Critical remarks on “On the compensation effect”

Abstract

A critical assessment of a published paper (by Agrawal) is presented. The procedure proposed and used by Agrawal to distinguish a false compensation effect from a true one is shown not to be COITeCt.In a recent paper, Agrawal [1] proposed some checkpoints for the drawing of conclusions regarding the occurrence of a true or false compensation effect (CE).In the introduction he shows that by applying the Arrhenius equationin the field of heterogeneous kinetics, both A and E have little physical significance. He sees the main cause of this in the lack of a definition for a "mole of solid". It is well known that the application of Eq. (1), valid for homogeneous gas-phase reactions, is an absolutely unjustified extrapolation [2]. Even if the validity of Eq.(1) were proved for certain heterogeneous reactions, the meanings of A and E would be rather obscure, due to the high complexity of the heterogeneous processes involving .solid-state reactions, gas evolution, diffusion, etc. How could the definition of a "mole of solid" help? If the rate constant k actually varies with Tin the way prescribed by Eq. (1), from the experimental data a magnitude can be derived having the dimension of temperature (let us call it the characteristic temperature O), denoted in Eq. (1) by E/R, since it can be transformed into energy per mole by multiplying it by R, or into energy per molecule by multiplying it by the Boltzmann constant. The definition of a mole of solid affects the amount of energy calculated from O, but it cannot clarify the physical meaning of this energy. If a linear relation between In A and Eis observed for a series of related reactions, it is referred to as a CE, formally involving an "isokinetic point", i.e. a temperature

The compensation effect

“…Therefore the correlation factor is not the deciding criteria for establishing c.e. as suggested by Zsak6 and Somasekharan [1].…”

“…from non-isothermal data, a plot of In k versus l/T(calculated from A and E) should at least exhibit a concurrence. The criticism of Zsak6 and Somasekharan [1] of my previous work involving the calculation of k from A and E is therefore unwarranted.…”

“…Their criticism was directed at four related issues: (i) questioning the use of Arrhenius equation and the definition of mole of solids; (ii) commenting on the calculation of k from A and E obtained from non-isothermal data; (iii) questioning the criterion In kiso ~ 0, and claiming that this depends on the units of k; and (iv)suggesting that a concurrence point depends on the correlation coefficient. I disagree with all the issues raised by Zsak6 and Somasekharan [1]. …”

Compensation effect in heterogeneous non-isothermal kinetics

“…Since we have no basis to presume that these formal parameters do determine a real rate constant as claimed by Eq. (2) [52,53], a plot of Ink values derived from lgA and E values cannot give more information than a lgA vs. E linear regression [54]. The single point of concurrence advocated by Agrawal cannot be the criterion of the true or false character of CE since all data are affected by experimental errors and even in the case of the truest CE the Ink vs. T -~ straight lines will intersect each other in a finite domain Alnk, z~T -~ [55].…”

Reply to remarks on “A new equation for modeling nonisothermal reactions”

“…No proofs or justifications are available in his quoted papers [6,7]. As discussed by Agrawal [8] (replying to another of Zsak6's [9] questions), many authors merely question the physical significance of the Arrhenius equation, but use it anyways. If Arrhenius equation is so objectionable and incorrect, then why isn't there a better equation available to replace it?…”