Oxygen atoms, concentration determination reactions

Relative concentrations of oxygen atoms were determined from the limiting amount of a primary or a secondary product (or the sum of these) in O atom reactions with the substance studied. The reactant was admitted in sufficient concentrations at various sites of the reaction vessel, and the limiting amount of reaction products was determined. A curve plotted as log (O)0/(O)r vs. X was used for determining the rate constant for the disappearance of atoms. 

The rate of CO2 formation due to reaction (V.33) is set proportional to Poofo where the surface concentration of adsorbed oxygen atoms is determined by a steady-state condition, i.e., the rate of formation of 0(ad) must be equal to the rate of consumption of 0(ad). Further analysis of the rate data shows that in reaction (V.38) predominantly O2 molecules adsorbed normal to the surface react with CO(g). Thus, one arrives at the rate law (40)... 

The most direct way to test the validity of a mechanism is to determine what intermediates are present during the reaction. If oxygen atoms were detected, we would know that Mechanism I is a reasonable description of NO2 decomposition. Likewise, the observation of NO3 molecules would suggest that Mechanism II is reasonable. In practice, the detection of intermediates is quite difficult because they are usually reactive enough to be consumed as rapidly as they are produced. As a result, the concentration of an intermediate in a reaction mixture is very low. Highly sensitive measuring techniques are required for the direct detection of chemical intermediates. 

We synthesized nine silicalites which had different concentrations of defect sites in the zeolite framework determined by isotope exchange method. These silicalites were treated with aluminium trichloride vapor under the same reaction conditions 923 K temperature, 1 h time, 11 kPa aluminium trichloride vapor pressure. Figure 1 shows the plots of the amount of aluminium atoms introduced into the framework against the amount of oxygen atoms on the defect sites. A... 

The relative 0 atom concentrations, for instance, that along the reaction vessel, may be determined from the limiting accumulation rate of one product (primary or secondary), since at a given temperature the fraction of oxygen atoms consumed by formation of this product is constant. 

In each of these equations the constant sought is found from the slope of the straight line and the intercept on the ordinate axis, though the intercept b and the slope values will be different for every case. A conclusion may be drawn that the final concentration of primary or secondary products, or the sum of these, as functions of the initial substance concentration should be known for determination of rate constants for oxygen atom reactions. But it is not indispensable to determine whether the products are primary, secondary, or the sum of these. 

The procedure adopted by Avramenko and Kolesnikova in an attempt to establish the primary reaction mechanisms consisted of the experimental determination for the various products formed of the shapes of the rate curves as a function of the initial concentration of the reactant olefins. These curves were then compared with the theoretical curves based on an assumed sequence of formation of products. Different shapes of rate curves were predicted for products formed with participation of a single oxygen atom (primary or secondary products), of two oxygen atoms ( quadratic products ), etc. 

The necessary condition is that the rates of formation of the products Pi and Pi per oxygen atom reacting with A1 and 42, respectively, are independent of whether only one or both A1 and A 2 are present. This condition is satisfied at sufficiently high pressures (where pressure-dependent fragmentation is suppressed) by most of the products of addition of oxygen atoms to olefins. If the initial concentrations (vl,) and (A2)j are sufficiently high so that they remain to a good approximation constant in the course of the reaction, the ratio of the rate constants is determined from the expression... 

Steps 1,3, and 5 cannot be slow as they are just proton transfers between oxygen atoms (Chapter 13). That leaves only steps 2 and 4 as possible rate-determining steps. The bimolecular addition of the weak nucleophile water to the low concentration of protonated ester (step 2) is the most attractive candidate, as step 4—the unimolecular loss of ethanol and re-formation of the carbonyl group—should be fast. What p value would be expected for the reaction if step 2 were the rate-determining step It would be made up of two parts. There would be an equilibrium p value for the protonation and a reaction p value for the addition of water. Step 1 involves electrons flowing out of the molecule and step 2 involves electrons flowing in so the p values for these two steps would have opposite charges. We know that the p value for step 2 would be about +2.5 and a value of about -2.5 for the equilibrium protonation is reasonable. This is indeed the explanation step 2 is the rate-deter-... 

The evidence for the proposed mechanism comes from kinetic, spectroscopic (multinuclear NMR), X-ray structure, and theoretical calculations. The kinetic rate law under optimum catalytic conditions is very complex. Under pseudo-first-order conditions, where the concentrations of both 9.35 and the hydroperoxide are much greater than that of allyl alcohol, the rate expression 9.5 is obeyed. In this expression the inhibitor alcohol is an inert alcohol such as isopropanol or f-butanol that is deliberately added to slow down the reaction for convenient rate measurements. The inert alcohol acts as an inhibitor, since it competes with both hydroperoxide and allyl alcohol for coordination to the Ti center. Note that expression 9.5 is consistent with the formation of an intermediate like 9.36, before the rate-determining oxygen atom transfer step. 

This rate law does not have the same form as the experimentally determined rate law. For one thing, it contains the concentration of the intermediate, an oxygen atom. We can remove [O] and obtain a rate law that agrees with the experiment results by making an additional assumption. We assume that the rates of the forward and reverse reactions in the first step are equal. That is,... 

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