Slow reaction higher pressures

In investigating the oxygen atom reaction with carbon monoxide 1 62 it was found that this reaction at pressures of 2.5 mm. Hg and higher obeyed essentially a bimolecular law, although a slow trimolecular reaction might have occurred as well. 

Gray and Darby (36) studied the reaction of 02 at low pressures (< 10-1 mm.) with NiO films, prepared from evaporated Ni films in a grease-free system, by following the change of conductivity of the film. It is not certain how closely the conditions in this system resemble those on the bulk oxide at much higher pressures but it is interesting to note that Gray and Darby ascribe the slow step in the desorption of 02 to the second order reaction... 

We could face two limiting situations an inconveniently slow reaction [15] and a process that is too fast to monitor [ 16]. One may choose to run experiments at a higher temperature for reactions that are too slow at around the usual 298 K. In this case, a limitation is imposed by the boiling point and vapour pressure of the solvent which may lead to evaporation and, consequently, volume change (see above). On the other hand, it may be necessary to carry out studies at low temperatures if the reaction is too fast at 298 K. In this case, potential problems include solutions freezing and condensation of atmospheric water vapour on the reaction vessel and optical surfaces, which may affect spectrophotometric measurements. 

Reaction between protonated acetone and methanol gives slow reaction at low pressure in the ICR, while it is unobserved at the higher pressure of the flowing afterglow, in agreement with accompanying ab initio data[139] ... 

Experiments at (high) vacuum make it possible, for well-defined surfaces, to obtain rate parameters that may be applicable to real catalysts at higher pressures 4, 5, 37). Fast processes can be measured, and this is certainly of fundamental interest in learning about the detailed pathways involved in a global reaction or even in a so-called elementary step. However, for reaction at ordinary pressures, in situ transient and other studies are usually sufficient to understand the observed kinetics, which depends essentially on the slow steps in the pathway. 

The reaction in the region above the second limit (i.e., at higher pressure in the slow reaction zone) is sufficiently rapid to be measured with conventional methods the exothermic reaction can support transient temperature excesses (gas temperature being heated above the ambient temperature) of several kelvin [24] as indicated in Fig. 5.17. 

The relatively low temperatures and partial pressures of most of the reagents, low catalyst concentration, and the intensity of the solar light flux do not favor high rates of heterogeneous catalytic and photocatalytic reactions in the atmosphere. However, due to the enourmous total volume of the atmosphere, even very slow reactions can result in chemical transformations of huge amounts of some atmospheric components. Note that most heterogeneous reactions are expected to occur in the lowest layer of the atmosphere, i.e. the troposphere, since the concentration of aerosols and their surface area are much higher here than in the upper layers of the atmosphere. 

Bimolecular reactions of CIO and CIO are slow and can be neglected. The termolecular reaction 1 is facilitated at higher pressures, that is, larger M, and low temperature.) C1202 has been shown to have the symmetric structure ClOOCl (McGrath et al. 1990). Photolysis of ClOOCl has two possible channels ... 

---