Heterogeneous systems chemical exchange

Dynamic parameters for heterogeneous systems have been explored in the liquid, liquid like, solid like, and solid states, based on analyses of the longitudinal or transverse relaxation times, chemical exchange based on line-shape analysis and separated local field (SLF), time domain 1H NMR, etc., as summarized in Figure 3. It is therefore possible to utilize these most appropriate dynamic parameters, to explore the dynamic features of our concern, depending upon the systems we study. 

Note that in any heterogeneous system which attained constant temperature-pressure conditions from below (from smaller to higher values), no reaction proceeds within the bulk of the ApBq layer. Inside the ApBq layer, the A and B atoms (or ions) can and do exchange of their positions but this act by no means represents any chemical reaction. 

Tracer techniques offer the unique possibility of studying the kinetics of chemical reactions in chemical equilibria in which one isotope is exchanged for another (isotopic exchange reactions, reaction enthalpy A/7 0, reaction entropy A5 0). Isotopic exchange reactions have foimd broad application for kinetic studies in homogeneous and heterogeneous systems. 

In the framework of this description an attempt to model an effect of spatial non-uniformity of real catalytic systems was made (Bychkov et al., 1997). It was assumed that reaction proceeds in a heterogeneous system represented by two active infinite plane surfaces and in the gas gap between them. Surface chemistry was treated as for the Li/MgO catalyst (see Table III). Because of substantial complexity of the kinetic scheme consisting of several hundred elementary steps, the mass-transfer was described in this case as follows. The whole gas gap was divided into several (up to 10) layers of the same thickness, and each of them was treated as a well-stirred reactor. The rate of particle exchange between two layers was described in terms of the first-order chemical reaction with a rate constant ... 

This is known as the Clausius inequality and has important applications in irreversible processes. For example, dS > (dQ/T) for an irreversible chemical reaction or material exchange in a closed heterogeneous system, because of the extra disorder created in the system. In summary, when we consider a closed system and its surroundings together, if the process is reversible and if any entropy decrease takes place in either the system or in its surroundings, this decrease in entropy should be compensated by an entropy increase in the other part, and the total entropy change is thus zero. However, if the process is irreversible and thus spontaneous, we should apply Clausius inequality and can state that there is a net increase in total entropy. Total entropy change approaches zero when the process approaches reversibility. 

The thermodynamic stability of heterogeneous systems in the condensed state requires the existence of mechanical, thermal, and chemical equilibria. Chemical equilibrium implies that the chemical potential of each component in the system is the same in both the droplet interphase and the massive phase. Therefore, the thermodynamic equilibrium condition imposes a continuous exchange of matter within the system. 

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