Relation between Real and Apparent Heats of Activation

a and the Relation between Real and Apparent Heats of Activation 

In any practical electrochemical experiment the absolute, but unknown, metal/solution p.d. at the reference electrode must normally vary with temperature on account of the single interface reaction entropy change. This leads to the now well-known situation that measurements of or io as a function of temperature can never give the true or real heat of activation for the electrode process. This was first pointed out by Temkin who showed that 

The relation between Temkin s energy of activation quantity, i.e., at zero metal-solution p.d. (V), and the apparent one AH (referred to by Temkin as the real heat of activation) that is both experimentally measured and practically significant, i.e., as obtained from the reciprocal temperature coefficient of In/o, d nio/d /T), is conveniently derived (cf. Refs. 25, 81) as follows. Ln io is written as 

obtaining the heat of activation in the usual way from d In io/d(l/T), it is found thatj 

The problem of the variation of reference-electrode potential can, of course, be avoided if the reference electrode is kept at constant T while the temperature of the working electrode is varied. However, then a thermal diffusion junction p.d. is introduced that depends on the temperature difference, as is well known. In deriving the result in Eq. (52) from Eq. (50), it is usually assumed that over relatively small ranges of T, itself does not vary with 7, i.e., ACp is small. This is not always the case, however, for reactions involving change of charge in hydroxylic, structured solvents (see Ref. 55). 

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