Thermodynamic irreversibility

This argument here has been oversimplified because the reaction is thermodynamically irreversible - after all, you cannot unboW an egg ... 

Diffusion in general, not only in the case of thin films, is a thermodynamically irreversible self-driven process. It is best defined in simple terms, such as the tendency of two gases to mix when separated by a porous partition. It drives toward an equilibrium maximum-entropy state of a system. It does so by eliminating concentration gradients of, for example, impurity atoms or vacancies in a solid or between physically connected thin films. In the case of two gases separated by a porous partition, it leads eventually to perfect mixing of the two. 

The electroanalytical chemist tends to use terms such as reversible and irreversible in a subtly different way, although the difference is more one of semantics rather than final understanding. Thus, the phrase totally irreversible is used by electroanalytical practitioners for reactions in the Tafel region [lT)l (RT/F)]. The physical electrochemist most certainly views such reactions as being thermodynamically irreversible, and distinctly so. However, it seems that the use of the term 101311/ irreversible may lead to misunderstanding, for it does not seem consistent with the ease of reversal in direction, which in practice can be made to occur even with the most (thermodynamically) irreversible reactions. 

The fundamental question in transport theory is Can one describe processes in nonequilibrium systems with the help of (local) thermodynamic functions of state (thermodynamic variables) This question can only be checked experimentally. On an atomic level, statistical mechanics is the appropriate theory. Since the entropy, 5, is the characteristic function for the formulation of equilibria (in a closed system), the deviation, SS, from the equilibrium value, S0, is the function which we need to use for the description of non-equilibria. Since we are interested in processes (i.e., changes in a system over time), the entropy production rate a = SS is the relevant function in irreversible thermodynamics. Irreversible processes involve linear reactions (rates 55) as well as nonlinear ones. We will be mainly concerned with processes that occur near equilibrium and so we can linearize the kinetic equations. The early development of this theory was mainly due to the Norwegian Lars Onsager. Let us regard the entropy S(a,/3,. ..) as a function of the (extensive) state variables a,/ ,. .. .which are either constant (fi,.. .) or can be controlled and measured (a). In terms of the entropy production rate, we have (9a/0f=a)... 

Reversibility in the context of chemical sensing means that the response follows concentration changes, both up and down. It does not have the usual thermodynamic meaning, despite the fact that a decrease of free energy is always the driving force in all interactions. Thus, sensors can be either thermodynamically reversible, as with ion-selective electrodes, or thermodynamically irreversible, as with enzyme electrodes if, however, they respond to a step up or a step down in the concentration... 

The second application of availability analysis is used to evaluate the nature and magnitude of thermodynamic irreversibilities in a methane reformer plant coupled to a high-temperature nuclear reactor. It is shown that a combination of thermal histograms and availability concepts are helpful not only in evaluating the net impact of irreversibilities in various chemical process steps on the steam power plant, but, more importantly, 1n suggesting process modifications that could improve the overall efficiency by avoiding unnecessary entropy production. 

A continuation of this application of the second-law analyses is an examination of the various irreversibilities in the reformer process for potential improvements. The chief sources of thermodynamic irreversibilities (with the associated exergy destruction) are (1) frictional losses, (2) heat transfer with a finite temperature difference, (3) chemical reaction far from equilibrium, and (4) diffusion. 

The Tafel equation implies that the overvoltage is a measure of the thermodynamic irreversibility of the electrode reaction, and it is associated with the slow step of the process. We distinguish some types of overvoltage depending on the type of slow reaction. 

The more recent adsorption-desorption isotherms of some organic vapours of different molecular diameter are shown in Figure 12.15. These measurements were made on different samples of VPI-5, each having been outgassed for 16 hours at 673 K. Although essentially of Type I, the isotherms reveal some degree of thermodynamic irreversibility with hysteresis extending back to very low p/p°. Evidently,... 

Thermodynamics of nonequilibrium (irreversible) processes is an extension of classical thermodynamics, mainly to open systems. Unfortunately, the Second Law of classical thermodynamics cannot be applied directly to systems where nonequilibrium (i.e., thermodynamically irreversible) pro cesses occur. For this reason, thermodynamics of nonequilibrium processes has used several principal concepts that are supplementary to the classical thermodynamics postulates. In contrast to the postulates, many of the con cepts in thermodynamics of nonequilibrium processes can be mathe maticaUy substantiated by considering, for example, the time hierarchy of the processes involved. 

In an open system, the entropy may change due to either increases caused by spontaneous thermodynamically irreversible internal processes in the system, djS, or exchanges between the system and the surrounding, dgS. In chemically reactive systems, djS may change as a result, for example, of spontaneous reactions inside the system, while dgS may change as a result of supply or extraction of heat and/or some reactants. 

