The Concept of Dynamic Equilibrium

Nutrients and waste materials are exchanged between fetal and maternal blood through the placenta. 

Recall from the previous chapter that reaction rates generally increase with increasing concentration of the reactants (unless the reaction order is zero) and decrease with decreasing concentration of the reactants. With this in mind, consider the reaction between hydrogen and iodine  

Dynamic equilibrium for a chemical reaction is the condition in which the 

Dynamic equilibrium is called dynamic because the forward and reverse reactions are still occurring however, they are occurring at the same rate. When dynamic equilibrium is reached, the concentrations of H2,12, and HI no longer change. They remain constant because the reactants and products form at the same rate that they are depleted. Note that just because the concentrations of reactants and products no longer change at equilibrium does not mean that the concentrations of reactants and products are equal to one another at equilibrium. Some reactions reach equilibrium only after most of the reactants have formed products. Others reach equilibrium when only a small fraction of the reactants have formed products. It depends on the reaction. 

Neariy all ck Mlcal leaclitis are at least theeretically reversible. Ii waiy cases, hewever, tbe reversibility is se sanii that it caa be igaerel 

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Thinking it Through When a reaction has reached equilibrium, it does not mean that all chemical activity has stopped. Rather, at equilibrium, the macroscopic view indicates constant (but seldom equal) concentrations for each substance, making Choice (D) the correct response. Choice (A) is a commonly held misconception, one that you will not choose if you remember the concept of dynamic equilibrium. It is also untrue that the total moles of products must equal the remaining moles of reactant, choice (B). The relative amounts of material present at equilibrium will depend greatly on the position of the equilibrium, revealed in quantitative problems by the value of the equilibrium constant. Choice (C) is based on another common misconception about equilibrium reactions. Addition of a catalyst, while it may increase the rate at which equilibrium is achieved, does not affect the position of equilibrium. 

The concept of dynamic equilibrium is also an important way to organize our thinking about reactions. If we want to achieve a particular outcome from a reaction that is at equilibrium, we may need to adjust the environment to... 

While equilibrium indicates a stable state, it is crucial to understand that equilibrium does not necessarily imply a completely static and unchanging state. We will often encounter the concept of dynamic equilibrium in our discussions of kinetics. In a dynamic equilibrium, backward and forward kinetic processes of equal but opposite rates occur. For example, a glass of water can be in dynamic equilibrium with 100% relative humidity air above it. In such a situation, the overall level of water in the... 

In all of the discussions so far, it has been implicitly assumed that the reactions go to completion. What this means is that the reactions are assumed to continue in the forward direction as written until one of the reactants is completely depleted. For many reactions, this assumption is reasonable. However, there are many other reactions that do not go to completion. Instead, the reaction only proceeds partway and an equilibrium state is reached where considerable concentrations of both the reactants and product species remain in coexistence. This connects back to the concept of dynamic equilibrium that we discussed in Chapter 2, when the forward and backward reaction rates reach a balancing point. 

Figure 1 Cartoon of the surfactant unimer distribution in aqueous solution and the spontaneous self-assembly of surfactant unimers into spherical micelles just above the cmc. The two sets of arrows represent the concept of dynamic equilibrium in which the exchange rates of unimer between the air/water interface and micelles are equal. The cross section of the spherical micelle shows the core region containing the tails, the interfacial region containing hydrated headgroups (and a fraction of the counterions for ionic micelles, not shown), and the surrounding aqueous region. Such iconic images of micelles are unrealistic because experiments show that micelles are fluids and the tails are almost randomly distributed, and headgroups move at near diffusion-controlled rates that do not define a smooth surface.

Classroom context. The lessons that we studied occurred over two days. On the first day, a double lesson (80 minutes) was given, starting with a recapitulation of the particulate nature of chemical reactions and factors that influence reaction rate, followed by activation energy, reaction profile diagrams, and the conditions for chemical equilibrium. The next day, a single lesson (40 minutes) elaborated the concept of dynamic equilibrium. No teaeher demonstrations or student practical work were included in the lessons. The topies were presented and discussed in an interactive way with Neil and his students asking many questions. 

The concept of dynamic equilibrium in chemical reactions was briefly mentioned in Section 14.8. Discussions on vapor pressure (Section 11.2) and solubility (Section 12.2) give detailed explanations of the role of dynamic equilibria. 

We will introduce basic kinetic concepts that are frequently used and illustrate them with pertinent examples. One of those concepts is the idea of dynamic equilibrium, as opposed to static (mechanical) equilibrium. Dynamic equilibrium at a phase boundary, for example, means that equal fluxes of particles are continuously crossing the boundary in both directions so that the (macroscopic) net flux is always zero. This concept enables us to understand the non-equilibrium state of a system as a monotonic deviation from the equilibrium state. Driven by the deviations from equilibrium of certain functions of state, a change in time for such a system can then be understood as the return to equilibrium. We can select these functions of state according to the imposed constraints. If the deviations from equilibrium are sufficiently small, the result falls within a linear theory of process rates. As long as the kinetic coefficients can be explained in terms of the dynamic equilibrium properties, the reaction rates are directly proportional to the deviations. The thermodynamic equilibrium state is chosen as the reference state in which the driving forces X, vanish, but not the random thermal motions of structure elements i. Therefore, systems which we wish to study kinetically must first be understood at equilibrium, where the SE fluxes vanish individually both in the interior of all phases and across phase boundaries. This concept will be worked out in Section 4.2.1 after fluxes of matter, charge, etc. have been introduced through the formalism of irreversible thermodynamics. 

