The Kinetic Compensation Effect

Definition By definition [1, p.l30], the compensation effect refers to the behaviour pattern in which a rise in (which will decrease the rate of reaction at any particular temperature) is partially or completely offset by an increase in A . In many cases the variation of these parameters corresponds to the equation  

Analysis Equation 12.1 is in fact the Arrhenius equation, written in logarithmic form, in which constants a and b correspond to magnitudes InA and 1/ RT). That is why the satisfaction of Eq. 12.1 for a set of measurements at close values of rate constants and temperatures signifies the validity of the Arrhenius equation, i.e., it is possible to represent any point in the plane InA = f[l/ RT)] by a set of interconnected parameters. In A and E. An alternative description of the compensation effect is the availability of the so-called isokinetic temperature, Tq, corresponding to the same reaction rate for different sets of In A and E. 

The real problem is the interpretation of the physical nature of the Arrhenius parameters and their interrelation under var3dng experimental conditions. In particular, an explanation needs to be provided for the increase in both of these parameters in the presence of an excess of gaseous product in the reactor. In the framework of traditional representations this problem remains unsolved. In particular, the transition state theory characterizes the A and E parameters as independent parameters. As compared to this approach, the CDV concept has evident advantages. 

Different Decomposition Modes As follows from the above kinetic equations (Sect. 3.6), the presence of a gaseous product in the reactor should cause variations of the A and E parameters in full conformity with Eq. 12.1. This can be shown using the example of the decomposition of a binary substance into two products in the equimolar and isobaric modes, when one of the products (A) is a low-volatility one. 

Taking into account Eqs. 3.23-3.30 and the equality of rate constants (In A = In A ) it can be shown that  

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An anion effect is also reported for the solid-state anations of cis-[Co(en)2(py)X]Y2 (X- = Cl-or Br- Y- = Br" or I"). Experimental conditions affect activation parameters, and the kinetic compensation effect is observed (that is, a is linearly correlated with log A). However, the correlation is different for [Co(en)2(py)X]Br2 than for [Co(en)2(py)X]I2.71 This result suggests that the chemical process of importance is anion-dependent. It is proposed that the Co—py bond strength is affected by polarization effects due to the surrounding ionic halides. 

Kinetic parameters from selected publication on CO2 reactivity are shown in Table 1. A wide span of apparent activation energies is published for biomass chars, 80.3 kJ/mol -318 kJ/mol and for coal chars, 79 kJ/mol -359.5 kJ/mol Some of the discrepancy can be explained as due to different experimental procedures (such as sample load, particle size and sample preparation) and the application of different analysing equipment. Concerning the latter one, the potential role of systematic errors in temperature measurements among various thermobalances is evident. Variations might also be explained by different extraction procedures, lack of accuracy caused by the approximations used in the different computational methods and the kinetic compensation effect. 

Thusius, O. (1971). Inorgo Chem. 10, 1106. A kinetic study illustrating the add compensation effect. 

Moro-oka el al. (42,230) have reported kinetic data for the oxidation reactions of acetylene, ethylene, propene, propane, and isobutene on up to twelve different oxides and also palladium and platinum metals. Calculated parameters for the two compensation effects mentioned by these authors [i.e., those oxidation reactions of propene for which the oxygen pressure dependency exponent was <0.6 (42) and the oxidation of acetylene (230)] and for all the data given in both references are given in Table V, D, E, and F, respectively. Although similar calculations were completed for several other selected groups of related reactions, no additional significant instances of obedience of data to Eq. (2) were detected. 

In order to avoid the effect of kinetic compensation effect, the Popescu non-model method based... 

From Eq(2), the integral term at certain temperature T is constant, which means the plot of the values of mechanism function G(a) versus llfi has to lead a straight line with an intercept of zero. Thus, by the calculating the values Of on each curve of different heating rate, the mechanism function can be determined by the plot of G(of) versus /p. If the plot of the values of G(of) versus llfi is a straight line with an intercept of zero, then the chosen G(or) is the mechanism function of the reaction. This method determines mechanism function with few assumption, and it also does not take the forms of k(T) into consideration, which avoids the effect of kinetic compensation effect, the analysis result has higher credibility. 

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

For catalytic reactions and systems that are related through Sabatier-type relations based on kinetic relationships as expressed by Eqs. (1.5) and (1.6), one can also deduce that a so-called compensation effect exists. According to the compensation effect there is a linear relation between the change in the apparent activation energy of a reaction and the logarithm of its corresponding pre-exponent in the Arrhenius reaction rate expression. 

Such behavior is known as the compensation effecf . The important point is that if we ignore the additional term in Eq. (18), we essentially force the kinetic parameters to satisfy Eq. (19) resulting in a correlation between the prefactor and the desorption energy according to the compensation effect ... 

The parameter E0 can be calculated to a good approximation as the average value between the forward and the reverse peaks, given that 0.3 < a < 0.7. In fact, in this case, the shift of the cathodic peak towards more negative potential values and the shift of the reoxidation peak towards more positive values, both caused by the kinetic effects, essentially compensate each other. 

A. Andreasen, T. Vegge, A.S. Pedersen, Compensation effect in the hydrogenation/dehydro-genation kinetics of metal hydrides, J. Phys. Chem. B 109 (2005) 3340-3344. 

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