Equilibrium A dynamic reaction system

Chemical equilibrium A dynamic reaction system in which the concentrations of all reactants and products remain constant as a function of time. 

Chemical change the change of substances into other substances through a reorganization of the atoms a chemical reaction. (1.9) Chemical equation a representation of a chemical reaction showing the relative numbers of reactant and product molecules. (3.8) Chemical equilibrium a dynamic reaction system in which the concentrations of all reactants and products remain constant as a function of time. (13)... 

Dynamic equilibrium — A dynamic equilibrium is reached when you have a reversible reaction taking place in a closed system. At equilibrium, the activities of all species participating in the reaction remain constant, although the reactions are still continuing. This is because the rates of the forward and the reverse reactions are equal. 

Chemical equilibrium A dynamic state in which the rates of forward and reverse reactions are identical a system in equilibrium will not spontaneously depart from this condition. Chemiluminescence The emission of energy as electromagnetic radiation during a chemical reaction. 

In contrast to a chemical reaction, dynamic surface processes are mainly characterised as "incomplete". This means that one or more parameters, necessary for the equilibrium state, are required to describe the distance of the instantaneous state of the entire system from equilibrium. A chemical reaction can be described by a degree of advancement e. The same procedure is possible for surface chemical reaction including mass transfer and processes of formation and dissolution of associates, e varies between +1 and -1 and depends on the equilibriiun state and affinity A. The differential quotient of affinity to the degree of advancement was defined by Yao (1981) as the ordering coefficient... 

It is a remarkable fact that the microscopic rate constant of transition state theory depends only on the equilibrium properties of the system. No knowledge of the system dynamics is required to compute the transition state theory estimate of the reaction rate constant... 

The assumptions of transition state theory allow for the derivation of a kinetic rate constant from equilibrium properties of the system. That seems almost too good to be true. In fact, it sometimes is [8,18-21]. Violations of the assumptions of TST do occur. In those cases, a more detailed description of the system dynamics is necessary for the accurate estimate of the kinetic rate constant. Keck [22] first demonstrated how molecular dynamics could be combined with transition state theory to evaluate the reaction rate constant (see also Ref. 17). In this section, an attempt is made to explain the essence of these dynamic corrections to TST. 

Static system The batch-wise employment of ion-exchange resins, wherein (since ion exchange is an equilibrium reaction) a definite endpoint is reached in which a finite quantity of all the ions involved is present. Opposed to a dynamic, column-type operation. 

Collisions between NO2 molecules produce N2 O4 and consume NO2. At the same time, fragmentation of N2 O4 produces NO2 and consumes N2 O4. When the concentration of N2 O4 is veiy low, the first reaction occurs more often than the second. As the N2 O4 concentration increases, however, the rate of fragmentation increases. Eventually, the rate of N2 O4 production equals the rate of its decomposition. Even though individual molecules continue to combine and decompose, the rate of one reaction exactly balances the rate of the other. This is a dynamic equilibrium. At dynamic equilibrium, the rates of the forward and reverse reactions are equal. The system is dynamic because individual molecules react continuously. It is at equilibrium because there is no net change in the system. 

Chemical reactions that are reversible are said to be in dynamic equilibrium because opposite reactions take place simultaneously at the same rate. A system that is at equilibrium can be shifted toward either reactants or products if the system is subjected to a stress. Changes in concentration, temperature, or pressure are examples of stresses. 

Let us consider a redox system at a static inert electrode. Whilst thermodynamics only describe the equilibrium of such a system (cf., Section 2.2.1.2.1), kinetics deal with an approach to equilibrium and assume a dynamic maintenance of that state. For that purpose the equilibrium reaction... 

In the absence of current (j = 0), the system is in equilibrium but the electrode reactions nonetheless proceed. Thus, similar to chemical equilibria, the electrode equilibria have a dynamic character. Under equilibrium conditions... 

The last unconventional approach considered in this chapter is low-pressure analyte pulse perturbation-CL spectroscopy (APP-CLS). This approach is highly dynamic as it relies on the combination of an oscillating reaction, which is a particular case of far-from-equilibrium dynamic systems, and a CL reaction. 

In a chemical context, therefore, the system A + B —C + D reaches a dynamic equilibrium when the rate of the forward reaction equals the rate of the backward reaction. At equilibrium, the concentrations of A, B, C and D remain constant. 

It is important that the catalysts are stable in each other s presence. Typically, kinetic resolution of the reaction is performed with an enzyme, which always will contain traces of water. Hence, MPVO catalysts and water-sensitive transition-metal catalysts cannot be used in these systems. The influence of the amount of the hydrogen acceptor in the reaction mixture during a dynamic kinetic resolution is less pronounced than in a racemization, since the equilibrium of the reaction is shifted towards the alcohol side. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

A chemical equilibrium results when two exactly opposite reactions are occurring at the same place, at the same time and with the same rates of reaction. When a system reaches the equilibrium state the reactions do not stop. A and B are still reacting to form C and D C and D are still reacting to form A and B. But because the reactions proceed at the same rate the amounts of each chemical species are constant. This state is a dynamic equilibrium state to emphasize the fact that the reactions are still occurring—it is a dynamic, not a static state. A double arrow instead of a single arrow indicates an equilibrium state. For the reaction above it would be ... 

The earth s subsurface is not at complete thermodynamic equilibrium, but parts of the system and many species are observed to be at local equilibrium or, at least, at a dynamic steady state. For example, the release of a toxic contaminant into a groundwater reservoir can be viewed as a perturbation of the local equilibrium, and we can ask questions such as. What reactions will occur How long will they take and Over what spatial scale will they occur Addressing these questions leads to a need to identify actual chemical species and reaction processes and consider both the thermodynamics and kinetics of reactions. 

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