodynamically unstable with regard to hydrolysis to HOBr and HOBrO 2 via transition state structure 14b. The transition state structure 14b is much more stable than 14a, and, hence, the formation of the "symmetrical anhydride" from bromous acid becomes an irreversible reaction for all practical purposes because 7 will instead be hydrolyzed as a "mixed anhydride" to afford HOBr and HOBrO 2. The mixed anhydride 8, BrO−BrO 2 , does not play a significant role in bromous acid disproportionation.
The Journal of Physical Chemistry, 1991
The oscillatory Belousov-Zhabotinsky reaction has been perturbed with sodium bromite solutions of various strengths. Experimental phase response curves have been compared with simulation calculations by using the original Oregonator, a seven-variable Oregonator model including reactions of Br2, the Showalter, Noyes, Bar-Eli (SNB) model, and a large reaction scheme containing 80 reactions with 26 kinetic active components. For HBr02-induced transitions to the excitable branch qualitative and almost quantitative agreements between experimental and calculated phase response curves are obtained with the seven-variable Oregonator and the largest model. The original Oregonator and the SNB model are not able to describe experimentally observed positive phase shifts. This work also shows that Br2 must be included in models that attempt to provide quantitative description of the Belousov-Zhabotinsky reaction.
The Journal of Physical Chemistry A, 2004
The aim of the present paper is to study radical reactions important in the mechanism of the Belousov-Zhabotinsky (BZ) reaction with its simplest organic substrate, oxalic acid, and to model the oscillatory system applying the newly determined rate constants. We considered five radical species in this BZ system: carboxyl radical, bromine atom, dibromine radical ion, and bromine monoxide and dioxide radicals. To study separately reactions of only three radicals, • CO 2 H, • Br, and • Br 2-, semibatch experiments were performed. The semibatch reactor contained oxalic acid, elemental bromine, and bromide ions in a solution of 1 M H 2 SO 4 at 20°C, and a continuous inflow of Ce 4+ generated carboxyl radicals. The carboxyl radicals initiate a chain reaction: first they react with elemental bromine and produce bromine atoms (CR1); then the bromine atoms react with oxalic acid, producing carboxyl radicals again (CR2). Consumption of elemental bromine in the chain reaction was followed with a bright Pt electrode. By measuring the stoichiometry of the chain reaction, it was possible to determine or estimate several rate constants. It was found that CR1 is a fast reaction with an estimated k value of more than 10 9 M-1 s-1. The rate constant of CR2 is 7 × 10 5 M-1 s-1 , and the k value for the Ce 4+-• CO 2 H reaction is 1.5 × 10 9. These values were obtained by comparing experiments with model calculations. Such simulations also suggested that a reaction of • Br 2with oxalic acid, analogous to CR2, plays a negligible role or no role here. Simulations of the oscillatory system applied rate constants, which were known from the literature, or determined here or in the first part of our work, and some unknown rate constants were estimated on the basis of analogous radical reactions. To obtain an optimal fit between experiments and simulations, only one rate constant was used as a variable parameter. This was the reaction of carboxyl radical with acidic bromate with an optimal k value of 1.0 × 10 7 M-1 s-1. Agreement between experimental and simulated oscillations was satisfactory at low bromine removal rates (that rate was controlled by a nitrogen gas flow), but a disagreement was found at higher flow rates. Possible reasons for this disagreement are discussed in the conclusion.
Physical Chemistry Chemical Physics, 2000
High-pressure liquid chromatography (HPLC) and measurements of the produced were performed in the CO 2 induction period of the classical BelousovÈZhabotinsky (BZ) reaction (malonic acidÈbromateÈcerium catalyst in sulfuric acid medium). It was found that oxalic acid is a Ñow-through intermediate of the reaction. This was conÐrmed with an independent qualitative test with thiobarbituric acid. The concentration of oxalic acid grows in the induction period together with that of bromomalonic acid and dibromomalonic acid intermediates. It is known that there are two negative feedback loops in the BZ reaction : one is via bromide and the other via organic free radicals. Oxalic acid and also are products of this second loop where CO 2 organic radicals react with radicals. The induction period was chosen for the present experimental BrO 2 studies because the above radicalÈradical reactions are most intense during that time. Based on the experimental results mechanistic proposals are made for the radical feedback loop. A method to accumulate multivalent organic acids present in very low concentrations in the BZ reaction was also developed. Applying this and a thermal decomposition method ethenetetracarboxylic acid (EETA) was identiÐed as an oxidation product of ethanetetracarboxylic acid (ETA).