Chemical Equilibrium A Dynamic Condition

A double arrow ( ) is used to show that a reaction is occurring in both directions. 

To see how this concept applies to chemical reactions, let s consider the reaction between steam and carbon monoxide in a closed vessel at a high temperature where the reaction takes place rapidly. 

When CO and H2O, the reactants, are mixed, they immediately begin to react to form the products, H2 and CO2. This leads to a decrease in the concentrations of the reactants, but the concentrations of the products. 

The reaction of HjO and CO to form CO2 and Hj as time passes, (a) Equal numbers of moles of H2O and CO are mixed in a closed container, (b) The reaction begins to occur, and some products (H2 and CO2) are formed, (c) The reaction continues as time passes and more reactants are changed to products. (d) Although time continues to pass, the numbers of reactant and product molecules are the same as in (c). No further changes are seen as time continues to pass. The system has reached equilibrium. 

The changes with time in the rates of the forward and reverse reactions for H2O(0) -i-CO(g) Hjfa) + C02(g) when equal numbers of moles of 

OBJECTIVE To learn about the characteristics of chemical equilibrium. 

Copyright 2011 Cengage Learning, Inc. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. 

---

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. 

Equilibrium is reached whether pure NO2, pure N2O4, or a mixture of NO2 and N2O4 is initially placed in a closed container. In any of these cases, conditions will eventually be reached in the container such that N2O4 is being formed and is decomposing at exactly the same rate. This leads to chemical equilibrium, a dynamic situation where the concentrations of reac-fanfs and producfs remain fhe same indefinitely, as long as the conditions are not changed. 

There is a lot of evidence that chemical equilibrium is dynamic. One example of this evidence is relevant to the ammonia equilibrium. Imagine carrying out two ammonia syntheses with the same starting conditions, but using D2 (deuterium) in place of H2 in one of them (Fig. 9.2). The two reaction mixtures reach equilibrium with almost exactly the same composition, except that D2 and ND3 are present in one of the systems instead of H2 and NH3. Suppose we now combine the two mixtures and... 

You may get the idea that all chemical reactions go to completion when you watch a hydrogen-oxygen explosion or watch a piece of wood burn in the fireplace. Chemicals do not always react to form products with the complete conversion of reactants, however. Whenever the point is reached at which the forward reaction is proceeding at the same rate as the reverse reaction, equilibrium is established and the amounts of reactants and products remain unchanged. But because equilibrium is a dynamic condition, the forward and reverse reactions are still happening so that each reactant or product is replaced as soon as it is consumed. 

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. 

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. 

Equilibrium Systems. Magda et al (12.) have carried out an equilibrium molecular dynamics (MD) simulation on a 6-12 Lennard-Jones fluid In a silt pore described by Equation 41 with 6 = 1 with fluid particle Interactions given by Equation 42. They used the Monte Carlo results of Snook and van Me gen to set the mean pore density so that the chemical potential was the same In all the simulations. The parameters and conditions set In this work were = 27T , = a, r = 3.5a, kT/e = 1.2, and... 

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... 

For most of the situations encountered in solvent extraction the gas phase above the two liquid phases is mainly air and the partial (vapor) pressures of the liquids present are low, so that the system is at atmospheric pressure. Under such conditions, the gas phase is practically ideal, and the vapor pressures represent the activities of the corresponding substances in the gas phase (also called their fugacities). Equilibrium between two or more phases means that there is no net transfer of material between them, although there still is a dynamic exchange (cf. Chapter 3). This state is achieved when the chemical potential x as... 

A condition of balance in a chemical system, at which no further change in reactant and product concentrations occurs. 2. If actions occurring within the reaction result in no net change in the reactant and product concentrations, the system is said to be in dynamic equilibrium. A chemical equilibrium is a dynamic equilibrium when the reaction rate in the forward direction is balanced by the rate in the reverse direction. The potential energy... 

As to what, exactly, might be the fluorinating agent in the proposed Radical Mechanism we have seen how some workers favour that of fluorine absorbed on or in the nickel fluoride layer, whereas others prefer that of high valence fluorides. In this respect, it is interesting to note how different workers interpret what are apparently very similar phenomena upon opening the electrical circuit of a previously conditioned ECF anode, e.g., Watanabe [169], and the decay of electrode potential, as an example of the first, and Sartori et al. [186], and the persistence of chemical activity, as an example of the second. Perhaps, in reality, the dynamic equilibrium which relates fluoride ion, nickel fluorides, and atomic... 

No final or absolute decision is therefore yet possible. By analogy with other cases of a similar type amongst organic compounds, the possibility of a dynamic isomerism must not be left out of consideration. It is quite possible that in sulphurous acid solution, molecules of the symmetrical and the unsymmetrical constitution may be present, side by side, in equilibrium with one another, and that even in solutions of the salts a similar condition of equilibrium may exist. Indeed, the distinctly contradictory nature of some of the chemical evidence favours this view. 

---