Chemical change observable phenomenon

The central topic of the book is the rates of chemical reactions or elementary steps. A rate rx states the number of moles of species i formed (if positive) or consumed (if negative) by a chemical reaction or reactions per unit volume and unit time. It is a process rate as distinct from a rate of change. The distinction is important. A rate of change is an observable phenomenon of nature, e.g., a change of concentration with time, and is the combined result of all contributing process rates. ... 

By carefully measuring mass before and after many chemical reactions, it was observed that, although chemical changes occurred, the total mass involved in the reaction remained constant. The constancy of mass in chemical reactions was observed so often that scientists assumed the phenomenon must be true for all reactions. They summarized this observation in a scientific law. The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction—it is conserved. This law was one of the great achievements of eighteenth-century science. The equation form of the law of conservation of mass is... 

The substances for which this phenomenon has been observed are invariably polycyclic aromatic hydrocarbon structures. No exciplex formation has been reported in the literature to involve drug molecules, but this remains a possibility in concentrated solution or perhaps in solid-state mixtures. The consequences of exciplex formation are a radiative or nonradiative return to the ground state without chemical change, or electron transfer leading to chemical reaction of the drug, the quencher, or both. Many photoaddition processes are postulated to proceed via exciplex formation with the quencher molecule becoming chemically bound. 

Enzymes bind their substrates by multiple non-covalent interactions on a specific surface. This way, a micro-heterogenization occurs and the local concentration of substrates is increased relative to the bulk solution. In addition, the chemical potential of specific groups may be drastically changed temporarily compared to aqueous solutions by the exclusion of water in the reactive site upon binding of substrate. Both aspects contribute to the observed phenomenon of high acceleration in reaction rate some examples are presented in Table 1-2. Enzymes often bind the substrate in the transition state better than in the ground state, which lowers the activation energy. 

Besides these two regimes, another regime, with a temporally periodic change of the chemical composition (chemical oscillation or self-oscillation), may also be observed. A famous example of this phenomenon is the Belousov-Zhabotinsky reaction. Another example of complex kinetic behavior in open chemical systems is the occurrence of multiple steady states due to the fact that for some components of the reaction mixture the rate of consumption and rate of production can be balanced at more than one point. This type of behavior has become the subject of detailed theoretical and computational analyses (Marin and Yablonsky, 2011 Yablonskii et al., 1991). Bespite the fact that there are many experimental data concerning such complex behavior, the steady-state regime with characteristics that are constant in time still is the most observed phenomenon. 

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