Electrochemical equilibria

Figure 3.1 Schematic representation of a non-sophisticated cell for equilibrium electrochemical measurements. The example shown is a Daniell cell comprising Cu +,Cu and Zn, Zn half cells. The need for the glass sleeves is discussed in Chapter 9.

In the simplest case, we can say that no current passes through the cell during an equilibrium electrochemical experiment, and the activities of the electroactive species in solution will obey the Nemst equation. 

Ozkaya (76) studied conceptual difficulties experienced by prospective teachers in a number of electrochemical concepts, namely half-cell potential, cell potential, and chemical and electrochemical equilibrium in galvanic cells. The study identified common misconceptions among student teachers from different countries and different levels of electrochemistry. Misconceptions were also identified in relation to chemical equilibrium, electrochemical equilibrium, and the instrumental requirements for die measurement of cell potentials. Learning difficulties were attributed mainly to failure of students to acquire adequate conceptual understanding, and the insufficient explanation of the relevant... 

If all interfaces remained at equilibrium, electrochemical devices would be limited in their possibilities. Substances could not be produced electrochemically neither would power production in fuel cells be possible. Net currents must flow across interfaces for devices to work. There must be net electronation or net dcclectronation. Interfaces need to move away from equilibrium and the corresponding Gal van i potential difference,... 

This paper provides an example of how accurate continuum models can open the door to the modeling of condensed-phase processes where solvation free energies have a very large influence on reaction energetics. It additionally offers a case study of how to first choose a model on the basis of experimental/tlieoretical comparisons over a relevant data set, and then apply tliat model with a greater expectation for its utility. The generality of this approach to other (equilibrium) electrochemical reactions seems promising. 

Fick s law of diffusion, J = —D9C/9x, applies when there is a source and a sink, with a vector current between then proportional to the concentration gradient. An equilibrium electrochemical reaction on open circuit is not a sink for reactants nor a source of product. It is a zero-flow blockage, albeit with balanced exchange currents. The flow of ions in the cell is a balance the drive V is balanced by an opposing concentration difference. 

Methane fuel has alternative and more complex equilibrium electrochemical oxidation routes and two of these are examined because of their resemblance to practical routes. The first is via a reformer and the second is via direct oxidation, now achievable in the laboratory. Both analyses involve approximate equilibrium constants, but the second direct oxidation calculation route is seen to be more in error, and the numerical answer less accurate, than that of the reformer route. 

Because the atmosphere at both the source and sink sides was the same, the chemical potential of sodium was equal and fixed by the Na COj phase in equilibrium with the same atmosphere at all three Pt electrodes, when no electric field was present. At this point, the equilibrium electrochemical reaction... 

The equilibrium electrochemical potential, or theoretical voltage requirement for the electrolyzer reaction (Eq. 8.42) is 3.13 V, significantly higher than the 2.20 V required by the diaphragm cell. However, better internal cell conductivities keep the operating potential to... 

At equilibrium, electrochemical potentials of ions are constant over the diffuse layer and these relations hold within the DL ... 

To this point, we have examined only systems at equilibrium, and we have learned that the potential differences in equilibrium electrochemical systems can be treated exactly by thermodynamics. However, many real cells are never at equilibrium, because they feature different electrolytes around the two electrodes. There is somewhere an interface between the two solutions, and at that point, mass transport processes work to mix the solutes. Unless the solutions are the same initially, the liquid junction will not be at equilibrium, because net flows of mass occur continuously across it. 

The key to inducing self-organization onto water-solid substrate interfaces is to achieve mild adsorption under controlled conditions. If adsorbate-substrate interactions are too strong, molecules cannot move around on the substrate surface. On the other hand, when adsorbate-substrate interactions are too weak, molecules desorb from surfaces. Relatively mild adsorption conditions between these extreme states leads to induction of 2D self-organization of molecules via rapid surface diffusion and acceleration of the adsorption/desorption equilibrium. Electrochemical potential management would be convenient for AISO, because it allows for precise control of adsorption strength in units of mV [11,13, 14]. 

Formation of ions during equilibrium electrochemical reaction depends on the pH of the solution and electrode potential. The relationship between electrode potential and pH of the solution can be represented by a phase diagram that is known as the Pourbaix diagram [5]. If a metal is made anodic in an aqueous solution, several reactions can occur depending on the change in free energy. For example, if zinc is made anode in water, the following possible reactions may take place ... 

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