Dynamic equilibrium reversible reactions

In such a state of dynamic equilibrium, both reactions continue, but there is no net change in the composition of the system. A reversible chemical reaction is in chemical equilibrium when the rate of its forward reaction equals the rate of its reverse reaction and the concentrations of its products and reactants remain unchanged. The chemical equation for the reaction at equilibrium is written using double arrows to indicate the overall reversibility of the reaction. 

The first step occurs rapidly and reversibly, a dynamic equilibrium is set up in which the rates of forward and reverse reactions are equal. Because the second step is slow and ratedetermining, it follows that... 

Overall, the partial pressure of N204 drops, and the forward reaction slows down. Conversely, the partial pressure of N02 increases, so the rate of the reverse reaction increases. Soon these rates become equal. A dynamic equilibrium has been established. 

We have learned much about equilibrium. It is characterized by constancy of macroscopic properties but with molecular processes continuing in a state of dynamic balance. At equilibrium we can conclude that every reaction that takes place does so at the same reaction rate as its reverse reaction. 

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. 

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,... 

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,... 

All chemical equilibria are dynamic equilibria. Although there is no further net change at equilibrium, the forward and reverse reactions are still taking place. 

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. 

This step also is bimolecular. The last two equations add up to give the reverse of the overall equation, 3 02(g) —> 2 03(g). The forward and reverse reactions jointly provide a mechanism for reaching dynamic equilibrium between the reactants and the products in the overall process. 

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. 

Dynamic equilibrium When the forward reaction and the backward reaction in a reversible reaction occur at the same rate. 

The threading-followed-by-capping method has been recently employed by Stoddart to prepare a [2]rotaxane under thermodynamic control [60]. In this approach, the dibenzylammonium ion 28 - which is terminated by an aldehyde function - is mixed with the dibenzo[24]crown-8 ether (20) to form a threaded species. Upon addition of a bulky amine, the aldehyde-terminated template can be converted into an imine in a reversible reaction establishing a dynamic equilibrium (see 29 and 30 in Scheme 17). 

Histone acetylation is a reversible amidation reaction involving defined e-amino groups of lysine residues (see Fig. 6) at the N-terminal tails of core histones. The highly dynamic equilibrium between the acetylated and non-acetylated states of lysine is maintained by two enzymatic groups, referred to as histone acetyltransferases (HATs) and histone deacetylases (HDACs). 

A chemical relaxation technique that measures the magnitude and time dependence of fluctuations in the concentrations of reactants. If a system is at thermodynamic equilibrium, individual reactant and product molecules within a volume element will undergo excursions from the homogeneous concentration behavior expected on the basis of exactly matching forward and reverse reaction rates. The magnitudes of such excursions, their frequency of occurrence, and the rates of their dissipation are rich sources of dynamic information on the underlying chemical and physical processes. The experimental techniques and theory used in concentration correlation analysis provide rate constants, molecular transport coefficients, and equilibrium constants. Magde" has provided a particularly lucid description of concentration correlation analysis. See Correlation Function... 

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... 

For instance, at room temperature when two moles of hydrogen gas (Ha) react with one mole of graphite (C), there is a complete conversion of the reactants into one mole of methane gas (CH4). However, if the reaction is carried out at high temperatures and constant pressure, it is foimd that the reaction does not proceed to completion and even after a prolonged time at that temperature and pressure, some hydrogen gas and graphite remain. The reaction thus reaches a state of chemical equilibrium where the rates of forward and reverse reactions are equal and a dynamic equilibrium is reached. 

Theoretically, all chemical reactions are reversible and, in the presence of enzymes, a dynamic equihbrium state will be reached rather than a total transformation of substrate molecules to product molecules. The dynamic equilibrium could be characterized by the Haldane equation ... 

As times passes, the rate of forward and reverse reaction becomes equal. At this point, a dynamic equilibrium is established. 

In the foregoing discussion, we have used the basic assumption that chemical reactions go to completion, or until at least one reactant is completely used up — that they are not reversible. Many reactions do go to completion, or so nearly so as to make no difference. But a huge number of reactions are reversible, and to such an extent that the products form and accumulate and then react with each other to re-form the reactants. The reaction ultimately goes to a position of dynamic equilibrium far from completion where the rate of the forward reaction is the same as the rate of the reverse reaction, and the reaction appears to have ceased. Under these conditions the experimenter observes the net rate of reaction, which is simply the difference between the rates of the forward and reverse reactions ... 

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