Electrochemical potential of the

When two dissimilar metals are connected, as illustrated in Fig. V-16, ]here is a momentary flow of electrons from the metal with the smaller work function to the other so that the electrochemical potential of the electrons becomes the same. For the two metals a and /3... 

If two metals with different work functions are placed m contact there will be a flow of electrons from the metal with the lower work function to that with the higher work fimction. This will continue until the electrochemical potentials of the electrons in the two phases are equal. This change gives rise to a measurable potential difference between the two metals, temied the contact potential or Volta potential difference. Clearly... 

Figure Bl.28.9. Energetic sitiration for an n-type semiconductor (a) before and (b) after contact with an electrolyte solution. The electrochemical potentials of the two systems reach equilibrium by electron exchange at the interface. Transfer of electrons from the semiconductor to the electrolyte leads to a positive space charge layer, W. is the potential drop in the space-charge layer.

Practical developers must possess good image discrimination that is, rapid reaction with exposed silver haUde, but slow reaction with unexposed grains. This is possible because the silver of the latent image provides a conducting site where the developer can easily give up its electrons, but requires that the electrochemical potential of the developer be properly poised. For most systems, this means a developer overpotential of between —40 to +50 mV vs the normal hydrogen electrode. 

In aqueous solutions it becomes somewhat more feasible to modify the entry of hydrogen into the steel. This can be achieved by the addition of inhibitors to the solution, by control of the electrochemical potential of the metal and by coatings. 

The applied voltage determines the difference in the electrochemical potential of the electrons or the ratio of the concentrations of the electronic species on both sides of the electrolyte ... 

Equation 6.34 expresses the fact that the electrochemical potential of an electron donor (Xj>0) is lowered with anodic (UWr>0) potential or with increasing work function (A<1 0). This favours adsorption. Similarly for an electron acceptor adsorbate (A.j<0) anodic potential (UWR>0) or increasing work function (Aelectrochemical potential of the adsorbate. This hinders adsorption. 

The Gibbs energy of an electroneutral system is independent of the electrostatic potential. In fact, when substituting into Eq. (3.7) the electrochemical potentials of the ions contained in the system and allowing for the electroneutrality condition, we can readily see that the sum of aU terms jZjF f is zero. The same is true for any electroneutral subsystem consisting of the two sorts of ion and (particularly when these are produced by dissociation of a molecule of the original compound k into x+ cations and x anions), for which... 

Electrolyte solutions ordinarily do not contain free electrons. The concept of electrochemical potential of the electrons in solution, ft , can stiU be used for those among the bound electrons that will participate in redox reactions in the solution. Consider the equilibrium Ox + ne Red in the solution. In equilibrium, the total change in Gibbs energy in the reaction is zero hence the condition for equilibrium can be formulated as... 

When the electrochemical potential of the electrons in the metal is counted from the point of reference just outside the metal, then at the electrode potential = 0 V it should also be equal to the value of A. At other potentials it wiU be determined by an equation analogous to Eq. (29.5) ... 

Thus, an unambiguous correlation exists between the values of electrode potential (electrochemical scale) and the Fermi levels or values of electrochemical potential of the electrons defined as indicated (physical scale) see the symbols at the vertical axes in Fig. 29.2. 

It follows from the Franck-Condon principle that in electrochemical redox reactions at metal electrodes, practically only the electrons residing at the highest occupied level of the metal s valence band are involved (i.e., the electrons at the Fermi level). At semiconductor electrodes, the electrons from the bottom of the condnc-tion band or holes from the top of the valence band are involved in the reactions. Under equilibrium conditions, the electrochemical potential of these carriers is eqnal to the electrochemical potential of the electrons in the solution. Hence, mntnal exchange of electrons (an exchange cnrrent) is realized between levels having the same energies. 

At electrode potentials more negative than approximately - 2.8 V (SHE), free solvated electrons appear in the solution as a result of (dark) emission from the metal. At this potential the electrochemical potential of the electrons according to Eq. (29.6) is about —1.6 eV, which is at once the energy of electron hydration in electron transfer from vacuum into an aqueous phase. 

Figure 29.4 shows an example, the energy diagram of a cell where n-type cadmium sulfide CdS is used as a photoanode, a metal that is corrosion resistant and catalytically active is used as the (dark) cathode, and an alkaline solution with S and S2 ions between which the redox equilibrium S + 2e 2S exists is used as the electrolyte. In this system, equilibrium is practically established, not only at the metal-solution interface but also at the semiconductor-solution interface. Hence, in the dark, the electrochemical potentials of the electrons in all three phases are identical. 

Figure 16.7 Schematic drawing of the asymmetric electrode pattern. The gold electrode was covered with a Fc-alkanethiol monolayer. The wetting of the gold electrode was switched from wetting to repulsive and vice versa by changing the electrochemical potential of the electrode.

Since the externally applied potential difference influences the nuclei and electrons of the electrodes in the same way, not only the vacuum levels but the entire electrostatic potentials of both electrodes, and therefore the Fermi levels, are shifted, too. Consequently, the electrochemical potentials of the electrons become... 

Since the capacitor without electrolyte shows a linear behavior of the electrostatic potential between the electrodes, (x), at the hypothetical moment when the electrolyte is added to the system (t = 0 s), the electrochemical potentials of the ions. 

In solid-state physics, the electrochemical potential of the electron pe(a) is mostly replaced by the equivalent energy of the Fermi level eF. While the electrochemical potential is usually related to one mole of particles, the Fermi energy is related to a single electron, so that... 

At the contact of two electronic conductors (metals or semiconductors— see Fig. 3.3), equilibrium is attained when the Fermi levels (and thus the electrochemical potentials of the electrons) are identical in both phases. The chemical potentials of electrons in metals and semiconductors are constant, as the number of electrons is practically constant (the charge of the phase is the result of a negligible excess of electrons or holes, which is incomparably smaller than the total number of electrons present in the phase). The values of chemical potentials of electrons in various substances are of course different and thus the Galvani potential differences between various metals and semiconductors in contact are non-zero, which follows from Eq. (3.1.6). According to Eq. (3.1.2) the electrochemical potential of an electron in... 

Oxidation-reduction electrodes. An inert metal (usually Pt, Au, or Hg) is immersed in a solution of two soluble oxidation forms of a substance. Equilibrium is established through electrons, whose concentration in solution is only hypothetical and whose electrochemical potential in solution is expressed in terms of the appropriate combination of the electrochemical potentials of the reduced and oxidized forms, which then correspond to a given energy level of the electrons in solution (cf. page 151). This type of electrode differs from electrodes of the first kind only in that both oxidation states can be present in variable concentrations, while, in electrodes of the first kind, one of the oxidation states is the electrode material (cf. Eqs 3.1.19 and 3.1.21). 

When two objects made of different metals or alloys are in contact with each other in the presence of an electrolyte (a medium that provides a transport mechanism), an electric current flows between them. The direction of the current s flow is determined by the electrochemical potential of the metals in contact the baser metal, having the higher electrochemical potential value, will be preferentially corroded, whereas the one having the lower electrochemical potential is more passive and will remain... 

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