Equilibrium potential hydrogen reaction

The areas bounded by solid lines correspond to regions of thermodynamic stability of certain substances that are named in the diagram. This stability is relative. The dashed line a in the diagram corresponds to the equilibrium potential of the hydrogen electrode. Metallic zinc, for which the reaction lines are below the line for the hydrogen electrode, can be oxidized while hydrogen is evolved (see Section 2.4.1). 

The equilibrium potential of reaction (15.21) is relatively readily established at mercury and graphite electrodes in alkaline solutions containing hydrogen peroxide. 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

Equation (3.3) gives the potential dependence of the reaction free energy of Reaction (3.2). Since this reaction equilibrium defines the standard hydrogen electrode potential, we now have a direct fink between quite simple DFT calculations and the electrode potential. In a similar way, we can now calculate potential-dependent reaction free energies for other reactions, such as O - - H" " + e OH or OH - -+ e HzO. 

Two things to notice are that fc, will depend on the reference electrode chosen and secondly E° is the equilibrium potential not for the standard hydrogen electrode but rather for the electrochemical equilibrium between H+ and H ds. This can be seen by explicitly writing out the equation corresponding to the reverse reaction ... 

The simultaneous transfer of four electrons is unlikely, and the overall reaction must contain several steps. An important intermediate is hydrogen peroxide H2O2, and its occurrence makes it difficult to establish even the equilibrium potential experimentally. The reaction is further complicated by the fact that in aqueous solutions almost all metals are covered by an oxide film in the potential range over which the reduction occurs. 

The electrode potential of an electrode reaction at equilibrium can be measured as the electromotive force of an electrochemical cell composed of both the reaction electrode and the normed hydrogen electrode. The potential of the reaction electrode thus measured is taken as the equilibrium potential of the electrode reaction relative to the normal hydrogen electrode. 

TABLE 6-L Die standard equilibrium potentials for redox electrode reactions of h rdrated redox particles at 25 C nhe = relative electrode potential referred to the normal hydrogen electrode. [Handbooks of electrochemistry.]... 

In electrochemistry, the chemical potential of hydrated ions has been determined from the equilibrium potential of ion transfer reactions referred to the normal hydrogen electrode. For the reaction of metal ion transfer (metal dissolution-deposition reaction) of Eqns. 6-16 and 6-17, the standard equilibriiun potential Sive in terms of the standard chemical potential, li, by Eqn. 

Figure 10-17 shows the polarization ciirves for the cathodic hydrogen reaction (cathodic electron transfer) on a p-type semiconductor electrode of galliiun phosphide. The onset potential of cathodic photoexcited hydrogen reaction shifts significantly from the equilibrium electrode potential of the same hydrogen reaction toward the flat band potential of the p-type electrode (See Fig. 10-15.). 

In order for this mixed electrode reaction shown in Fig. 11-2 to proceed, the Fermi level efod of the iron electrode must be higgler than the Fermi level erh-zhj) of the hydrogen redox reaction and also must be lower than the Fermi level for the transfer reaction of iron ions. In other words, the potential E of the iron electrode must be lower than the equilibrium potential of the... 

E is the standard equilibrium potential, i. e. the potential corresponding to unit activity and RTF. The dissolution reaction leads to the development of an electrical double layer at the iron-solution interface. The potential difference of the Fe/Fe " half cell cannot be measured directly, but if the iron electrode is coupled with a reference electrode (usually the standard hydrogen electrode, SHE), a relative potential difference, E, can be measured. This potential is termed the single potential of the Fe/Fe electrode on the scale of the standard hydrogen couple H2/H, the standard potential of which is taken as zero. The value of the equilibrium potential of an electrochemical cell depends upon the concentrations of the species involved. 

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