At the cathode electrode, the thermodynamically irreversible four-electron oxygen reduction reaction (ORR) is the dominant electrochemical process (reaction 2) ... 

Several works have discussed the implication of the FR and JE on thermodynamic irreversibility. It should be mentioned, that underlying the deriva-... 

The directionality of a step s, will be denoted by a label, as either = 5, or - Sj. The sign denotes a reversible step, whose net rate may be either positive (i.e., in the forward direction) or negative (i.e., in the reverse direction). The sign - denotes an irreversible step that is either thermodynamically irreversible or known to proceed with a net positive rate (i.e., in the forward direction). 

Liquid simulation studies have been essential in assessing the applicability of various fluctuation relations to real physical systems. These are important relations in nonequilibrium statistical mechanics that are valid far from equilibrium and can be used to derive Green-Kubo relations for transport coefficients.223,224 They show how thermodynamic irreversibility emerges from... 

It should perhaps be re-emphasized that even Planck-power input as hydrogen entails some entropy increase and therefore is thermodynamically irreversible, consistently with the Second Law of Thermodynamics while still thwarting the heat death. The heat death is thus thwarted via dilution of entropy as an island Universe [1] expands indefinitely, which is consistent with the Second Law [59-63] — not via destruction of entropy, which is not Planck-power input as hydrogen represents input at positive but far less than maximum entropy. Thus Planck-power input (if it exists) defeats the heat death predicted by the Second Law of Thermodynamics [59-63] even though it does not defeat the Second Law itself. 

In order to avoid the possibility of misunderstanding, it may be pointed out here that many changes which occur spontaneously in nature, e g., expansion of a gas, evaporation of a liquid and even chemical reactions, can be carried out reversibly, at l t in principle, as described in 8a However, when they do occur spontaneously, without external intervention, they are thermodynamically irreversible. 

Figure 21-1. Interconversion of phosphoenolpyruvate (PEP) and pymvate. The conversion of PEP to pyruvate is thermodynamically irreversible in the cell. To convert pyruvate back to PEP for gluconeogenesis, pyruvate must enter the mitochondrion, be carboxylated to oxaloacetate (OAA), and reduced to malate. After exiting the mitochondrion, malate is oxidized back to OAA and converted to PEP by the action of phosphoenolpyruvate carboxykinase.

Figure 21-2. The reactions that make up the thermodynamically irreversible steps in glycolysis and gluconeogenesis. These steps make up potential futile cycles that must be carefully regulated.

As far as I can tell by talking with contemporary thermo-dynamicists, especially those who grew up with the traditions of classical thermodynamics, these revolutionary ideas have had very little effect on them. But the impacts of the Shannon and Jaynes papers on others has been most dramatic. A few months ago I ordered a computer search of one particular data base. We looked for all papers published between 1970 and 1975 in which Shannon or Jaynes or both appeared as references. There were over 400 literature citations in such fields as systems theory, biology, neurology, meteorology, statistical mechanics, thermodynamics, irreversible processes, reliability, geology, psychiatry, communications theory and even urban studies, transportation and architecture. 

Such process is referred to as the elastic aftereffect and can be found in solid-like systems that reveal an elastic behavior. Elastic aftereffect is mechanically reversible the removal of applied stress results in gradual decrease of strain to zero due to the energy stored in the elastic element. The object thus restores its original shape. At the same time, in contrast to the case of a truly elastic body, the deformation of an object that follows Kelvin s model is thermodynamically irreversible due to the dissipation of energy in the... 

The thermal design and some aspects of the mechanical design of a shell-and-tube heat exchanger are empirically based, as discussed above. However, there are many criteria for mechanical selection [5], many experience-based criteria that can avoid or minimize operating problems [155], and other design considerations such as identification of thermodynamic irreversibilities [15, 110], thermoeconomic considerations [111], system optimization, and process integration [112], In industrial applications, thermoeconomic optimization should be... 

The driving forces, resulting in thermodynamic irreversibility, that are inherent in any distillation process are ... 

The treatment of thermodynamically reversible processes is of great importance in connection with the second law. However, in practice we are concerned with thermodynamically irreversible processes, since these are the processes that occur in nature. Therefore it is important to consider the relationships which apply to irreversible processes. 

Transport effects together with nonequilibrium effects, such as finite-rate chemical reactions and phase changes, have their roots in the molecular behavior of the fluid and are dissipative. Dissipative phenomena are associated with thermodynamic irreversibility and an increase in global entropy. 

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