Bromates, reactions

Thiocyanate ion frequently reacts in oxidation with a rate-determining step reminiscent of those with the halides, and this oxidation is no exception. A study based on measurements of the limiting polarographic currents for thiocyanate in the presence of hydrochloric acid and bromate gives clear evidence of first-order dependence on bromate and on thiocyanate ion concentrations and of second-order dependence on hydrogen ion concentration. The authors report the Arrhenius parameters as, AH = 11.4 kcal.mole and AS = 28.0 caLmole . deg . One strange, and unexplained, anomaly is present. The experimental first-order coefficient for bromate reaction appears to be constant over about one half-life for equimolar initial thiocyanate and bromate concentrations. Yet these rate coefficients depend linearly on thiocyanate ion concentration for [SCN ] > [BrOj ], but non-linearly below. It seems improbable that the postulated mechanism with first steps (1 )-(3), is capable of explaining this, viz. 

IIIC) D Alba, F., Serravalle, G. Oxidation of Malonic Acid by Potassium Bromate (Reaction... 

F.R. Duke and J.M. Shlegel, Acid-Base Reactions in Fused Salts. The Dichromate Bromate Reaction, J. Phys. Chem. 67 (1963) 2487-2488. 

T. Fukasawa, S. Kawakubo, and A. Unno, Flow-Injection Spectropho-tometric Determination of Trace Vanadium Based on Catalysis of the Gallic Acid Bromate Reaction. Anal. Chim. Acta, 183 (1986) 269. 

All the component reactions investigated are found to be exothermic. Initial temperature rise of bromide + bromate reaction was found to be the highest (0.55°C/min) while that of cerous + bromate + malonic acid was to be found to be quite low (0.085°C/min). Thus in the first stage, when the reaction was mixed, the latter reaction involving autocatalysis predominates and the temperature rise is very slow. On the other hand, when Br + BrOj reaction involving inhibition reaction becomes dominant, there is a sharp rise in temperature. The thermochemical behaviour is thus in conformity with the FKN mechanism (Br -control mechanism). 

Oscillating chemical reactions continue to be investigated in sulphate media. Addition of a-monobromo-ketone, one of the products of the BrO 3-Ce v-cyclohexanone and Br03 -Ce -cyclopentanone systems has been shown to decrease and, in some instances, suppress the initial induction period. The inhibitory effects of Cl ions have also been described. In the gallic acid-bromate reaction catalysed by [Fe(phen)3]+/ + the enol is the reactive form of the organic substrate. The mechanism proposed is essentially that of... 

In aquation of the cations [M(NH3)5Br] + (M = Cr or Co) in alcohol-water mixtures, solvent composition variation has much more effect on A.S than on iiH. This is attributed to solvation-shell ordering effects in the transition state. Enhancement of reactivity in the reaction of cobalt(ii) with chlorophyllic acid in methanol on addition of lithium nitrate is attributed to inhibition of transition-state solvation. The effective radius of a transition state can be guessed from the variation of rate constant with dielectric constant. This approach has been used for the bromide-bromate reaction. In contrast to this concentration of attention on the transition state, it may be noted that it is the stabilization of the reactant in relation to water structure that is thought to control the variation of the racemization rate of the ( + )-[Co(phen)3] + cation in t-butyl alcohol-water mixtures. ... 

The oxidation of tetrametliylthiourea (TTTU) by bromine and acidic bromate ion in aqueous media has been studied. The bromate reaction, which has an induction period followed by formation of bromine, involves formation of tetramethythiourea sulfenic acid, which is oxidized to tetramethylurea and sulfate ion as final products. There was no evidence for formation of the sulfinic and sulfonic acids, which implicated the sulfoxylate anion as a precursor to formation of sulfate ion. ... 

The [H+] term in bromate reactions referred to above has been confirmed in the reaction with bromide. A similar observation has been made in the reaction with the mercuryfi) dimer species. In this case the mechanism is... 

We studied the autocatalytic minimal bromate reaction, which can be oscillatory, but was studied in a bistable regime. A proposed mechanism for this reaction, and participating species, are listed in Table 10.1. 

Table 10.1. Reaction mechanism for the minimal bromate reaction, from (5)...

In the experiment to be described we study the electrochemical displacement of a non-linear chemical system, the minimal bromate reaction, from nonequilibrium stationary states and from equilibrium. In the following chapter we shall relate such measurements to the thermodynamic and stochastic theory of potentials governing fluctuations in electrochemical systems in stationary states far from, near to and at equilibrium. 

For the first purpose we choose a chemical reaction system with some ionic species, as for example the minimal bromate reaction, for which we presented some experiments in Chap. 10. The system may be in equilibrium or in a nonequilibrium stationary state. An ion selective electrode is inserted into the chemical system and coimected to a reference electrode. The imposition of a current flow through the electrode coimection drives the chemical system (CS) away from its initial stationary state to a new stationary state of the combined chemical and electrochemical system (CCECS), analogous to driving the CS away from equilibrium in the same maimer. A potential difference is generated by the imposed current, which consists of a Nernstian term dependent on concentrations only, and a non-Nernstian term dependent on the kinetics. We shall relate the potential difference to the stochastic potential for this we need to know the ionic species present and their concentrations, but we do not need to know the reaction mechanism of the chemical sj tem, nor rate coefficients. 

The effects of added M + ions, M = Mn, Ni, Cu, or Zn, on rates of aquation and of racemization of [Cr(ox)3] are attributed to their effects on the hydration shell around the complex ion rather than to simple ion-pairing. The markedly different effects of different alkali-metal cations on base hydrolysis of the [Co(NH3)5Br] + cation" and on the cerium(rv) oxidation of methylmandelic acid " may have a similar explanation. Non-electrostatic factors are also important in salt effects on the formate plus chromate and iodide plus bromate reactions." Observed salt effects on rates of hydrolysis of a variety of anions such as [BFi]-, [SiFe] , and [S20 ] ... 

Kinetics of the bromide/bromate reaction are discussed in J.Chem.Educ.47 970ym5. The manganese dioxide-catalysed decomposition of potassium chlorate is demonstrated in the same journal (i6iV/.40(1963)78). 

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