The pre-equilibrium treatment. Unimolecular reactivities

The foregoing discussion emphasizes the desirability of treating one-electron electrochemical reactions as involving an electron-transfer step occurring within a precursor state previously formed in the interfacial region. It is therefore useful to separate the overall observed rate constant, kob, into a precursor equilibrium constant, Kp (cm) [eqn. (4b)], and a unimolecular rate constant, ket (s-1), for the elementary electron-transfer step, related by [7] 

This relation will be valid for both inner- and outer-sphere pathways provided that electron transfer, rather than precursor-state formation, is the rate-determining step. The precursor equilibrium constant can be expressed as 

For processes involving strongly adsorbing reactants, Tp is often sufficiently large to be measured by surface electroanalytical methods (e.g. chronocoulometry, double-layer capacitance), enabling Kt to be evaluated. This allows ket to be determined from kob using eqn. (10). Alternatively, kel can often be evaluated directly in such circumstances by using electrochemi- 

The measurement of ket for single electron-transfer reactions is of particular fundamental interest since it provides direct information on the energetics of the elementary electron-transfer step (Sect. 3.1). As for solution reactants, standard rate constants, k t, can be defined as those measured at the standard potential, E, for the adsorbed redox couple. The free energy of activation, AG, at E°a is equal to the intrinsic barrier, AG t, since no correction for work terms is required [contrast eqn. (7) for solution reactants] [3]. Similarly, activation parameters for surface-attached reactants are related directly to the enthalpic and entropic barriers for the elementary electron-transfer step [3], 

In order to understand the manner in which the interfacial region influences the observed kinetics, especially in terms of the theoretical models discussed below, it is clearly important to gain detailed information on the spatial location of the reaction site as well as a knowledge of the mechanistic pathway. Information on the latter for multistep processes can often be obtained by the use of electrochemical perturbation techniques in order to detect reaction intermediates, especially adsorbed species [13]. Various in-situ spectroscopic techniques, especially those that can detect interfacial species such as infrared and Raman spectroscopies, are beginning to be used for this purpose and will undoubtedly contribute greatly to the elucidation of electrochemical reaction mechanisms in the future. 

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