Ferrous-ferric outer-sphere

After more than two decades of investigation, it is pertinent to ask to what extent the predictions of this type of theory, and its agreement with experiment, have changed. The system which has received most study is the aquo ferrous-ferric outer-sphere symmetrical exchange ... 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Micellar Efllecls on Inorganic Reactions.—Electron transfer between ferric ion and phenothiazine is inhibited by cationic micelles and accelerated by up to 10 -fold in anionic micelles of sodium lauryl sulphate. " In both cases the rate-surfactant concentration profile can be simulated accurately. Anionic micelles only cause a small effect on the reactivity of ruthenium(iii) tris(bipyridyl) with molybdenum(iv) octacyanide but accelerate the reaction between ferrous ion and tris(tetramethylphenanthroline)iron(ra). In the latter case a plot of surfactant concentration is linear with the reciprocal of the observed rate constant. Fast outer-sphere electron-transfer reactions may decrease in rate constant by up to four orders of magnitude when one of the reactants is solubilized in an anionic micelle. When this partner is neutral the inhibition is reduced somewhat by added salt, but when it is cationic the effect may be attenuated by competitive binding of Na or HsO and exclusion of reactant from the micelle. Oxidation of diethyl sulphide is catalysed by micelles of sodium lauryl sulphate containing carboxylate-ions by the mechanism shown. (Scheme 3). The rate advantage is quantitatively accounted for by the entropy term, and hexanoate is forty-fold more effective than acetate. Electron-transfer between the anionic trans-1,2-diaminocyclohexane... 

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