Activation free energy oxidants

It therefore seems reasonable that the deviations of the activation free energies for highly exoergic electrochemical and homogeneous reactions, illustrated in Figures 2 and 5, may arise partly from the same source, i.e., from values of for the oxidation half reactions that are unexpectedly small. That is not to say that other factors are not responsible, at least in part, for these discrepancies. Nonadiabaticity, work terms, specific solvation, and other environmental effects may all play important roles depending on the reactants. For example, there is evidence to suggest that the true rate constant for outer-3+/2+... 

The formal rate constants kfh and kb h are potential dependent. At any given potential, in order for the transition from the oxidized form to occur, it will be necessary to pass over an activation free-energy barrier, AGf, as illustrated by the Morse curves in Figure 2.14. The rate of reaction will be proportional to exp (-AGf /RT). 

Since anions and cations adsorb at oxide electrodes positive and negative to the pzc, respectively, electrostatic work terms (double layer corrections) should contribute to the activation free energy barrier for adsorbed electroactive ions depending on the position of the reaction site. Not much attention has been paid to this phenomenon yet. Trasatti and co-workers... 

Figure 12. Quadratic free energy relationship in the low endergonic region for the oxidation of methylbenzenes by Fe(phen)3". (a) Experimental variation of the observed rate constant compared with the theoretical variations (solid curve) (- - -) activation or (—) diffusion controls, (b) Experimental relationship between the activation free energy and the driving force compared with...

Moreover it was possible to calculate from the free energy curve an activation free energy for the oxidative process of the order of 30 KJmol and a reaction free energy of about -75 KJmol F... 

We have found a free energy for the oxidation at 1000 K of the order of -75 KJmol and an activation free energy of about 30 KJmol . A plausible mechanism for this chemical reaction was given, that may explain at microscopic level the phenomenological first order kinetics with respect to NO3 found experimentally. 

Comparison of the kinetics for the most feasible stereochemical pathways of the alternative la—>4 and lb 2 routes clearly shows that the octadienediyl-Ni" complex is preferably generated via the ethylene-assisted coupling of two -butadienes along lb—>2. Thus, the thermodynamically favorable [Ni (ix -butadiene)2(ethylene)] form of lb also represents the catalytically active species for oxidative coupling. The [Ni (ix -s/ ,ri (C )>A-cis,-octadienediyl)-(ethylene)] species 2 is almost exclusively formed in a thermoneutral process (AG= -0.1 kcal mol 1) that requires a moderate activation free-energy of 12.8 kcal mol This indicates the oxidative coupling as a facile, reversible step. 

The oxidation and snbsequent reduction of Tyrz are important at all stages of this process but the question of how the tyrosine is reduced has been a major issue. The two leading hypotheses are illustrated in Scheme 1. Pathway A involves a hydrogen atom transfer (HAT) but has been criticised in favour of a more conventional electron transfer (ET) process, pathway B. DFT calculations were used to try and determine which pathway is more likely. However, the detailed structure of the active site was unknown at that time. There is now a fairly low resolution structure available but in any case, trying to model the Mn4 system would be a daunting computational task. Instead, the important feature of the reaction was identified [16] as the oxidation of Mn(III) to Mn(IV) which experimentally has an activation free energy of about 12 kcal mol ... 

As shown in Scheme 11, the next step includes H2 coordination, oxidative addition, and aldehyde reductive elimination. As compared to propene, there are no significant effects from functional groups. Starting from the most stable acyl complex 6Ba, H2 coordination with the formation of 7B is endergonic by 62.3 kJ/mol. The subsequent oxidative addition from 7B to 8B is also endergonic by 28.1 kJ/mol with the activation free energy of 34.8 kJ/mol. The reductive elimination from 8B to... 

The Ha coordination and oxidative addition to 15e are computed to be ender-gonic processes (53.2 and 33.9 kJ/mol), and the competitive CO coordination for the formation of (HaC=CHCO)Co(CO)4 (15g) is exergonic by —8.3 kJ/mol. Thus, the formation of 15g is more favored thermodynamically, and also dominant in the possible equilibrium between (HaC=CHCO)Co(CO)4 (15g) and (HaC=CHCO)Co (CO)3( 7 -Ha) (15f). As shown in Scheme 17, the isomerization from 15d to 15e and the CO dissociation from 15g to 15e as well as Ha coordination from 15e to 15f have very close activation free energies (44.8—49.4 kJ/mol). Therefore, each of them arotmd 15e can be the rate-determining step under the variation of the reaction condition (temperature, pressure, and solvent). 

Wang JX, Springer TE, Liu P et al (2007) Hydrogen oxidation reaction on Pt in acidic media adsorption isotherm and activation free energies. J Phys Chem C 111 12425-12433... 

The free energy on the right-hand side of both of the above equations can be considered as the chemical component of the activation free-energy change that is, it is only dependent upon the chemical species and not the applied voltage. We can now substitute the activation free energy terms above into the expressions for the oxidation and reduction rate constants, which give... 

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