Oxygen atoms, concentration determination rate constants

The rate constants for the disappearance of oxygen atoms were determined using the method described earlier.61 To obtain the value of k + k"ro (Af)(0)o the relative concentrations of atoms along the vessel length, i.e., the dependence of (0)o/(0)a should be determined. 

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 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. 

If electronically excited species can be made to give up their excess energy to some active material, then their concentration may be determined by measurement of the rate of heat liberation. The method is, of course, well established for the measurement of oxygen and hydrogen atom concentrations, and the most accurate experimental technique is to use the isothermal hot-wire calorimeter developed by Tollefson and LeRoy.42 The amount of power needed to maintain a catalytic probe at a constant temperature is reduced if heat is liberated at the probe, and no correction is needed for heat losses. The flow of energy-rich species, [Pg.325]

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... 

Hydrogen atom abstraction by CraqOO from an oxygen-hydrogen bond was demonstrated in the reaction with a substituted, cationic, water-soluble phenol shown in Fig. 9 and abbreviated as ArOH" 132). Upon one-electron oxidation, this phenol generates a stable radical ArO that absorbs strongly at 400 nm, = 2.6 x 10 cm The reaction with CraqOO " has a 2 1 [CraqOO ]/[ArOH ] stoichiometry consistent with the sequence of reactions 16-17. The kinetics of the first, rate-determining step are independent of acid concentration at a constant ionic strength, k = 1.24 s , and exhibit... 

An additional aspect in studying the half-reactions arises when a reaction is not treated as pseudo first order reaction. This is the case when the substrate is in approximately the same concentration as the metal complex. Caradonna et al. for instance studied the required treatment for such a case. Due to the accumulation of the product in the course of the reaction, the back reaction is not negligible anymore. To the best of our knowledge, analyses of the back reaction have never been mentioned in the literature with respect to oxygen atom transfer reactions of model complexes and this is due to several reasons. Either the reaction rate is too small compared to the forward reaction so that experimental difficulties in determining the reaction rate constant emerged. Or, as was emphasized by Enemark et No bis(dithi-olene) molybdenum or tungsten eomplex was observed to reduce PhjPO . ... 

We neglect the term K3NO compared with K202 in the denominator15 and use the relation between the rate coefficients and the equilibrium constants finally we assume that the concentration of atomic oxygen is determined by the dissociation equilibrium of molecular oxygen... 

Reactions (1) and (2) essentially convert solar radiant energy into thermal energy. The parameters which determine the rate of ozone formation (UV photon flux, atomic and molecular oxygen number density and the total gas number density) are not constant with altitude and so the ozone concentration and hence Tg varies with altitude. The net result is that Tg increases thoughout the stratosphere until a maximum is reached at the stratopause whence Tg begins to decrease again. 

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