Chemical equilibrium definition

If chemical equilibrium between the melt and the solid is assumed throughout the melting column, the definition of the partition coefficient (D) ... 

One could go on with examples such as the use of a shirt rather than sand reduce the silt content of drinking water or the use of a net to separate fish from their native waters. Rather than that perhaps we should rely on the definition of a chemical equilibrium and its presence or absence. Chemical equilibria are dynamic with only the illusion of static state. Acetic acid dissociates in water to acetate-ion and hydrated hydrogen ion. At any instant, however, there is an acid molecule formed by recombination of acid anion and a proton cation while another acid molecule dissociates. The equilibrium constant is based on a dynamic process. Ordinary filtration is not an equilibrium process nor is it the case of crystals plucked from under a microscope into a waiting vial. 

The overall effect of the preceding chemical reaction on the voltammetric response of a reversible electrode reaction is determined by the thermodynamic parameter K and the dimensionless kinetic parameter . The equilibrium constant K controls mainly the amonnt of the electroactive reactant R produced prior to the voltammetric experiment. K also controls the prodnction of R during the experiment when the preceding chemical reaction is sufficiently fast to permit the chemical equilibrium to be achieved on a time scale of the potential pulses. The dimensionless kinetic parameter is a measure for the production of R in the course of the voltammetric experiment. The dimensionless chemical kinetic parameter can be also understood as a quantitative measure for the rate of reestablishing the chemical equilibrium (2.29) that is misbalanced by proceeding of the electrode reaction. From the definition of follows that the kinetic affect of the preceding chemical reaction depends on the rate of the chemical reaction and duration of the potential pulses. 

Every gas has a definite temperature (at a particular pressure) at which m = 0. Below this temperature m is positive and above this temperature m is negative. This temperature (at a particular pressure) at which m = 0, i.e., the gas neither cooled down nor heated upon adiabatic expansion and below which Chemical Equilibrium... 

Charge transfer resistance, 1056 Charge transfer overpotential, 1231 Charge transfer, partial. 922. 954 Charges in solution, 882 chemical interactions, 830 Charging current. 1056 Charging time, 1120 Chemical catalysis, 1252 Chemical and electrochemical reactions, differences, 937 Chemical equilibrium, 1459 Chemical kinetics, 1122 Chemical potential, 937, 1058 definition, 830 determination, 832 of ideal gas, 936 interactions, 835 of organic adsorption. 975 and work function, 835... 

Equations 27 and 28 permit a simple comparison to be made between the actual composition of a chemical system in a given state (degree of advancement) and the composition at the equilibrium state. If Q K, the affinity has a positive or negative value, indicating a thermodynamic tendency for spontaneous chemical reaction. Identifying conditions for spontaneous reaction and direction of a chemical reaction under given conditions is, of course, quite commonly applied to chemical thermodynamic principle (the inequality of the second law) in analytical chemistry, natural water chemistry, and chemical industry. Equality of Q and K indicates that the reaction is at chemical equilibrium. For each of several chemical reactions in a closed system there is a corresponding equilibrium constant, K, and reaction quotient, Q. The status of each of the independent reactions is subject to definition by Equations 26-28. 

To evaluate die position of the equilibrium, p is determined by the equation pAeq = pA a (reactant acid) —pAa (product acid). For the above example pA), = 17 — 10 = 7, and based on the definition of pA, Aeq = 10 7 for this equilibrium. Thus the equilibrium lies far to the reactant side that is, very litde isopropoxide ion or ethyl ammonium ion is present at equilibrium. This technique is applicable for virtually any acid-base equilibrium. The three required steps are to (a) write a balanced equation that describes the equilibrium to be analyzed, (b) identify the species which is acting as an acid on each side of the equilibrium and write down its pAa, and (c) subtract the pAa of the product acid from the pAa of the reactant acid to give the pAeq for the equilibrium in question. It is a requirement that the pAy s of the acids on each side of the equilibrium are known or can be estimated reasonably well. Furthermore, the pA eq that is determined refers to the equilibrium in the direction it is written. It is therefore important to write the chemical equilibrium as you wish to analyze it. 

The measurements of water quality parameters (oxidation-reduction potential, pH, temperature, conductivity, dissolved oxygen, and turbidity) and the collection of field screening data with field portable instruments and test kits constitute a substantial portion of field work. Field measurements, such as pH, stand on their own as definitive data used for the calculations of solubility of chemical species and chemical equilibrium in water, whereas others serve as indicators of well stabilization or guide our decision-making in the field. Table 3.8 shows the diversity of field measurement... 

The concept of chemical components. Each species can be expressed as the product of a set of chemical components that define the equilibrium problem and a formation constant. Morel (1983) has expressed the definition of the chemical components as a set of chemical entities that permits a complete description of the stoichiometry of the system . For the example of the hydroxy-aluminium species given above Al3+ and H+ are the chemical components. As will be seen in the section on surface complexes the components are not necessarily elements or species. The components concept is important for understanding how to set up chemical equilibrium problems with various computer models. 

When a chemical reaction is proceeding, it is, by definition, not at equilibrium and thus not reversible. Thus, entropy changes in chemical reactions cannot be obtained from heat effects in calorimetric experiments. Entropy changes can be obtained by studying chemical equilibrium (Chapter 7) or by opposing the tendency of the reaction to proceed with an applied electric potential (Chapter 10). 

See, for example, Chap. 9 in K. Denbigh, The Principles of Chemical Equilibrium, Cambridge University Press, Cambridge, 1981. ThelUPAC recommendation for the symbol to represent rational activity coefficients is yx, which is not used in this book in order to make the distinction between solid solutions and aqueous solutions more evident. In strict chemical thermodynamics, however, all activity coefficients are based on the mole fraction scale, with the definition for aqueous species (Eq. 1.12) actually being a variant that reflects better the ionic nature of electrolyte solutions and the dominant contribution of liquid water to these mixtures. (See, for example, Chap. 2 inR. A. RobinsonandR. H. Stokes,Electrolyte Solutions, Butterworths, London, 1970.)... 

