The reaction between S(1V) and 0 2 in aqueous solution is of importance because of its involvement in flue gas desulfurization processes and in acid rain formation. Despite considerable research, a complete mechanism for the reaction has not been established. In this work a detailed study of the uncatalyzed reaction in the region of pH 4.5 has been carried out. The overall rate law was found to be -d[02]/dt = kobs [HS03-]2[H+]-2[02]o with kobs of 3.6 x lo6 M s-' at 25 "C and ionic strength 0.05 M. The yield of the intermediate S20?-formed duringthe reaction has been determined over the pH range 4.1-6.3. By comparison with the yield of S~0 7~-produced in the reaction of HSO3-with HS05-, it is concluded that S~0 7~-formed in the HS03--02 reaction comes from the reaction between HSO5-and HSO3-, and therefore HSO5-must be an intermediate in the HSO3--O2 reaction in this pH region. From the above yields it is concluded that 60% of the reaction proceeds through the intermediate HS05-. A detailed mechanism consistent with the overall rate law of the reaction has been proposed. A key feature is the proposal that the chain initiation occurs through the reaction of HSO5-with HSO3-to form S O iand Sod-. In order to study the individual rate laws for the initiation, propagation and termination reactions, a relaxation technique was applied. In these experiments, the reaction at a steady state rate was perturbed by suddenly introducing a change in the concentration of HSO3-or H+, and the relaxation of the reaction to the new steady state was recorded. Treatment of these experimental data not only confirmed the HSO3-dependence of the rates of initiation, propagation, and termination in the proposed mechanism but also gave the pH dependence of these reactions. A study of the remarkable inhibitory effect of methanol produced strong support for the proposed initiation reaction. The effect of the introduction of chain initiator S20g2-was also found to be consistent with the mechanism. Relative values of rate constants of reactions in the proposed mechanism have been evaluated from the present experimental data. Evaluation of absolute values of these rate constants requires one additional piece of experimental information, which, depending on the choice, yields a range of values. An arbitrary choice is used to give rough values of the rate constants for initiation, Propagation and termination and typical values for the steady state concentrations of chain carriers independent of the bisulfite concentration.

The decomposition of nitromethane was studied over the temperature range 1000−1100 K in reflected shock waves. CH3NO2 and the reaction products were analyzed by gas chromatography. The derived gross rate constant and activation energy for the disappearance of CH3NO2 is consistent with that of Glänzer and Troe. A reaction mechanism consisting of 99 chemical reactions was developed to simulate the experimental data of the present study and that of Hsu and Lin. Good agreement between experiments and simulations was achieved. It appears that significant amounts of CH3NO2 are destroyed through secondary reactions that involved highly reactive free radicals (H, OH, and CH3), suggesting the need for redeterming the true unimolecular decay rate constant for CH3NO2. For improvement of the performance of the model, several other rate constants also need to be determined. The final section is a preliminary report on a spectrophotometric technique for measuring the loss of nitromethane due to pyrolysis by recording its absorption of UV radiation, directed axially along a small diameter shock tube. Although only semiquantitative data were obtained, this novel procedure merits discussion.

The Mn2+-catalyzed oxidation of HSO3 - by O2 has been studied in the pH region 4.5 and at bisulfite ion concentrations from 1.5 × 10-3 to 1.2 × 10-2 M. The reaction was found to obey a three-term rate law:  −d[O2]/dt = k α[HSO3 -]2 + k β[HSO3 -][Mn2+] + k γ[Mn2+]2 with k α = 3.6 × 10-3 M-1 s-1, k β = 1.23 M-1 s-1, and k γ = 98.6 M-1 s-1 at pH 4.50, 25 °C, and ionic strength 0.050 M. The kinetic behavior of the reaction resembles markedly that of the uncatalyzed reaction. The rate of the reaction is independent of oxygen concentration, S2O7 2- and HSO5 - are intermediates in the reaction, and the reaction is catalyzed by S2O8 2- and strongly inhibited by methanol. The experimental results can be quantitatively explained by the addition to the uncatalyzed reaction mechanism of a chain propagation reaction involving Mn2+ and SO5 •-. Among several alternatives, the manganese propagation may be represented as follows:  (I) Mn2+ + SO5 •- → Mn(III) + HSO5 -; (II) Mn(III) + HSO3 - → Mn2+ + SO3 •-. The resulting mechanism leads to the three-term rate law where the first term is the uncatalyzed rate and the second term can be predicted quantitatively from the uncatalyzed rate and the last term. It was inferred from the yield of S2O7 2- that, unlike the reaction between SO5 •- and HSO3 -, there is little or no branching of the first reaction to form the SO4 •- radical. The ratio of the rate constant of (I) to that of the reaction of SO5 •- with HSO3 - in the uncatalyzed reaction mechanism has been determined to be 124. The quantitative agreement between the experimental data and predictions for the effect of S2O8 2- has substantiated the validity of the proposed mechanism. The pH dependence of the reaction rate can be almost entirely accounted for by the pH dependence of the reaction between HSO5 - and HSO3 -, indicating that reaction I is independent of the hydrogen ion concentration in the pH region studied. While methanol inhibits the reaction, the quantitative discrepancy between predictions and experiments suggests that the reactions with alcohols are more complex than previously thought. Further work in this area is needed to understand fully the reaction mechanism when alcohol is present.

The thermal decomposition mechanisms of gaseous nitromethane, methyl nitrite, dimethylnitramine and 1,3,3,-trinitroazetidine (TNAZ) have been reanalyzed using sensitivity and principal component analyses. For each system an adequate, much simplified mechanism was developed, and the fragmentation/reaction sequence leading to the final products was made transparent. The critical roles of free radicals (CH 3 , H, OH, HCO, HNO, CH 3 O, etc.) at different stages of these pyrolyses were then identified. The predicted product distributions for these compounds were calculated to facilitate the assessment of their explosive performance parameters. Comparison of the products generated by nitromethane with those produced by methyl nitrite showed that the contribution of a nitro-nitrite isomerization reaction is negligible in the nitromethane pyrolysis.