Symmetry factor temperature independent

Figure 4. Schematic representation of Tafel lines for the cathodic and anodic directions of the reaction in Eq. (1), occurring in an isothermal WE-RE cell without transference, at various temperatures. It is assumed that Pc=Pa = 0-5 and that both symmetry factors are independent of temperature. EARO, Experimentally accessible region of overpotential.

Note that the transfer coefficient obtained here is not in any way related to the symmetry factor. It follows from the quasi-equilibrium assumption and should therefore be a true constant, independent of potential and temperature, as long as the assumptions leading to Eq. 43F are valid. 

Thus from Refs. 2,41, and 42, b has a temperature-independent component, K, which for the h.e.r. at Hg has the value 40 mV. Here, in Eq. (14), is the symmetry factor which must be evaluated from the derivative of b with respect to T thus... 

Of special interest for the topic of the present chapter is the observation of Weaver that while the double-layer-corrected AS quantities are ligand sensitive, they are found to be independent of potential. This is not the case for the atom and electron transfer process involved in the hydrogen evolution reaction at Hg studied by Conway, et where an appreciable potential dependence of AS is observed, corresponding to conventionally anomalous variation of the Tafel slope with temperature. Unfortunately, in the work with the ionic redox reactions, as studied by Weaver, it is only possible to evaluate the variation of the transfer coefficient or symmetry factor with temperature with a limited variety of redox pairs since Tafel slopes, corresponding to any appreciable logarithmic range of current densities, are not always easily measurable. Alternatively, evaluation of a or /3 from reaction-order determination requires detailed double-layer studies over a range of temperatures. 

Chapter 2, by B. E. Conway, deals with a curious fundamental but hitherto little-examined problem in electrode kinetics the real form of the Tafel equation with regard to the temperature dependence of the Tafel-slope parameter 6, conventionally written as fe = RT/ aF where a is a transfer coefficient. He shows, extending his 1970 paper and earlier works of others, that this form of the relation for b rarely represents the experimental behavior for a variety of reactions over any appreciable temperature range. Rather, b is of the form RT/(aH + ctsT)F or RT/a F + X, where and as are enthalpy and entropy components of the transfer coefficient (or symmetry factor for a one-step electron transfer reaction), and X is a temperature-independent parameter, the apparent limiting... 

In Fig. 7, nt-EACEF-E dependences are presented for electrode reactions with various thermodynamic properties, all characterized by temperature-independent symmetry factors, with P. Pa- y and Pc-Pa- 0-5. The case considered in Fig. la is presented in Fig. 6 in more detail. 

All the theoretical considerations regarding the effect of temperature on the rate of electrode reactions presented thus far were based on the assumption of temperature-independent symmetry factors. The treatment... 

When Fig. 13 is compared to Fig. 6, the basic difference is in the values of the slopes of the lines presented. In Fig. 13 the slopes of the lines are defined by where Ph is the temperature-independent part of the symmetry factor. Thus, p < p. It should be noted that the values of the symmetry factor obtained from the slopes of the Tafel lines (Fig. 12) are different from those obtained from the slopes of the relations... 

When the two wells are of similar energies, and the crystal structure allows, the above will no longer be the situation. We may then expect a number of consequences There may be a measurable displacement of the hydrogen between the two sites induced by such factors as a change in temperature, application of an electric field, and irradiation with light both tautomers may be present at symmetry-independent sites in the crystal different tautomers may be present in different crystal modifications and the presence of molecular substituents that do not directly affect the properties of the hydrogen bond may influence the tautomerism via the crystal structure. 

The electrical properties of PTFE are dominated by its extremely low dielectric constant (2.1) This value is invariant over a broad range ol temperatures (—40 to 250 °C) and frequencies (5 Hz to 10 GHz). Similarly, PTFE has an unusually low dissipation factor, which is also quite independent of temperature and frequency This behavior results from the high degree of dipolar symmetry of the perfluonnated and unbranched chains The dielectric strength, resistivity, and arc resistance are very high... 

Nevertheless, immiscibility and Al-Si ordering are minimized when specimens anneal at high temperatures for long periods and then are rapidly quenched. In natural and synthetic specimens that have experienced such cooling histories, the effects of cationic substitutions can be analyzed independent of other factors. For instance, a number of authors have examined transition temperatures from CUm to CT symmetry in completely disordered alkali feldspars (Nai- cK cAlSisOg), and the results (Fig. 26) indicate that the critical temperature decreases linearly with K content, since the larger K cation inhibits structural collapse (Kroll et al. 1980, 1986 Salje 1985, Harrison and Salje 1994... 

---