Aqueous solutions limiting reactant problems

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

We recognize this as a problem involving a limiting quantity because the quantities of two reactants are given. Because this reaction takes place in aqueous solution, the number of moles of water present before and after the reaction is unknown. 

Because of the correlation between rate coefficient and activation energy, the reactions which are classified as fast tend to belong to certain categories. Thus, for example, fast gas-phase reactions usually involve free radicals or atoms—a fact which increases the problems considerably since it is very difficult to generate radicals of known concentration, mix them with the other reactant and then monitor the concentrations of radicals and products at known time intervals after the start of the reaction. In solution the upper limit to the reaction velocity is governed by the rate at which the partners can diffuse together gas reactions are not so frequently diffusion-controlled because of the considerably higher diffusion coefficients in the gas phase. The fastest reaction studied in aqueous solution is the neutralization reaction, formally... 

Diffusion into the bulk. This is determined by the diffusion coefficient in the liquid (D,). Diffusion within the bulk aqueous phase is much slower than gas-phase diffusion and can be rate-limiting under conditions of high reactant concentrations where the rate of the chemical reaction is high. This appears to have been a problem in some experimental studies of some aqueous-phase reactions relevant to the atmosphere where either bulk solutions or large droplets and reactant concentrations higher than atmospheric were used (Freiberg and Schwartz, 1981). 

The major problem associated with aqueous catalysis is the limited and often very low solubility of certain organic reactants in water. Much work is needed to find practical solutions for these hydrophobic reactants. Possibilities deserving further attention include the application of fluorous biphasic catalysis or nonaqueous ionic liquid catalysis. The potential of organic reactions compatible with or even promoted by water is not yet fully exploited. 

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