Site changes, chemical reactions

The lattice model that served as the basis for calculating ASj in the last section continues to characterize the Flory-Huggins theory in the development of an expression for AHj . Specifically, we are concerned with the change in enthalpy which occurs when one species is replaced by another in adjacent lattice sites. The situation can be represented in the notation of a chemical reaction ... 

Modular structures can be built from slabs of the same or different compositions, the slabs widths can be random or regular, and the slabs themselves can be ordered in a variety of ways (Fig. 4.19). Composition variation can then occur by variation of the amount of each slab type present, by variation in the degree of order present, or, when slabs have the same composition, by changes in atom ratios introduced at the slab boundaries. In addition, the planar boundaries that divide up a modular structure create new coordination polyhedra in the vicinity of the fault that are not present in the parent structure. These may provide sites for novel chemical reactions or introduce changes in the physical properties of significance compared to those of the parent structure. 

Protons in soil are associated with soil water molecules to form hydronium ions, which can also be on an exchange site in soil (see Figure 8.5). Additionally, protons are bonded with different strengths to both organic and inorganic components in soil. All of these are potential sources of protons for chemical reactions. They also produce buffering of soil pH such that the pH of soil is hard to change. 

Interpretation of KIEs on enzymatic processes (see Chapter 11) has been frequently based on the assumption that the intrinsic value of the kinetic isotope effect is known. Chemical reactions have long been used as models for catalytic events occurring in enzyme active sites and in some cases this analogy has worked quite well. One example is the decarboxylation of 4-pyridylacetic acid presented in Fig. 10.9. Depending on the solvent, either the zwitterionic or the neutral form dominates in the solution. Since the reaction rates in D20/H20 solvent mixtures are the same (see Section 11.4 for a discussion of aqueous D/H solvent isotope effects), as are the carbon KIEs for the carboxylic carbon, it is safe to assume that this is a single step reaction. The isotope effects on pKa are expected to be close to the value of 1.0014 determined for benzoic acid. This in mind, changes in the isotope effects have been attributed to changes in solvation. 

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