Dissociation reactions, activation energy

Comparison of True Activation Energies in Reactions of Carbon with Oxygen-Containing Gases and the Dissociation Energy of an 0 Atom from the Reactant after Rossberg )... 

The halogen atom consumed in (2) is regenerated in (3) and so the chain is continued. It can be shown13 that for this mechanism the over-all activation energy is equal to the activation energy of reaction (2) plus half the heat of dissociation D of the halogen molecule as given by reaction (1) and amounts in this case to 22,600 calories. Thus it appears that an atom-catalyzed chain is about as probable as a bimolecular addition of bromine to ethylene. 

It follows from Table 6 that the Eg values, calculated for a dissociative addition to siloxane bonds of silica surface at the interaction with (CH3)3SiX (X = N3, NCS and NCO) compounds, are by 10 to 20 kJ-mole lower than the activation energies of reactions of these reactants with the silica surface OH groups. The lowest Eg value in the Aji process is displayed by the reaction with X = NCS that features the weakest Si X bond and a substituent with the highest electron acceptor ability, while the reactions with X = NCO and X = N3 occur to the close activation energies, which, probably, is related to the close strength of Si-X bond in these reactants. 

There has been a longstanding controversy in EOM studies over whether the effect of alloys on electrode performance is due solely to the bifunctionality provided by the substituted metal, or whether these are extra electronic effects that need to be considered [75,25,91]. From the calculated dissociation and activation energies of reactions 5 and 6, we are able to conjecture as to the relative... 

Figure 3.5 Schematic potential energy profiles for three types of unimolecular reaction, (a) Isomerization (b) dissociation where there is a high energy barrier and a large activation energy for reaction in both the forward and reverse directions (c) dissociation where the potential energy rises monotonically as for rotational ground state species, so that there is no barrier to the reverse association reaction (Steinfeld et al., 1989).

The production of free radicals and other excited neutral species within the plasma can enhance the rate of reaction in the gas and with the surface by reducing the activation energy for reaction and/or dissociative adsorption on the surface. For example, CF does not adsorb on silicon, however, both of the major plasma products... 

Bond energies (actually bond dissociation energies) primarily indicate the susceptibility of the bond to thermal scission, and therefore give evidence of the thermal stability of the macromolecule. The vulnerability of a bond to other reagents, on the other hand, depends mainly on the ionic character of the bond and whether there remain unoccupied orbitals or electron pairs in the molecule, as these will lower the activation energy of reaction with the reagent. The resistance to reduction, oxidation, hydrolysis, etc., decreases with increasing atomic number in every period. Thus hydrocarbons, Cn H in + 2 are not hydrolyzed, but silanes, Si H2n + 2, certainly are. In the latter case there are only four completely filled orbitals of a possible maximum coordination number of six. 

As long as the structures of ttansition state and dissociated state are close, changes in metal-atom interactions will lead to the Bronsted-Evans-Polanyi relation between activation energy and reaction energy of a surface elementary reaction. Interestingly, microscopic reversibility imphes that the Bronsted-Evans-Polanyi proportionality constant for recombination is typically 0.1. This implies that the ratio of the energy of the surface fragments in the transition state compared to the dissociated state is a constant and on the order of 90%. 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

Steps 2 and 4 are proton transfer reactions and are very fast Nucleophilic addi tion to the carbonyl group has a higher activation energy than dissociation of the tetra hedral intermediate step 1 is rate determining... 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

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