Dynamic equilibrium chemical equilibrium

Chemical Equilibrium Chemical Equilibrium describes the state in which the rates of forward and reverse reactions are equal and the concentrations of the reactants and products remain unchanged with time. This state of dynamic equilibrium is characterized by an equilibrium constant. Depending on the nature of reacting species, the equilibrium constant can be expressed in terms of molarities (for solutions) or partial pressures (for gases). The Equilibrium constant provides information about the net direction of a reversible reaction and the concentrations of the equilibrium mixture. 

Dynamic Chemical Equilibrium Dynamic chemical equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction. Dynamic Chemical Equilibrium When dynamic chemical equilibrium is reached, the concentrations of the reactants and products become constant. 

Trend of kinetic curves for each participant of the reaction is modeled in Fig. 1.7. It is seen, that in some time the concentrations of the substances become time-undependable, which corresponds to the equilibrium condition. Time-dependences of the rate of each stage are also shown here, and that illustrates the dynamic character of the chemical equilibrium reaching equilibrium condition does not mean the end of the reaction. In equilibrium condition the total rate of the reaction is equal to zero because of the equality of the rates of direct and reverse reaction, which have quiet definite values. 

Fluctuations of observables from their average values, unless the observables are constants of motion, are especially important, since they are related to the response fiinctions of the system. For example, the constant volume specific heat of a fluid is a response function related to the fluctuations in the energy of a system at constant N, V and T, where A is the number of particles in a volume V at temperature T. Similarly, fluctuations in the number density (p = N/V) of an open system at constant p, V and T, where p is the chemical potential, are related to the isothemial compressibility iCp which is another response fiinction. Temperature-dependent fluctuations characterize the dynamic equilibrium of themiodynamic systems, in contrast to the equilibrium of purely mechanical bodies in which fluctuations are absent. 

Stratospheric ozone is in a dynamic equilibrium with a balance between the chemical processes of formation and destruchon. The primary components in this balance are ultraviolet (UV) solar radiation, oxygen molecules (O2), and oxygen atoms (O) and may be represented by the following reactions ... 

Chemists picture equilibrium as a dynamic balance between opposing reactions. An understanding of the Law of Chemical Equilibrium can be built upon this basis. 

Another difficulty is that spontaneous chemical reactions do not go to completion. Even if a spontaneous reaction is exothermic, it proceeds only till it reaches equilibrium. But in our golf ball analogy, equilibrium is reached when all of the golf balls are on the lower level. Oui analogy would lead us to expect that an exothermic reaction would proceed until all of the reactants are converted to products, not to a dynamic equilibrium. 

It is found that after the elapse of a sufficient time interval, all reversible reactions reach a state of chemical equilibrium. In this state the composition of the equilibrium mixture remains constant, provided that the temperature (and for some gaseous reactions, the pressure also) remains constant. Furthermore, provided that the conditions (temperature and pressure) are maintained constant, the same state of equilibrium may be obtained from either direction of a given reversible reaction. In the equilibrium state, the two opposing reactions are taking place at the same rate so that the system is in a state of dynamic equilibrium. 

It can be shown, (Gibbs, Scientific Papers, I. J. J. Thomson, Applications of Dynamics to Physics and Chemistry), that a chemical equilibrium can be modified by the action of capillary forces. Thus, a state of equilibrium in solution may conceivably be modified if the latter is in the form of thin films, such as soap bubbles. Since, according to Freundlich (Kapillarchemie, 116), there is at present no direct evidence of the existence of such modification (which would no doubt be exceedingly, though possibly measurably, small) we shall not enter any further into the matter here. 

Why Do We Need to Know This Material The dynamic equilibrium toward which every chemical reaction tends is such an important aspect of the study of chemistry that four chapters of this book deal with it. We need to know the composition of a reaction mixture at equilibrium because it tells us how much product we can expect. To control the yield of a reaction, we need to understand the thermodynamic basis of equilibrium and how the position of equilibrium is affected by conditions such as temperature and pressure. The response of equilibria to changes in conditions has considerable economic and biological significance the regulation of chemical equilibrium affects the yields of products in industrial processes, and living cells struggle to avoid sinking into equilibrium. 

Like physical equilibria, all chemical equilibria are dynamic equilibria, with the forward and reverse reactions occurring at the same rate. In Chapter 8, we considered several physical processes, including vaporizing and dissolving, that reach dynamic equilibrium. This chapter shows how to apply the same ideas to chemical changes. It also shows how to use thermodynamics to describe equilibria quantitatively, which puts enormous power into our hands—the power to control the And, we might add, to change the direction of a reaction and the yield of products,... 

The criteria that identify a dynamic chemical equilibrium are ... 

Like phase changes, chemical reactions tend toward a dynamic equilibrium in which, although there is no net change, the forward and reverse reactions are still taking place, but at matching rates. What actually happens when the formation of ammonia appears to stop is that the rate of the reverse reaction,... 

Chemical reactions reach a state of dynamic equilibrium in which the rates of forward and reverse reactions are equal and there is no net change in composition. 

A catalyst speeds up both the forward and the reverse reactions by the same amount. Therefore, the dynamic equilibrium is unaffected. The thermodynamic justification of this observation is based on the fact that the equilibrium constant depends only on the temperature and the value of AGr°. A standard Gibbs free energy of reaction depends only on the identities of the reactants and products and is independent of the rate of the reaction or the presence of any substances that do not appear in the overall chemical equation for the reaction. 

Like all chemical equilibria, this equilibrium is dynamic and we should think of protons as ceaselessly exchanging between HCN and H20 molecules, with a constant but low concentration of CN and H30+ ions. The proton transfer reaction of a strong acid, such as HCl, in water is also dynamic, but the equilibrium lies so strongly in favor of products that we represent it just by its forward reaction with a single arrow. 

In a balanced chemical equation (commonly called a chemical equation ), the same number of atoms of each element appears on both sides of the equation, chemical equilibrium A dynamic equilibrium between reactants and products in a chemical reaction, chemical formula A collection of chemical symbols and subscripts that shows the composition of a substance. See also condensed structural formula empirical formula,- molecular formula structural formula. 

Other reactions do not go to completion because they reach dynamic equilibrium. While reactant molecules continue to form product molecules, product molecules also interact to re-form reactant molecules. The Haber reaction and many precipitation reactions, described later in this chapter, are examples of reactions that reach d3Tiamic equilibrium rather than going to completion. We treat chemical equilibria in detail in Chapters 16-18. 

As an indispensable source of fertilizer, the Haber process is one of the most important reactions in industrial chemistry. Nevertheless, even under optimal conditions the yield of the ammonia synthesis in industrial reactors is only about 13%. This Is because the Haber process does not go to completion the net rate of producing ammonia reaches zero when substantial amounts of N2 and H2 are still present. At balance, the concentrations no longer change even though some of each starting material is still present. This balance point represents dynamic chemical equilibrium. 

The dynamic dissociation model resembles the association (or dissociation) model in that electrically conducting species are assumed to he nonassociated species, and it differs from the association model in that in the dynamic dissociation model the dissociation process itself is the electrically conducting process, while in the association model, the amount of the dissociated species is constant according to the chemical equilibrium. 

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