Rate coefficients ratios

The rate coefficient ratio k22/k23 has been determined by Klein and Wolfs-berg5 0 from the photochlorination of hydrogen in the presence of tritium-labelled HC1. For the temperature range 0-62°... 

The kinetics of the reactions of six amines with phenacyl bromide in acetonitrile have been studied conductimetrically at several temperatures and activation parameters were calculated.129 The relative reactivities of the amines were explained in terms of inductive and steric effects. Relative reactivities have been determined conductimetrically for the 2-propanolysis of a series of chlorides and they he in the order PhSCl > PhCOCl > PhS02Cl > PhCH2Cl PhCl, with rate coefficient ratios 9.5 x 104 1 7.14 x 10 2 4.7 x 10-3 10 26.130 (The value for chlorobenzene was estimated.)... 

It was possible to examine the effect of ionic strength on the rate coefficient ratios kjki, k3/k2 and k2/k2. In Fig. 4 the results are plotted in the form log K/K0 versus ionic strength where K, K0, are the appropriate rate coefficient ratios at... 

Again it is possible to replace the fractionation parameter for the transition state by the rate coefficient ratio in isotopically pure solvents, so that (Gold, 1960)... 

Table 1-1. Factors Contributing to the Predicted Rate Coefficient Ratio qSKIE = fc[Cl"(H20) + CH3Cl]/fe[Cr(D20) + CH3C1]...

Ratios of rate coefficients. Unlike most other kinetic quantities, rate-coefficient ratios of steps involving free radicals as reactants can be estimated on the basis of thermochemical data. The estimates are crude, no better than giving approximate orders of magnitude, but this often suffices. However, the estimates are reliable only if the molecular species involved are indeed small. The basis of such estimates is as follows ... 

Replacement of CA./CB, in eqn 10.97 with this expression and use of eqns 10.95 for the rate coefficient ratios gives eqn 10.94. That equation in terms of mole fractions and with -rA/(-rB) = yAlyB = yA/(l-yA) gives... 

Again, values of fH fx were substituted by known values of f for HC1. The product ratio must be dependent on the rate coefficient ratio and on [X-], provided the sole source of halogenoacetate is the reaction pathway whose rate depends on [X-], viz. 

Rate coefficient ratios fe x/ H were evaluated by extrapolation of experimental product ratio data to zero ionic strength, with the aid of eqn. (52b). The rate coefficient was easily obtained by extrapolation of kinetic data obtained for dilute perchloric acid solutions. Values of x could then be computed from the ratios x / h ... 

To conclude this section, the treatment outlined gives a remarkably good account of the experimental observations over a wide range of H2 + N2 + O2 compositions at 470—500 °C, and to a sightly lesser extent at somewhat h her and lower temperatures. The agreement between 470 and 500 °C (r.m.s. deviation < 5 %) is such as to generate considerable confidence in the validity of the treatment and in the rate coefficient ratios given in Table 18. 

The room temperature photochemical reaction between H2 and Br2 has been investigated by Bodenstein and Jung . They used the sun as a light source and compared the rate of HBr production in systems containing H2 and Bf2 with the rate in systems containing H2, Bf2 and a large excess of HBr. They interpreted their results according to an atomic chain mechanism and were able to extract the rate coefficient ratio, k jk, = 8.6. 

These results show an isotope effect in reaction (2) which has also been observed at low temperatures. By combining ki with the equilibrium constant K for Brj dissociation, a value of 8.3+0.7 was obtained for the rate coefficient ratio k fk. This compares very favorably with the value 8.4+0.6 obtained at lower tempera-... 

Jones compared his data with theoretically calculated values of the rate coefficient ratio from transition state theory. The linear activated complex model proposed by Wheeler et predicts ratios substantially smaller than those observed. A triangular complex is suggested as an improved theoretical model based on the considerations of Magee... 

Bigeleisen et al. measured the relative rates of chlorine atom attack on H2 and HD to evaluate kinetic isotope effects. Experimentally, mixtures of H2, HD and CI2 were photolyzed with wavelengths longer than 3900 A and at temperatures ranging from 243 to 250 °K. Measuring mass spectrometrically the isotopic composition of the hydrogen gas both before and after photolysis permitted calculation of the rate coefficient ratio... 

The experimental results are compared with theoretical calculations using strictly empirical as well as Sato potential energy surfaces and a transition state theory formulation of the rate coefficient ratio. No one set of potential parameters could be found to fit all the data but it is not possible to attribute the deviations either to experimental error or the theory. 

Mixtures of CI2 with various isotopic hydrogens were irradiated with visible light from a mercury arc. The isotopic content of the residual hydrogen molecules was then determined either mass spectrometrically (stable isotopes) or with a Geiger counter (radioactive species). The rate coefficient ratios with respect to H2 were calculated from these data and their temperature dependence furnished ratios of pre-bxponential factors and differences in Arrhenius activation energies. Table 10 summarizes the results. 

The fluorination of hydrogen-methane mixtures in both inconel and fused silica reaction vessels has been studied by Mercer and Pritchard They noted rather severe complications from surface interactions but were able to evaluate the rate coefficient ratio k jk = 2.4 0.2 for... 

Goy et have studied the reaction of CN radicals with hydrocarbons and hydrogen the CN radicals being produced by photolysis of ICN at 2537 A. Relative rate coefficients for H abstraction were obtained as a function of temperature for only C2H6-CH4 mixtures. For systems containing ethane-propane and propane alone no temperature dependence of the rate coefficient ratios were observed. For... 

The reaction between two HS radicals has been studied by Darwent et Darwent and Roberts , during the course of an investigation of the reactions of H atoms with hydrocarbons, had occasion to consider the fate of HS radicals produced by the photolysis of H2S. They estimated the rate coefficient ratio for... 

In using the tables of the present chapter it should be borne in mind that while the results are presented in the form of absolute Arrhenius parameters, the bulk of the data have been derived from measurements of rate coefficient ratios and the absolute values are usually based on estimates of the rate coefficients of the reference reaction. This applies particularly to the radical reactions where the measurements are made relative to the radical combination reaction. Details of the assumptions involved with many of the reference reactions are discussed under the headings of the appropriate radicals. 

Mass spectrometric analysis of the product ratio CD4/CD3H enables a determination of the rate coefficient ratio... 

In view of the rate coefficient ratio k u/kn, by co-oxidation it is possible to produce caprolactone with good selectivities compared with aldehyde and ketone. Since the overall kinetic scheme for the reaction is known and all the kinetic coefficients have been estimated, it has been possible to make a computer simulation [60]. 

Let us evaluate this ratio at 35 km (237 K). From Figure 5.2, [O3] = 2 x 1012 molecules cm-3 at this altitude. We will see later that a representative N02 concentration is 109 molecules cm-3. The rate coefficient ratio at this altitude is no2 i o/ o+o3 — 9000. Thus, the ratio of the two rates is... 

---