Spontaneous process reversal

Understand the meaning of spontaneous process, reversible process, irreversible process, and isothermal process. (Section 19.1)... 

What is meant by a reversible process Are naturally occurring spontaneous processes reversible Why is the concept of reversible process important ... 

The more negative the value of AG, the more energy or useful work can be obtained from the reaction. Reversible processes yield the maximum output. In irreversible processes, a portion of the useful work or energy is used to help carry out the reaction. The cell voltage or emf also has a sign and direction. Spontaneous processes have a positive emf the reaction, written in a reversible fashion, goes in the forward direction. 

Thermodynamic, second law The entropy of the universe increases in a spontaneous process and remains unchanged in a reversible process. It can never decrease. 

Equation (5.47) gives the criterion for reversibility or spontaneity within subsystem A of an isolated system. The inequality applies to the spontaneous process, while the equality holds for the reversible process. Only when equilibrium is present can a change in an isolated system be conceived to occur reversibly. Therefore, the criterion for reversibility is a criterion for equilibrium, and equation (5.47) applies to the spontaneous or the equilibrium process, depending upon whether the inequality or equality is used. 

Every process has a preferred direction, which is referred to in thermodynamics as the spontaneous direction. Left to itself, a process follows Its spontaneous direction. For example, the spontaneous direction for water movement Is downhill, from higher altitude to lower altitude. A spontaneous process can be reversed only by the action of some outside force. Water runs uphill only If an external agent, such as a pump, forces it to do so. 

For an isolated (adiabatic) system, AS > 0 for any natural (spontaneous) process from State a to State b, as was proved in Section 6.8. An alternative and probably simpler proof of this proposition can be obtained if we use a temperature-entropy diagram (Fig. 6.13) instead of Figure 6.8. In Figure 6.13, a reversible adiabatic process is represented as a vertical line because AS = 0 for this process. In terms of Figure 6.13, we can state our proposition as follows For an isolated system, a spontaneous process from a to b must lie to the right of the reversible one, because AS = Sb Sa> 0. 

Figure 6.17 also indicates that U = 0.78 V is the highest electrode potential, for which the ORR process is not activated. Conversely, it is the highest potential for which the water dissociation process (reverse of process (28)) is activated. For U > 0.78V water spontaneously dissociates on the Pt surface and blocks surface sites, a process which is likely to be the origin of experimental observations such as changes in Tafel slopes, and a drop in ORR activity [86]. 

Note also that if infinitesimally small currents are allowed to flow (see Fig. 7.175) through the galvanometer G in one or another direction, (i.e., when the electrode reactions occurring at both electrodes are reversed from their spontaneous direction), the polarity of the electrodes remains unchanged. Thus the sign of the electrode potential remains in this convention invariant, irrespective of whether the electrode processes proceed in the spontaneous or reverse direction, and thus are written as... 

The carboxylation of acetone with HCO to form acetoacetate (the reverse of Eq. 13-44) is not a thermodynamically spontaneous process (AG0 ... 

Now let s consider an isolated system consisting of both the system that interests us and its surroundings (again like that in Fig. 7.14). For any spontaneous change in this isolated system, we know from Eq. 12 that A> 0. If we calculate for a particular hypothetical process that AStot < 0, we can conclude that the reverse of that process is spontaneous. A reversible process is one for which AStot = 0, which implies that a reversible process is not spontaneous a reversible process has no tendency to occur in either the forward or the reverse direction. 

We have defined a spontaneous process as one that proceeds on its own without any external influence (Section 8.13). The reverse of a spontaneous process is always nonspontaneous and takes place only in the presence of some continuous external influence. Consider, for example, the expansion of a gas into a vacuum. When the stopcock in the apparatus shown in Figure 17.1 is opened, the gas in bulb A expands spontaneously into the evacuated bulb B until the gas pressure in the two bulbs is the same. The reverse process, migration of all the gas molecules into one bulb, does not occur spontaneously. To compress a gas from a larger to a smaller volume, we would have to push on the gas with a piston. 

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