Provide clear definition of the design problem. Collect sufficient engineering data. Get a comprehensive picture of chemistry and reaction conditions, thermal effects and chemical equilibrium, as well as about safety, toxicity and environmental problems. Examine the availability of physical properties for components and mixtures of significance, identify azeotropes and key binaries. Define the key constraints. 

Table 8.1 describes the steps of the methodology in more detail. The procedure starts with the Problem definition production rate, chemistry, product specifications, safety, health and environmental constraints, physical properties, available technologies. Then, a first evaluation of feasibility is performed by an equilibrium design. This is based on a thermodynamic analysis that includes simultaneous chemical and physical equilibrium (CPE). The investigation can be done directly by computer simulation, or in a more systematic way by building a residue curve map (RCM), as explained in the Appendix A. This step will identify additional thermodynamic experiments necessary to consolidate the design decisions, mainly phase-equilibrium measurements. Limitations set by chemical equilibrium or by thermodynamic boundaries should be analyzed here. 

Note that when water is added to CaSO f Os, the latter dissolves until its rate of dissolution is equal to its rate of precipitation. This is, by definition, the chemical equilibrium point. If, at this point, a certain amount of NaCl (a very water-soluble salt) is added, it suppresses the single-ion activity coefficient of Ca2+ and SO2". Hence, the... 

Several approximations that allow simple estimates of bond parameters are presented as a demonstration that predictions based on quantum potentials are of correct order, and not as an alternative to well-established methods of quantum chemistry. In the same spirit it is demonstrated that the fundamental thermodynamic definition of chemical equilibrium can be derived directly from known quantum potentials. The main advantage of the quantum potential route is that it offers a logical scheme in terms of which to understand the physics of chemical binding. It is only with respect to electron-density distributions in bonds that its predictions deviate from conventional interpretations in a way that can be tested experimentally. 

Dissociation constants, which are chemical equilibrium constants for dissolution of acids, are defined in a manner similar to the definition of pH in the case of water. As discussed before, a weak acid such as H3PO4 goes through step-by-step ionization in water given by Eqs. 4.7-4.9. In each step, the dissociation constant is experimentally found to be... 

The reaction temperature profile is of particular importance because the reaction rate responds vigorously to temperature changes. Figure 82 plots lines of constant reaction rate illustrating its dependence on temperature and ammonia concentration in the reacting synthesis gas. The line for zero reaction rate corresponds to the temperature-concentration dependence of the chemical equilibrium. From Figure 82 it is apparent that there is a definite temperature at which the rate of reaction reaches a maximum for any given ammonia concentration. Curve (a) represents the temperature-concentration locus of maximum reaction rates. To maintain maximum reaction rate, the temperature must decrease as ammonia concentration increases. 

From now on, we shall consider a state of chemical equilibrium in the light of its ability to take part in reversible transformations its essential character will no longer consist in the absence of all change it will consist rather in the separation of states which are the seat of a transformation of definite direction from those states which are the seat of a transformation in the opposite direction we may then characterize such a state of chenfical equilibnum as one where two reactions the inverse of each other, limit each other. 

The definition of Eh, and thus Pe, is given by the Nemst equation, in which the Eh of a solution is related to concentrations of aqueous redox couples at chemical equilibrium and the voltage of a standard hydrogen electrode ( ). For example, when concentrations of aqueous Fe and Fe " " are at equilibrium. Eh is defined as... 

The subsequent discussion is based on the theorem that all reactions in a homogeneous system come to a stop when a certain definite chemical equilibrium has been established between the substances which take part in the reaction. In other words, that such reactions never proceed so far that the substances entering into the reaction have disappeared entirely. All the... 

The equilibrium constant approach works well when single simple reactions occur, but not when there are competing reactions. The formal definition of chemical equilibrium is that the total Gibbs free energy is at a minimum ... 

We have specified all the equilibrium relations. The last step in the derivation is to connect the chemical equilibrium with the electrochemical potential this can be done using the definition of electrochemical potential. The electrochemical potential of an ion was defined for first time by Guggenheim [16] as follows the difference in the electrochemical potential of an ion between two phases is defined as the work of transferring reversibly, at constant temperature and constant volume, 1 mol from one phase to the other. Hence [15]... 

According to IUPAC the definition solvents polarity is the overall solvation capability (or solvation power) for (1) educts and products, which influences chemical equilibrium, (2) reactants and activated complexes ( transition states ), which determines reaction rates, and (3) ions or molecules in their ground and first excited state, which is responsible for light absorptions in the various wavelength regions. This overall solvation capability depends on the action of all, non-specific and specific, intermolecular solute-solvent interactions, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute [53],... 

In spite of the outlined above formulation of chemical equilibrium problem in terms of rigorous thermodynamics (equilibrium constants defined as quotients of activities) which is well known and does not pose any special difficulty when it is compared with formulation in terms of conditional equilibrium constants (defined as quotients of concentrations), the former approach is not very popular, and many equilibrium constants of surface reactions reported in published papers were defined in terms of concentrations. Even praise of use of concentrations rather than activities in modeling of adsorption can be found in recent literature. Many publications do not address this question explicit, and then it is difficult to figure out how the equilibrium constants of surface species were defined K, or Accordingly, the equilibrium constants of surface species reported in tables of Chapter 4 constitute a mixture of constants defined in different ways (K, or The details regarding the definition of equilibrium constants can be found (but not always) in the original publications. 

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