Bimolecular association reactions pressure dependence

The studies of bimolecular association reactions are of special interest because they may be expected to show, at sufficiently low concentrations, the same type of dependence of rate on total concentration as is displayed by unimolecular reactions. Indeed the simplest of such systems, the recombination of atoms at normal gas concentrations, never follow simple second-order kinetics but are rather at the extreme end of the concentration-dependent rate law and their kinetics is found experimentally to follow third-order kinetics. From the discussion of the pressure dependence of unimolecular decompositions (Table XI.2) we would expect the region of total concentration dependence to shift to lower and lower concentrations as the number of atoms in the product molecule increases. This is in quali-... 

Energy transfer limitations have long been recognized to affect the rates and mechanisms of fission and association reactions (Robinson and Holbrook, 1972 Laidler, 1987). In addition, it is increasingly being recognized that many exothermic bimolecular reactions can exhibit pressure-(density)-dependent rate parameters if they proceed via the formation of a bound intermediate. When energy transfer limitations exist, the rate coefficients exhibit non-Arrhenius temperature dependencies—i.e., the plots of ln(k) as a function of l/T are curved. 

In order to better understand the detailed dynamics of this system, an investigation of the unimolecular dissociation of the proton-bound methoxide dimer was undertaken. The data are readily obtained from high-pressure mass spectrometric determinations of the temperature dependence of the association equilibrium constant, coupled with measurements of the temperature dependence of the bimolecular rate constant for formation of the association adduct. These latter measurements have been shown previously to be an excellent method for elucidating the details of potential energy surfaces that have intermediate barriers near the energy of separated reactants. The interpretation of the bimolecular rate data in terms of reaction scheme (3) is most revealing. Application of the steady-state approximation to the chemically activated intermediate, [(CH30)2lT"], shows that. 

According to the literature [2,23], we suggest that the first OPTPD (for m/z = 18) or DTG peak for all studied oxides is caused by unimolecular desorption of intact H2O molecules that is, the reaction equation is the first order (x= 1 in Equation (37.12)). All other peaks at higher temperatures were assigned to associative desorption of water molecules in a second order reaction (as bimolecular process), that is, (x = 2 in Equation (37.12)). Maybe some part of water observed at T above 550-600 K can be due to water desorbed through the unimolecular process (e.g., water desorption from micropores, defects, and primary particle volume), but this contribution is not dominant [23]. Besides, the amount of intact water is minor in the OPTPD measurements performed at 10 Torr as the amount of adsorbed molecules depends linearly on the pressure. 

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