Thermodynamics - Equilibrium Potentials

There was opposed to Henry Le Chatilier a tradition that knew nothing of thermodynamics. He was reproached for encouraging the encroachment of physics and mathematics into a science which could perhaps ignore them. But in scientific matters, it is always a ticklish matter to make ignorance a weapon in your arsenal. 

The above quotation from Georges Urbain has been reprinted from a book written by the corrosion scientist who more than any other person has contributed to the thermodynamic basis for modem corrosion science and technology, namely the Belgian professor Marcel Pourbaix (1904—1998) [3.1]. 

In corrosion theory and partly in practical corrosion engineering, the concept of electrode potential caimot be avoided. In order to understand corrosion and in many cases to take the appropriate steps to prevent it, a reasonably clear understanding of the term electrode potential is necessary. In this chapter it is the intention to establish such an understanding and present the formal quantitative expressions of electrode potentials, particularly under reversible or equilibrium conditions. Further, equilibrium diagrams of various metals are introduced. These diagrams show whether corrosion and other reactions are theoretically possible or not at given values of potential and pH of the environment. 

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At each phase boundary there exists a thermodynamic equilibrium between the membrane surface and the respective adjacent solution. The resulting thermodynamic equilibrium potential can then be treated like a Donnan-potential if interfering ions are excluded from the membrane phase59 6,). This means that the ion distributions and the potential difference across each interface can be expressed in thermodynamic terms. 

The production of HjOj, either as an intermediate in the four-electron reduction of Oj to water, or as a reaction product. In the latter case the thermodynamic equilibrium potential of the reaction, i.e.,... 

The onset of H pd OHad exchange at 0.1 V on the Pt monolayer island-modified Ru(0001) surface in either scan direction is the basis for our estimate that the thermodynamic equilibrium potential for H pd OHad exchange on Ru(OOOl) is at about this value (see the preceding section). Therefore, at a potential of 0.1 V, Hupd and OHad are about equally stable. With increasing potential, they become more weakly... 

The rate-determining step, rds, for a given mechanism may change with potential. However, there is no violation of the principle of microscopic reversibility inspection of Fig. 9 shows that, at any potential, the transition state is always the same for the forward and backward reactions in eqn. (112). Transition states at different electrode potentials, on the other hand, need not be the same (see Fig. 8). Furthermore, there is no reason why the potential E, determined by kinetic factors, at which the transition of rds occurs, should be the same as the thermodynamic equilibrium potential Ee. 

Note, however, that EM is not determined by the thermodynamic equilibrium potentials Ef and E2 but by the kinetics of the respective reactions, i.e. by the respective anodic and cathodic component curves in Fig. 13(a) with the condition indicated by eqn. (190). These curves may be altered by mass transport conditions, surface area and, specific properties and consequently the mixed potential EM may be susceptible to those kinetic factors, unlike the equilibrium potential of each partial electrode reaction which is fixed by thermodynamics and the activities in the bulk solution. 

Kinetic factors may induce a variation of electrode potential with current the difference between this potential and the thermodynamic equilibrium potential is known as the overvoltage and the electrode is said to be polarized. In a plating bath this change of potential can be attributed to the reduced concentration of depositing ions in the double layer which reduces the rate of transfer to the electrode, but the dissolution rate from the metal increases. Since the balance of these rates determines the electrode potential, a negative shift in the value occurs the concentration polarization Olconc)- Anodic effects are similar but in the opposite direction. 

The cell voltage depends on the thermodynamic equilibrium potentials of the anode and cathode reactions, kinetic overvoltages, and ohmic resistances caused by the cell hardware. The cell voltage can be calculated using Eq. (16)... 

Kinetic or activation overpotential is defined as the potential in excess of the thermodynamic equilibrium ( °) potential of the half-reaction that is necessary to drive the reaction, and often plays an important role in determining overall reaction rates. A means of increasing the reaction rate is to specifically improve the kinetics of the surface reactions by the addition of catalyst materials which can reduce these overpotentials. In metal electrodes, the modification of the surface with a catalyst increases the current at a given applied potential. Similarly in photoelectrodes, addition of a catalyst can ensure efficient catalytic turnover of photogenerated minority charge carriers at the semiconductor-electrolyte interface. 

To take an example, suppose one is measuring the electrode deposition of copper, the reversible (i.e., the situation in thermodynamic equilibrium) potential for a copper ions in solution is found by experiment to be 0.337 V. 

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