Enthalpy, activation reaction

The mechanism of the reaction A - B consists of two steps, with the formation of a reaction intermediate. Overall, the reaction is exothermic, (a) Sketch the reaction profile, labeling the activation energies for each step and the overall enthalpy of reaction, (h) Indicate on the same diagram the effect of a catalyst on the first step of the reaction. 

In the literature tetrathiosulfuranes have been discussed as possible intermediates in the thermal decomposition of sulfanes and other polysulfur compounds. High-level ab initio MO calculations have in fact revealed that such species are local minima on the potential energy hypersurface [34]. However, recent results show that both the Gibbs reaction energies as well as the activation enthalpies for reactions of the type... 

From the chemical point of view, the solvent in which the CL experiment is carried out can have a dramatic influence on the efficiency of the CL reaction as solvation can alter the shapes, the depths, and the densities of the vibrational states of the potential surfaces representing the ground states of products and reactants and the lowest excited singlet state of the potential fluorophore. The alteration of the intersections of these potential energy surfaces can affect the enthalpies of reaction and the enthalpies of activation for dark and lumigenic reactions. In some cases, these changes will favor CL (if AH decreases relative to AHa) and in some cases, they will make it thermodynamically unfavorable for CL to occur. 

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

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

The parabolic model is, in essence, empirical because the parameter a is calculated from spectroscopic fa and v ) and atomic (/q and /q) data, while the parameter bre (or Ee0) is found from the experimental activation energies E(E= RT a(A/k)), where A is the pre-exponential factor typical of the chosen group of reactions, and k is the rate constant. The enthalpy of reaction is calculated by Equation (4.6). The calculations showed that = const, for structurally similar reactions. The values of a and bre for reactions of different types are given in Table 4.16. 

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] ... 

A clear-cut dependence of the activation energy on the heat (enthalpy) of the reaction, which is equal, in turn, to the difference between the dissociation energies of the ruptured (Z> ) and the formed (D j bonds, was established for a great variety of radical abstraction reactions [1,2,16]. In parabolic model, the values of Dei and Def, incorporating the zero-point energy of the bond vibrations, are examined. The enthalpy of reaction AHe, therefore, also includes the difference between these energies (see Equation [6.7]). 

Enthalpies, Activation Energies, Rate Constants, and Geometric Parameters of TS of Reaction R02 with Aromatic Amines Calculated by Equations (15.13)—(15.15)... 

Enthalpies, Activation Energies, Rate Constants, Increment A H, aid Geometrical Parameters of TS Reactions of Phenoxyl Radicals with Cumene (Reaction 10) Calculated by I PM Method... 

Enthalpies, Activation Energies, and Rate constants of Reactions of Phenoxyl Radicals (A O ) with 2,6-bis(1,1-dimethylethyl)-4-methylphenol (lonol) Calculated by IPM Method, Equations See in Chapter 6, for the Values of a, bre, and A, See Table 18.3... 

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