What Are Network Solids

Consider the heat of solution of a process in which an ionic compound is the solute and water is the solvent. For example, what happens when solid NaCl dissolves in water In solid NaCl, the Na and CP ions are held together by strong positive-negative (electrostatic) forces, but when a small crystal of NaCl dissolves in water, the three-dimensional network of ions breaks into its individual units. (The structure of solid NaCl is shown in Figure 2.12.) The separated Na+ and CP ions are stabilized in solution by their interaction with water molecules (see Figure 4.2). These ions are said to be hydrated. In this case water plays a role similar to that of a good electrical insulator. Water molecules shield the ions (Na+ and CP) from each other and effectively... 

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (ry), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an inter-particle or inter-molecular network, which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case, flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes a fluid. Examples include oil well drilling muds, greases, lipstick, toothpaste and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear-thinning behaviour as shear rate increases. 

Our basic assumption is that the discovery of new catalysts can be accelerated by developing a framework for understanding catalysis as a phenomenon and by pinpointing what are the most important parameters characterizing the chemical properties of the catalyst. We will concentrate in this book on heterogeneous catalysis, that is, catalysts where the processes take place at the surface of the solid. We will develop a systematic picture of the surface-catalyzed processes from the fundamental link to surface geometry and electronic structure to the kinetics of the network of elementary reactions that constitute areal catalytic process. The end result is a theory of variations in catalytic activity and selectivity from one catalyst to the next that will... 

Note that diffusion occurs only in one direction because the silica-tetrahedra are not free to move. What is actually happening is that the three-dimensional network of tetrahedra is being rearranged to form cmother structure. This illustrates the fact that the actual structure and composition of the two reacting species are the major factor in determining the nature and speed of the solid state reaction. 

What is of value here — what is generally called freedom — is thus not a matter of unconstrained choice. Rather it is an achieved ability to be who one is within a network of relationships, a network that affords opportunities to, even while it constrains, the individual. Such networks are located in the world of things indeed, as I argued above, they are often formed around such things. Arendt talks of the web of human relationships as being bound to the objective world of things (Arendt, 1958, pi 83). Without this bond, human relationships lack solidity and are liable to wither away. To preserve networks of relationships that foster and sustain people as independent practical reasoners, the world must be a place for such networks. The creation and preservation of such a world in turn needs people who are independent practical reasoners — people who think and act for themselves and who take responsibility for their actions. 

It has just been argued that the conductivities of simple ionic liquids, on the one hand, and liquid sihca and water, on the other, are vastly different because a fused salt is an unassociated liquid (it consists of individual particles) whereas both molten silica and water are associated liquids with network structures. What is the situation with regard to the viscosities of fused salts, water, and fused silica Experiments indicate that whereas water and fused NaCl have similar viscosities not far above the melting points of ice and solid salt, respectively, fused silica is a highly viscous liquid (Table 5.46). Here then is an interesting problem. 

Let us assume for a moment, however, that we had reliable (and direct) thermochemical information about trimethylarsine and tetramethyldiarsine. How valid is it to assume all As—As bonds have comparable dissociation enthalpies Clearly we know enough not to assume the As—As bond in AS4 is the same as in tetramethyldiarsine. The former is immediately identified as strained and so we are to use the more stable allotrope with an infinite network of nonplanar hexagons. What about Sb—Sb bonds If we assume constancy of the dissociation enthalpies of these bonds (excepting the strained Sb4), how about using the bond enthalpy from elemental solid antimony The enthalpy of dimerization of antimonin. CsHjSb, to form the tricyclic Diels-Alder or [4-1-2] cycloaddition product has been determined to be — 30.5 +1.3 kJ mol by a careful study of the monomer-dimer equilibrium (equation 19). 

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