Transfer activity coefficient electrostatic contribution

Transfer activity coefficient The electrostatic, or nonspecific, contribution to the transfer activity coefficient can be obtained by estimating the free-energy change involved in transferring a sphere of radius r and charge Zfi from one solvent to another of different dielectric constant. According to the Bom equation, " when 1 mole... 

EXAMPLE 4-1 Calculate the expected electrostatic contribution to the transfer activity coefficient from differences in dielectric constant for ethanol and water (dielectric constants 24.3 and 78.3) at 25°C for a 1 1 electrolyte with an average ionic radius of 1.5 x 10 cm. 

It is a serious drawback that it is not possible to determine the transfer activity coefficient of the proton (or of any other single-ion species) directly by thermodynamic methods, because only the values for both the proton and its counterion are obtained. Therefore, approximation methods are used to separate the medium effect on the proton. One is based on the simple sphere-in-continuum model of Born, calculating the electrostatic contribution of the Gibb s free energy of transfer. This approach is clearly too weak, because it does not consider solvation effects. Different ex-trathermodynamic approximation methods, unfortunately, lead not only to different values of the medium effect but also to different signs in some cases. Some examples are given in the following log yH+ for methanol -1-1.7 (standard deviation 0.4) ethanol -1-2.5 (1.8), n-butanol -t-2.3 (2.0), dimethyl sulfoxide -3.6 (2.0), acetonitrile -1-4.3 (1.5), formic acid -1-7.9 (1.7), NH3 -16. From these data, it can be seen that methanol has about the same basicity as water the other alcohols are less basic, as is acetonitrile. Di-... 

The quantity a is the anodic transfer coefficient-, the factor l/F was introduced, because Fcf> is the electrostatic contribution to the molar Gibbs energy, and the sign was chosen such that a is positive - obviously an increase in the electrode potential makes the anodic reaction go faster, and decreases the corresponding energy of activation. Note that a is dimensionless. For the cathodic reaction ... 

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