Activation energies and reaction enthalpy

TABLE 4. Room temperature rate constants and preexponential A factors (in cm3 molecule 1 s 1), as well as activation energies and reaction enthalpies (in kJ mol 1) for reactions of chlorine atoms with methane and halomethanes. 

FIGURE 12-3 Activation Energies and Reaction Enthalpies, (a), (b), Large E, slow reaction, (c) Small Ea, fast reaction, (a), (b), hH < 0, large equilibrium constant ... 

Topic 4.3.5 Relationships of kinetics and thermodynamics of reversible reactions Correlation between Activation Energy and Reaction Enthalpy... 

Several empirical correlations are known for rate constants and activation energies of bimo-lecular radical reactions [1 4], Evans and Polyany [5] were the first to derive the linear correlation between the activation energy and the enthalpy of reaction of R,X with Na. Later Semenov [1] generalized this empirical equation for different free radical reactions in the following form ... 

Nonlinear hyperbolic dependence between the activation energy and the enthalpy of reaction was proposed by Rudakov [8] ... 

Fig. 3-9. Observed activation energies, reaction activation energies, and adsorption enthalpies in the cracking of n-alkanes over ZSM-5 (adapted from [8]).

An excellent study has been made of the temperature dependence of the steady-state kinetic parameters of the fumarase reaction [6]. By studying the temperature dependence over a wide range of pH, the apparent activation energies and standard-enthalpy changes associated with the pH-independent steady-state parameters, the lower bounds of the rate constants, and the ionization constants of the groups at the active site were obtained. The results are summarized in Table 9-1. In this case the temperature dependence of all parameters appears quite normal. The standard-enthalpy changes of... 

Apparent Activation Energies and Standard Enthalpy Changes for the Fumarase Reaction"... 

Table 4 Activation (AH ) and reaction enthalpies (AH) as well as Gibbs free energies (AG and AG) of the (2+2) reaction pathways in comparison to the (3+2) addition (in kcal/mol at 298.15 K)...

The foregoing discussion deals with the stmcture effect on A (or entropy change). The stmcture effect on the activation energy (or reaction enthalpy change) is described by the Evans-Polanyi relation, with just two parameters Eo and a) for each single event type, which generally are obtained from model-compound experiments. 

With accurate estimates of kp now available, the possibility of correlating the coefficient with the structural characteristics of the propagating radicals and monomers is receiving attention. While some progress has been made in relating activation energy to reaction enthalpy corrected for polar and resonance factors [17], more work is required for quantitative predictions. 

The parabolic model postulates a non-linear relationship between the activation energy and the enthalpy of a reaction (see Eq. 7.18). The contribution of enthalpy to file activation energy A h can be estimated as the difference A e =... 

Regarding the influence of temperature, a clear distinction is made between the elementary steps, for which Arrhenius law is always trae and leads to an activation energy, and the common reactions (in general non-elementary) for which Arrhenius law s experimental application leads to a temperature coefficient . This coefficient can sometimes, but by no means always, be Unked to magnitudes related to the mechanism s steps, such as activation energies and/or enthalpies. This temperature coefficient is then called the apparent activation energy. 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

GP 11] [R 5] Numerically iterated parameter values for the Langmuir-Hinshelwood kinetics were listed, including activation energy, oxygen reaction rate, and enthalpy (2.0-7.0 mmol 1 hydrogen 3.6 mmol 1 oxygen 48-70 °C) [121]. 

Enthalpies, Activation Energies, and Rate Constants of Reaction RH + N 02 —> R + HONO Calculated by the IPM Method [168]... 

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