Electrolyte chemical reaction effects

The polyelectrolyte catalysis of chemical reactions involving ionic species has been the subject of extensive investigations since the pioneering studies of Morawetz et al. [12] and Ise et al. [13-17]. The catalytic effect or the ability of poly-electrolytes to enhance or retard reaction rates is mainly due to concentration or exclusion of either or both of the ionic reactants by the polyions added to the reaction systems. For example, the chemical reaction between ionic species carrying the same charge is enhanced in the presence of polyions carrying the opposite charge. This enhancement can be attributed to an increase in the local concentration... 

Figure 3,55 Cyclic vollammograms of Re(Bipy)(CO)3CI in CH3CN/0.l M letrabutylammonium hexafluorophosphate as supporting electrolyte at a button Pt electrode, and with a sweep rate of 200 mV s (a) The switching potential characteristics of the coupled chemical reactions in the ahsence of C02. The lettered redox processes are discussed in the text. (b> The effect of saturating the solution with C02. From Sullivan et al. (1985).

Four different regimes of the I-V curve for moderately doped silicon electrodes in an HF electrolyte are shown in Fig. 3.2. These regimes will now be discussed in terms of the charge state of the electrode, the dependence on illumination conditions, the charge transfer, the mass transport, and accompanying chemical reactions. Transient effects are indicated in Fig. 3.2 by a symbol with an arrow. 

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

The electrochemical rate constants of the Zn(II)/Zn(Hg) system obtained in propylene carbonate (PC), acetonitrile (AN), and HMPA with different concentrations of tetraethylammonium perchlorate (TEAP) decreased with increasing concentration of the electrolyte and were always lower in AN than in PC solution [72]. The mechanism of Zn(II) electroreduction was proposed in PC and AN the electroreduction process proceeds in one step. In HMPA, the Zn(II) electroreduction on the mercury electrode is very slow and proceeds according to the mechanism in which a chemical reaction was followed by charge transfer in two steps (CEE). The linear dependence of logarithm of heterogeneous standard rate constant on solvent DN was observed only for values corrected for the double-layer effect. 

Solvation is a process in which solute particles (molecules or ions) in a solution interact with the solvent molecules surrounding them. Solvation in an aqueous solution is called hydration. The solvation energy is defined as the standard chemical potential of a solute in the solution referred to that in the gaseous state.11 The solvation of a solute has a significant influence on its dissolution and on the chemical reactions in which it participates. Conversely, the solvent effect on dissolution or on a chemical reaction can be predicted quantitatively from knowledge of the solvation energies of the relevant solutes. In this chapter, we mainly deal with the energetic aspects of ion solvation and its effects on the behavior of ions and electrolytes in solutions. 

As described above, the role of ion solvation is crucial in the dissolution of electrolytes. Ion solvation also has significant effects on chemical reactions and equilibria. Ion-solvent interactions that may participate in ion solvation are shown in Table 2.3 [8],... 

So far, the discussion of concentrated electrolyte solutions has presumed that ionic relaxation is complete and so is a static correction. Dynamic electrolyte theories are still in their infancy and, in view of the rate of ionic relaxation compared with chemical reaction rates for dilute electrolytes (Sect. 1.6), such effects are probably not very important in concentrated electrolyte solutions containing reactants. The Debye— Falkenhagen [92] theory predicts a change in the relaxation time of electrolyte solutions with concentration, though experimental confirmation is scant [105]. At very high concentrations, small changes in the relaxation time ( 25%) of solvent relaxation can be identified (see also Lestrade et al. [106]). 

As for any chemical reaction, an electrode reaction is driven by a gradient in tree energy. Since charged particles such as electrons and/or ions are involved in electrode reactions at the electrode—solution interface, the electrode—electrolyte potential difference has a linear effect on the position of the free energy surfaces as depicted in Fig. 4. The effect of the high electrical field (i.e. 107 V cm-1) at the interface on uncharged species will be neglected for simplicity. 

We turn our attention in this chapter to systems in which chemical reactions occur. We are concerned not only with the equilibrium conditions for the reactions themselves, but also the effect of such reactions on phase equilibria and, conversely, the possible determination of chemical equilibria from known thermodynamic properties of solutions. Various expressions for the equilibrium constants are first developed from the basic condition of equilibrium. We then discuss successively the experimental determination of the values of the equilibrium constants, the dependence of the equilibrium constants on the temperature and on the pressure, and the standard changes of the Gibbs energy of formation. Equilibria involving the ionization of weak electrolytes and the determination of equilibrium constants for association and complex formation in solutions are also discussed. 

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