Chemical reactions macroscopic view

The probability distribution in Figure 11.6 indicates that there are two stable states for the chemical reaction system of Equation (11.25). Since the system is open to species A, B, and C, these states are non-equilibrium steady states (NESS). A more careful discussion of the terminology is in order here. The concept of an NESS has different meanings depending on whether we are considering a macroscopic or a microscopic view. This difference is best understood in comparison to the term chemical equilibrium. From a macroscopic standpoint, an equilibrium simply means that the concentrations of all the chemical species are constant, and all the reactions have no net flux. However, from a microscopic standpoint, all the concentrations are fluctuating. 

When we consider a chemical reaction, there are two viewpoints a macroscopic one and a molecular level one. It was only about a hundred years ago when the reality of molecules was established. In 1905, Einstein proposed a theory of Brownian motion, and later (1908-1912) Perrin proved it by experimental work. They showed that Brownian motion is caused by the collision of molecules on small particles (micrometer size). Although some scientists at the time considered that molecules only had a virtual existence that was useful to explain chemical phenomena, since then, no scientist has doubted the existence of molecules. Since that time a molecular point of view has become very popular in chemistry, although it is rather difficult to see molecules directly even with the present technology. 

When doing stoichiometric calculations, the assumption often made is that the reaction goes to completion. This is a convenient assumption when focusing on calculations involving mole ratios and limiting reagents, but there are many examples of commercially and biologically important chemical reactions that do not go to completion. Rather, appreciable amounts of reactants and products remain in the reaction mixture once equilibrium is reached. When viewed macroscopically, the concentrations of all reactants and products remain constant, but not necessarily equal, over time. 

Thinking it Through When a reaction has reached equilibrium, it does not mean that all chemical activity has stopped. Rather, at equilibrium, the macroscopic view indicates constant (but seldom equal) concentrations for each substance, making Choice (D) the correct response. Choice (A) is a commonly held misconception, one that you will not choose if you remember the concept of dynamic equilibrium. It is also untrue that the total moles of products must equal the remaining moles of reactant, choice (B). The relative amounts of material present at equilibrium will depend greatly on the position of the equilibrium, revealed in quantitative problems by the value of the equilibrium constant. Choice (C) is based on another common misconception about equilibrium reactions. Addition of a catalyst, while it may increase the rate at which equilibrium is achieved, does not affect the position of equilibrium. 

Chemical transformations can be performed in a gas, liquid, or solid phase, but, with good reasons, the majority of such reactions is carried out in the liquid phase in solution. At the macroscopic level, a liquid is the ideal medium to transport heat to and from exo- and endothermic reactions. From the molecular-microscopic point of view, solvents break the crystal lattice of solid reactants, dissolve gaseous or liquid reactants, and they may exert a considerable influence over reaction rates and the positions of chemical equilibria. Because of nonspecific and specific intermolecular forces acting between the ions or molecules of dissolved reactants, activated complexes as well as produets and solvent molecules (leading to differential solvation of all solutes), the rates, equilibria, and the selectivity of chemical reactions can be strongly influenced by the solvent. Other than the fact that the liquid medium should dissolve the reactants and should be easily separated from the reaction products afterwards, the solvent can have a decisive influence on the outcome (i.e., yield and product distribution) of the chemical reaction under study. Therefore, whenever a chemist wishes to perform a certain chemical reaction, she/he has to take into account not only suitable reaction partners and their concentrations, the proper reaction vessel, the appropriate reaction temperature, and, if necessary, the selection of flic right reaction catalyst but also, if the planned reaction is to be successful, flic selection of an appropriate solvent or solvent mixture. 

The pore is the central structural motif in host-guest functional materials, such as zeolites, and is also the ubiquitous object in nature from microscopic to macroscopic points of view. Most importantly, the porous structure provides inherently accessible space, for guest, where chemical reaction or physical sorption could take place therefore, they are always addressed as open frameworks. Accordingly, if one of the open frameworks could be selectively synthesized, assembled, or constructed for molecules, supramolecules, or polymers, they should form continuously... 

The chemical equations that we have been learning to write are symbolic descriptions of chemical reactions. But to interpret these equations, we must think about them from another point of view, in terms of the actual substances and processes they represent. As often happens in chemistry, we can do this at either the microscopic or the macroscopic level. The microscopic interpretation visualizes reactions between individual molecules, and that interpretation is the one we... 

Chemical reactions in solids cannot be interpreted using the theory of gas phase reactions, since a solid phase can be considered as a macroscopic continuum and not a system of independent, discrete entities. Changes in such systems imply changes in the internal structures and symmetries of the whole macroscopic system. Chemical reactions are intimately interconnected with diffusion in condensed matter (Goselle, 1984). From a theoretical point of view it is hard to make a distinction between phase transitions and chemical reactions. Transformations of solids can be classified into two groups (Budnikov Gisling, 1965) with or without change in composition of the phases. 

In this chapter we will discuss some of the basic concepts which are used to describe adsorption phenomena of pure and mixed gases on the surface of solids. We here prefer a physical point of view, restricted to physisorption phenomena where adsorb molecules (admolecules) always are preserved and are not subject to chemical reactions or catalysis. Also, we always have industrial applications of physisorption processes in mind, i. e. we prefer simple and phenomenological concepts based on macroscopic experiments often being embedded within the framework of thermodynamics. That is, we prefer to take only those aspects of the molecular situation of an adsorption system into account which have been or at least can be proved experimentally and are not subject to mere speculation. 

This last usually involves driving force of a chemical reaction or a sequence of chemical reactions. Active transport is found mainly in biological membranes. From a macroscopic point of view, there are several types of fabrication methods, producing different membrane systems. These include ... 

In various models deahng with the reaction mechanism of mechanochemical reactions, it was often arbitrarily assumed that the hydrolysis of ATP precedes the mechanochemical work. This is mostly based on the intuitive view that the chemical energy is first derived from ATP and then is transformed to the mechanical energy. However, this is a concept obtained from the macroscopic view of the overaU reaction, and hence, it may not be very useful for resolving the sequences of events occurring as the partial reactions. Furthermore, it was rather difficult to visualize the mechanism in which the energy derived from the hydrolysis of a chemical bond at the active site of an enzyme is conserved and then subsequently transformed. 

TheoreticaE - " and experimentaE studies of chemical reaction dynamics and thermodynamics in bulk liquids have demonstrated in recent years that one must take into account the molecular structure of the liquid to fully understand solvation and reactivity. The solvent is not to be viewed as simply a static medium but as playing an active role at the microscopic level. Our discussion thus far underscores the unique molecular character of the interface region asymmetry in the intermolecular interactions, nonrandom molecular orientation, modifications in the hydrogen-bonding network, and other such structural features. We expect these unique molecular structure and dynamics to influence the rate and equilibrium of interfacial chemical reactions. One can also approach solvent effects on interfacial reactions at a continuum macroscopic level where the interface region is characterized by gradually changing properties such as density, viscosity, dielectric response, and other properties that are known to influence reactivity. 

In describing chemical reactions, we often take a microscopic view and focus on the entities—atoms, ions, or molecules—that make up the substances involved. However, when doing chemistry, we often think of reactions in more macroscopic terms because, in the laboratory, we handle... 

Expressions (27) and (29) show how the rates of reaction (26) and its reverse, reaction (28), depend upon the concentrations. Now we can apply our microscopic view of the equilibrium state. Chemical changes will cease (on the macroscopic scale) when the rate of reaction (26) is exactly equal to that of reaction (28). When this is so, we can equate expressions (27) and (29) ... 

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