Reversibility cells and

Reactions in Reversible Cells.—It is of importance in many respects to know what is the reaction occurring in a reversible cell, and some different types of cells will be considered for the purpose of illustrating the procedure adopted in determining the cell reaction. The Daniell cell, for example, is... 

Such a cell can operate rigorously as a reversible cell, and is termed a cell without a liquid junction. Such cells are ideal since, by operating reversibly, they furnish exact thermodynamic quantities. 

At constant temperature and pressure, the relations between the e.m.f. E, of a reversible cell, and the free energy, entropy and enthalpy are ... 

In spite of the somewhat hesitating conclusions, our aim was not perhaps to deny the possibility of exergy-based analysis at all. For instance in the case of separation processes, some information can be drawn even from a simple model, as shown above. Another problem is, how the information is further used. One should always bear in mind that the underlying ideas are rather simplistic, going back to the traditional, past-century s concepts of Carnot cycles, semipermeable membranes (in case of diffusion processes), reversible cells, and the like. Their role in the development of thermodynamics is incontestable. But in their rudimentary form, they hardly can stand as a base of contemporary engineer s reasoning, however sophisticated be the methods of computation. 

Studies grouped under this general heading are concerned with the kinetics and mechanisms of electrode reactions. They are to be distinguished from thermodynamic studies, based upon reversible cells and electrode potentials in which, by definition, systems at equilibrium are investigated. 

Owing to the size of the subject, it has proved necessary in the following chapters, to be selective in the choice of material presented. Earlier chapters are concerned with ionics and its applications. Here are considered ion interactions in solution, acid-base equilibria, transport phenomena, and the concept of reversible electrode potential. This last named leads to the development of reversible cells and their exploitation. Here one is dealing with electrochemical thermodynamics - with the rapid attainment of equilibrium between species at an electrode surface and charged species in solution. 

Cell Volta.ge a.ndIts Components. The minimum voltage required for electrolysis to begin for a given set of cell conditions, such as an operational temperature of 95°C, is the sum of the cathodic and anodic reversible potentials and is known as the thermodynamic decomposition voltage, is related to the standard free energy change, AG°C, for the overall chemical reaction,... 

Fig. 1.22 Spontaneous corrosion of zinc in acid illustrated by the reversible cell ZnlZn IH30", H2 Pt. The individual potentials of the electrodes are determined by a reference electrode (ReQ and a Luggin capillary to minimise the IR drop in the solution...

Equations 20.176 and 20.179 emphasise the essentially thermodynamic nature of the standard equilibrium e.m.f. of a cell or the standard equilibrium potential of a half-reaction E, which may be evaluated directly from e.m.f. meeisurements of a reversible cell or indirectly from AG , which in turn must be evaluated from the enthalpy of the reaction and the entropies of the species involved (see equation 20.147). Thus for the equilibrium Cu -)-2e Cu, the standard electrode potential u2+/cu> hence can be determined by an e.m.f. method by harnessing the reaction... 

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