You can t get very far into acid-base chemistry before you run into acid-base equilibria. So, let s get started with acids and bases by first revisiting the concept of dynamic equilibria. As we said in chapter 7, a dynamic equilibrium exists in a system comprised of (at least) two states when the populations of the two states are constant, even though the members of the system are constantly changing from one state to another. We illustrated this principle with vapor pressure. Now let s consider some chemical examples. Most chemical reactions are reversible. 

Figure 21 The concept of dynamic covalent chemistry the presence of a template shifts the equilibrium and amplifies a specific member of the DCL (Adapted from ref 38 with permission).

Protein Turnover. The failure of induced enzymes to acquire label from bacterial proteins or to liberate their own amino adds has led to the conclusion that there is no turnover of miteins in growing cells. This is in contradiction to the generally held interpretation of the Schoen-heimer concept of dynamic equilibrium. From observations on the incorporation of labeled amino acids, fats, and carbohydrates into various parts of animals, Schoenheimer and his collaborators postulated that there are essentially no inert biochemical structures, but that apparently stable structures are continually broken down and renewed. 

The Young-Dupre relation is the boundary condition that allows the determination of the shape of the surface between two fluids in contact with a solid wall, such as a liquid drop on a solid wall within a gas (dew) or an air bubble adhering to a sohd wall within a liquid. It is important to emphasize that the Young-Dupre law is applicable only to the case where the triple line is at static equilibrium on the sohd. When the triple line moves (if for example the dew drop slides down the wah), the concept of dynamic contact angle shoirld be substituted for that of static contact angle for better results. ... 

The concept of non-equilibrium solvation has been introduced to describe the solvent polarization in processes involving dynamic, or sudden, variations of solute charge distribution of the solute, and it takes into account that during the time-scale of a fast event not aU the degrees of freedoms of the solvent molecules (nuclear, translational, rotational,vibrational electronic) are able to respond to the variations of the charge distribution of the solute (see Appendix). 

This section will deal with the above interfacial aspects starting with the equilibrium aspects of surfactant adsorption at the air/water and oil/water interfaces. Due to the equilibrium aspects of adsorption (rate of adsorption is equal to the rate of desorption) one can apply the second law of thermodynamics as analyzed by Gibbs (see below). This is followed by a section on dynamic aspects of surfactant adsorption, particularly the concept of dynamic surface tension and the techniques that can be applied in its measurement. The adsorption of surfactants both on hydrophobic surfaces (which represent the case of most agrochemical solids) as well as on hydrophilic surfaces (such as oxides) will be analyzed using the Langmuir adsorption isotherms. The structure of surfactant layers on solid surfaces will be described. The subject of polymeric surfactant adsorption will be dealt with separately due to its complex nature, namely irreversibility of adsorption and conformation of the polymer at the solid/liquid interface. 

The simplest explanation for why the vapor pressure of a solution is lower than that of the pure solvent is related to the concept of dynamic equihbrium itself. Consider the following representation of a liquid in dynamic equilibrium with its vapor. Here the rate of vaporization is equal to the rate of condensation. 

Having separated the dynamical from equilibrium (or, more accurately, quasi-equilibrium) effects, one can readily discover the origin of the activation free energy and define the concept of the potential of mean force by analysis of the expression for the TST rate constant, k in (A3.8.3). The latter can be written as [7]... 

The system is ideal, with equilibrium described by a constant relative volatility, the liquid components have equal molar latent heats of evaporation and there are no heat losses or heat of mixing effects on the plates. Hence the concept of constant molar overflow (excluding dynamic effects) and the use of mole fraction compositions are allowable. 

The usual emphasis on equilibrium thermodynamics is somewhat inappropriate in view of the fact that all chemical and biological processes are rate-dependent and far from equilibrium. The theory of non-equilibrium or irreversible processes is based on Onsager s reciprocity theorem. Formulation of the theory requires the introduction of concepts and parameters related to dynamically variable systems. In particular, parameters that describe a mechanism that drives the process and another parameter that follows the response of the systems. The driving parameter will be referred to as an affinity and the response as a flux. Such quantities may be defined on the premise that all action ceases once equilibrium is established. 

In practice, the concept of temperature is most useful when determining whether two bodies are in thermal equilibrium. Firstly, we need to appreciate how these equilibrium processes are always dynamic, which, stated another way, indicates that a body simultaneously emits and absorbs energy, with these respective amounts of energy being equal and opposite. Furthermore, if two bodies participate in a thermal equilibrium then we say that the energy emitted by the first body is absorbed by the second and the first body also absorbs a similar amount of energy to that emitted by the second body. 

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