Kinetics nemstian processes

In most of the cases, the main electrochemical or electrocatalytic reaction is coupled with homogeneous chemical reactions that can be defined either in the bulk of the solution or in a Nemstian heterogeneous film. Whether one or the other takes place, the result is the modification of the kinetic constant of the main process. The topic is so general that we can only discuss some... 

Thus, the i-E curve, (1.5.11), has the usual nemstian shape, but E f2 is shifted in a negative direction (since K 1) from the position that would be found for process 1.5.2 unperturbed by the homogeneous equilibrium. From the shift of E1/2 with log Cy, both q [= —inl0.059) dEii2jd log C ] and K can be determined. Although these thermodynamic and stoichiometric quantities are available, no kinetic or mechanistic information can be obtained when both reactions are reversible. 

With respect to the heterogeneous electron-transfer process, reversible (nemstian) systems are always at equilibrium. The kinetics are so facile that the interface is governed solely by thermodynamic aspects. Not surprisingly, then, the shapes and positions of reversible waves, which reflect the energy dependence of the electrode reaction, can be exploited to provide thermodynamic properties, such as standard potentials, free energies of reaction, and various equilibrium constants, just as potentiometric measurements can be. On the other hand, reversible systems can offer no kinetic information, because the kinetics are, in effect, transparent. 

The equivalent circuit corresponding to an uncomplicated electrochemical reaction (i.e., a one-step CT process) is shown in Figure 15.1. An important advantage of ac voltammetry is that it allows relatively easy evaluation of the solution resistance ( J and double layer capacitance (C4). These elements can be separated from the and components, which together make faradaic impedance. Without simplifying assumptions, the analysis of faradaic impedance even for a simple ET reaction is rather complicated (9). The commonly used assumptions are that the dc and ac components of the total current can be uncoupled, and the dc response is Nemstian because of the long dc time scale. The latter assumption is reasonable because ac voltammetry is typically used to measure fast electrode kinetics. The ac response of the same electrochemical process may be quasi-reversible on the much shorter ac time scale. Quasi-reversible ac voltammograms are bell-shaped. 

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