Reverse of a process

We saw in Section 6.11 that the first law of thermodynamics implies that, because enthalpy is a state function, the enthalpy change for the reverse of a process is the negative of the enthalpy change of the forward process. The same relation applies to forward and reverse chemical reactions. For the reverse of reaction A, for instance, we can write... 

Although from the thermodynamic point of view one can speak only about the reversibility of a process (cf. Section 3.1.4), in electrochemistry the term reversible electrode has come to stay. By this term we understand an electrode at which the equilibrium of a given reversible process is established with a rate satisfying the requirements of a given application. If equilibrium is established slowly between the metal and the solution, or is not established at all in the given time period, the electrode will in practice not attain a defined potential and cannot be used to measure individual thermodynamic quantities such as the reaction affinity, ion activity in solution, etc. A special case that is encountered most often is that of electrodes exhibiting a mixed potential, where the measured potential depends on the kinetics of several electrode reactions (see Section 5.8.4). 

It is well known that the transfer of nonpolar molecules from nonpolar to polar surroundings results in a decrease in the partial molar volume of the solute. The dimerization studies also show that there is a similar volume decrease when two monomers form a dimer. This volume decrease is of the order of 20 cm3 mol-1. It is difficult to understand how there can be first a volume decrease when the nonpolar molecules are transferred from the nonpolar to the polar environment and then a further volume decrease when two molecules come together and partly reverse the first transfer. It is a little dangerous to speak of the partial reversal of a process we know so little about. It is believed that the hydrophobic hydration is a cooperative phenomenon, in which the exact microstructure of water is very important for the occupied volume. How this microstructure changes when two molecules associate in a hydrophobic interaction is not par-... 

The first factor determines the tendency for dissolution to occur while the second and third, which are closely related, determine the rate of dissolution. The use of the standard electrode potentials as a measure of nobility is well known. The recognition that the exchange current density is a measure of the reversibility of a process and therefore a quantity characteristic of the reactivity of the system is more recent (13,32). As indicated by the Tafel relations, the exchange current density is a direct measure of the rate of the electrode reaction for any given value of the activation overvoltage (33). The values of iG may then be taken as a criterion for the electrochemical activity of a system. 

Reversible process AG = AHjys — = 0 1 Irreversible process AG = AHjys — <0 f [19.17] Relating the free-energy change to the reversibility of a process at constant temperature and pressure... 

There is a close relationship between the reversibility of a process and whether it is spontaneous or at equilibrium. Recall Figure 19.3, in which we showed the spontaneous melting of ice at T > 0°C and tiie spontaneous freezing of hquid water at T < 0°C. Bofli of these processes are irreversible. At T = 0°C ice and water are in equilibrium, and they can convert back and forth reversibly. These observations are examples of two very important concepts regarding reversible and irreversible processes ... 

We saw in Chapter 1 that the criterion of thermodynamic reversibility is the reversal of a process by an infinitesimal change in the external conditions. 

Cyclic voltammograms were obtained at sweep rates of 1-10 V s The criteria of (i) separation of less than 80 mV between cathodic and anodic peaks, (ii) close to unity ratio of the intensities of the cathodic and anodic currents, and (iii) constancy of the peak potential on changing sweep rate in the cyclic voltammograms were used to establish the reversibility of a process. For reversible processes, the halfwave potential value was calculated from the average of the potential values for anodic and cathodic peaks. 

Biological reactions in vivo rarely operate under conditions even remotely approaching those of reversibility. For a living organism, the rate of a process is usually more important than the attainment of equilibrium, and large driving... 

This cleavage is a retro aldol reaction It is the reverse of the process by which d fruc tose 1 6 diphosphate would be formed by aldol addition of the enolate of dihydroxy acetone phosphate to d glyceraldehyde 3 phosphate The enzyme aldolase catalyzes both the aldol addition of the two components and m glycolysis the retro aldol cleavage of D fructose 1 6 diphosphate... 

A completely reversible processes is hypothetical, devised solely to find the ideal work associated with a given change of state. Its onlv con-neclion with an actual process is that it brings about the same change of state as the actual process, allowing comparison of the actual work of a process with the work of the hypothetical reversible process. 

When a process is completely reversible, the equahty holds, and the lost work is zero. For irreversible processes the inequality holds, and the lost work, that is, the energy that becomes unavailable for work, is positive. The engineering significance of this result is clear The greater the irreversibility of a process, the greater the rate of entropy production and the greater the amount of energy that becomes unavailable for work. Thus, every irreversibility carries with it a price. 

The constitution of laudanidine was determined by Spath and Bernhauer, who showed that with diazomethane it yields Maudanosine, m.p. 87-8°, and, the reversal of this process, viz, partial demethylation of synthetic Maudanosine by Spath and Burger provided a complete synthesis of Maudanine, which is laudanidine. 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

Repression of genes is associated with reversal of this process under the control of histone deacetylases (HDACs). Deacetylation of histones increases the winding of DNA round histone residues, resulting in a dense chromatin structure and reduced access of transcription factors to their binding sites, thereby leading to repressed transcription of inflammatory genes. 

It is a necessary consequence of the reversibility of osmotic processes that the osmotic pressure is independent of the nature of the septum used to measure it. For, suppose there are two semiperineable septa [a] and [/3], and let the osmotic pressures of a solution when separated from pure solvent under a given pressure by these septa be Pa and Pp. Then if we separate a volume 8V of solvent through [a], the work Pa V is spent on the system, and if the solvent is readmitted through [3] the work PpSV is done by the system. The isothermal cycle being now completed, we have ... 

We shall now calculate the diminution of free energy which results from the admixture of Ni mols of [1] and N2 mols of [2], both in the liquid state. The simplest method is an application of equation (13) of 52, which states that the work done in the isothermal and reversible execution of a process is equal to the diminution of free energy ... 

Now consider an isolated system consisting of both the system that interests us and its surroundings (again like that in Fig. 7.15). For any spontaneous change in this isolated system, we know from Eq. lib that ASun > 0. If we calculate for a particular hypothetical process that A5tot < 0, we can conclude that the reverse of that process is spontaneous. 

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