High-pressure rate constant

Figure A3.4.9. Pressure dependence of the effective unimolecular rate constant. Schematic fall-off curve for the Lindemaim-FIinshelwood mechanism. A is the (constant) high-pressure limit of the effective rate constant...

The successful application of HPLC was made possible largely by (a) the development of pump systems that can provide constant flow rates at high pressure and (b) the identification of suitable pressure-resistant chromatographic media. Traditional soft gel media utilized in low-pressure applications are totally unsuited to high-pressure systems due to their compressibility. 

Interpreting this mechanism requires the further postulation that for low rank coals, ku is much larger than any other kus. Therefore, the measured k4 approximates ku, and correlates with the behavior of experimental rate constants with pressure. For high rank coals, ku ku— ku, and the measured... 

A first approximate value of k2 can be found from the pressure at which the observed first order rate constant has fallen to half the constant high pressure value (Problem 4.19), and this first approximate value can be used to obtain a first approximate value for k. Thereafter k2, e0 and s are found by successive iterations and best fit of experiment with theory. 

It is interesting to note the effect of chromium content on reaction rate at high pressures (,—500 p.s.i.g.). Experiments (5) were carried out with normal air-activated catalysts (Figure 4). Catalysts were used with chromium contents ranging from 0.7 to 0.0005 wt. % of the total catalyst. Results of one-hour ethylene polymerization tests at 132°C. and 450 p.s.i.g. with these catalysts, activated at 500°C., are given. As the concentration of chromium was decreased, catalyst charge was increased to compensate for poisoning of catalyst sites by trace impurities and to keep total rate of production about constant. 

Slater has carried out a classical investigation of unimolecular reaction rates at high pressures. The model used is that the molecule decomposes whenever a given coordinate q reaches a critical extension q0. He obtains for the velocity constant... 

Nonlinear behavior is also observed in the wide-range (0.1-2.5 GPa) pressure dependence of the ESPT rate of DCN2 in alcohols [44[. At low pressure, the proto-lytic photodissociation rate slightly increases, reaching the maximum value. With further pressure increase this rate decreases below the initial value at atmospheric pressure (Fig. 13.11). To explain the unique nonexponential dependence of ESPT rate constants on pressure, as well as temperature, Huppert et al. have developed an approximate stepwise two coordinate proton-transfer model that bridges the high-temperature nonadiabatic proton tunneling limit with the rate constant... 

Table 11.8 2 shows the variation of rate constants with pressure. Many high-pressure reactions are done neat, but if a solvent is used, the influence of pressure on that solvent is important. The melting point generally increases at elevated pressures, and this influences the viscosity of the medium (the viscosity of liquids... 

As indicated earlier, the flow-rate of a dual-head, special-drive reciprocating pump depends on the downstream back-pressure because of the compressibility of the liquid and the elasticity of the chambers. In the Haters Model 6000 pump, the output Of the pressure signal amplifier is fed to the motor speed control circuit to increase the pump speed slightly and keep constant the flow-rate at high pressure. The amount of compensation is adjusted by a potentiometer to allow for the difference in the compressibilities of the solvents. 

Radiative Association Reactions The study of radiative association reactions, (Eq. 2.2), has been of considerable interest [6-8] in chemical kinetics, planetary and interstellar chemistry, flames, and a variety of other areas. The kinetic study makes it possible to model the formation of complex molecular species in the interstellar science. At the very low molecular number densities in interstellar environments, the probability of formation of the products of association reactions by collisional stabilization is very low. Therefore, the radiative association process becomes an extremely important one for the production of the complex molecular species observed by astronomical physicist. The methodology is either flowing afterglow (FA) or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. For the study of the apparent bimolecular rate constant for formation of association products as a function of pressme of a third body (N), the pressure should be set up to be sufficiently high in order to release the energy in the associated complex. Under the high pressure conditions collisional stabilization has competed with and usually dominated over radiative associatioiL As a result, the radiative association rate was then extrapolated from the intercept of a plot of apparent rate constant versus pressure of a third body, N. 

The kinetics of the overall reaction have been studied, over a range of temperatures and pressures, by observing the decay of OH radicals in the presence of known concentrations of C2H4. Particularly noteworthy are the experiments of Fulle et who measured the rate constant at pressures up to 150 bar of helium at temperatures between 300 and 700 K. Based on a short extrapolation, they derived a temperature-independent value of k(7) = 1.0 x 10 " cm molecule s in the limit of high pressure. At the pressures and temperatures found in the Earth s atmosphere, channel (R12e) predominates and the... 

Kinetic studies of the alcoholysis of /3,/3-dimethylacryloyl chloride (49) with 14 alcohols in benzene at a reactant ratio of 1 1 have shown that the reaction probably proceeds via a charge-transfer complex. Reaction rates at high pressures (200, 500 and 1000 bar) of the reaction between a series of quinolines and benzoyl chloride in MeCN have been determined, together with those with 4-methyl- and 4-nitro-benzoyl chloride. Rate constants increased with increasing pressure, and it was concluded that those reactions proceed via an associative 5 n2 mechanism. ... 

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