Activation energy barrier effects

Figure 8-8 shows the analogous situation for a chemical reaction. The solid curve shows the activation energy barrier which must be surmounted for reaction to take place. When a catalyst is added, a new reaction path is provided with a different activation energy barrier, as suggested by the dashed curve. This new reaction path corresponds to a new reaction mechanism that permits the reaction to occur via a different activated complex. Hence, more particles can get over the new, lower energy barrier and the rate of the reaction is increased. Note that the activation energy for the reverse reaction is lowered exactly the same amount as for the forward reaction. This accounts for the experimental fact that a catalyst for a reaction has an equal effect on the reverse reaction that is, both reactions are speeded up by the same factor. If a catalyst doubles the rate in one direction, it also doubles the rate in the reverse direction. 

The kinetic isotope effect (KIE) produced in the photorearrangement of 4-me-thoxyphenyl acetate to 2-hydroxy-5-methoxyacetophenone has been measured by determining the isotope ratios before and after irradiation (k, and ka, respectively) in the starting material and the rearranged products. A value of KIE = kjk, different from unity would indicate that the rearrangement proceeds along an activation energy barrier. This is neither the case for the [48,49] nor for the case of isotope, both included in the carbonyl moiety [49]. In fact, the obtained values are KIE ( C) = 0.9988 0.0051 and KIE ( 0) = 1.0000 0.0023. 

The height of the energy barrier between the forward and reverse states is the product of the particle volume, V, and the anisotropy constant Kefr (which is, to some extent, a function of particle size). Superparamagnetic relaxation occurs when the thermal energy of the particles exceeds the activation energy barrier between the spin states and so allows rapid, spontaneous fluctuations between these states. The effect of these spin reversals is that the observed magnetic field is reduced or even absent. 

The role of a catalyst is to lower the activation energy barrier and hence to increase the reaction rate according to (B.7). On the other hand, inhibitors increase the activation energy and/or cancel the catalytic effect, resulting in lowering of the reaction rate. 

Note that this inhibition mechanism readily accounts for the noninhibiting properties of fluorides, since the high stability of HF provides an excessively high activation energy barrier for a reaction with H atoms to take place. The much lower effectiveness of chloride as compared with bromide inhibitors is probably due to the HCI reaction being very close to thermo-neutral, hence it is likely that the reaction can also proceed in the back direction to generate H atoms. 

The rate of nucleation of particles or clusters of size x can be written as the product of the number of clusters of size x and the probability that another molecule gets to the interface by overcoming kinetic barriers which provide an activation energy barrier, Ag. This latter term includes viscosity and diffusion effects of the bulk liquid medium as well as solvent association reactions that deplete monomers. Ifx is the critical size then the nucleation rate, Jx is... 

Computational studies investigate reaction mechanisms and pathways by constructing potential energy profiles. This involves exploring reaction thermodynamics and kinetics, by examining reactants and products as well as the transition states geometries and activation energy barriers. Like those seen in structure prediction, most current studies implement effective core potentials and density functional theory to perform calculations.However, ECPs can be paired with MP2 to account for electron correlation thus far, this approach has only been used for smaller chemical systems. " Eurthermore, solvation methods such as the polarizable continuum model can be employed to examine... 

The decomposition reaction below is exothermic (AH° = - 234 kJ/mole) as well as having an increase in entropy. Thermodynamically, at 1500°C, an increase in entropy will have a large effect on AG (remember AG = AH - TA5). Kinetically, almost any activation energy barrier will be overcome at 1500°C. Acetylene would likely decompose into its elements ... 

Due to their high activation energy barriers, these elementary reactions have been shown [13,14] to have a negligible effect on the overall kinetics of the WGSR. Furthermore, these reactions present some difficulties in the analysis that are best avoided in this initial report. 

Thus, one can see that it seems reasonable to assume that the main effect of a catalyst or a solvent is to soften or to harden the reacting molecules, and because of Eqs. (40) and (41), this modifications have a direct effect on the activation energy barrier, even if the transition state adopts the same structure than the one adopted by the reacting molecules in the absence of a catalyst or a solvent. 

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