Activation energy different magnitudes

The minimum value of the relative kinetic energy of molecular motion that is required for a chemical reaction to occur is called the activation energy Differences in activation energies are largely responsible for the large range in the magnitudes of chemical reaction rates. 

The conductivity of solid dielectrics is roughly independent of temperature below about 20°C but increases according to an Arrhenius function at higher temperatures as processes with different activation energies dominate [ 133 ]. In the case of liquids, the conductivity continues to fall at temperatures less than 20°C and at low ambient temperatures the conductivity is only a fraction of the value measured in the laboratory (3-5.5). The conductivity of liquids can decrease by orders of magnitude if they solidify (5-2.5.5). 

This 580 to 280 K decrease in reaction temperature for trimethylsilane formation corresponds to a decrease in the reaction activation energy from 33 kcal/mol to -16 kcal/mol. Alternatively, if the rates of these processes could be measured at a common temperature, they would differ by more than 5 orders of magnitude. The fact that trimethylsilane is evolved at either 580 K or 280 K and not at temperatures in between suggests that there are distinctly different active sites for forming this product. The ratio of these active sites is a fimction of the temperature at which die surface is ion bombarded, and the transition from high to low temperature product evolution correlates directly with a factor of 1.5 to 2 increase in the Cu/Si... 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

An estimate for the activation energy can be obtained by the consideration that for r = 2, the charge reduction reaction must proceed at a rate similar to that for the single solvent molecule dissociation, which means that the activation free energies for the two reaction are of similar magnitude. Neglecting the difference between AG and AH we obtain,... 

It is seen that the dimer is decomposed into free radicals by two to four orders of magnitude more rapidly than one molecule of hydroperoxide. Due to the difference in their activation energies, this difference increases with lowering of temperature. 

It is apparent that these reactions are close in their enthalpies, but greatly differ in the rate constants. The peroxyl radical reacts with p-cresol by four orders of magnitude more rapidly than with ethylbenzene. Such a great difference in the reactivities of RH and ArOH is due to the different activation energies of these reactions, while their pre-exponential factors are close. This situation was analyzed within the scope of the parabolic model of transition states (see Chapter 6 and Refs. [33-38]). 

Errors of this magnitude make the useful prediction of free energies a difficult task, when differences of only one to three kcal/mol are involved. Nevertheless, within the error limits of the computed free energy differences, the trend is that relative to 8-methyl-N5-deazapterin or 8-methyl-pterin, the compounds methyl substituted in the 5, 6 or 7 positions are thermodynamically more stable when bound to DHFR largely by virtue of a hydrophobic effect, i.e. methyl substitution reduces the affinity of the ligand for the solvent more than it reduces affinity for the DHFR active-site. The stability of ligand binding to DHFR appears to be optimal with a 6-methyl substituent additional 5-methyl and/or 7-methyl substitution has little effect... 

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