Activation energy, carbonate decomposition

Stability and decomposition kinetics of aspirin both as a solid and in solution continue to be studied. The topochemical decomposition pattern of aspirin tablets has been explored.175 The degradation of aspirin in the presence of sodium carbonate and high humidity was studied by x-ray diffraction.176 The activation energy of decomposition by water vapor in the solid state was found to be 30 kcal/mol.177 The effect of common tablet excipients on aspirin in aqueous suspension was also studied.178... 

Judd and Pope [36] conclude that because the activation energies for decompositions of CaCOj, SrCO, and BaCOj are all close to the corresponding enthalpies of dissociation (apparent values of , are 180,222 and 283 kJ mol and A//, 178, 235 and 269 kJ mol, respectively) the mechanisms of decomposition in all three substances are the same as that proposed by Hills [18] for calcite. Strontium carbonate [37] generally resembles the calcium salt in that an increase in sample size results in a decrease in reaction rate. Differences in behaviour were ascribed [37] to the occurrence of a crystallographic transformation and to fusion. 

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

Endotliermic Decompositions These decompositions are mostly reversible. The most investigated substances have been hydrates and hydroxides, which give off water, and carbonates, which give off CO9. Dehydration is analogous to evaporation, and its rate depends on the moisture content of the gas. Activation energies are nearly the same as reaction enthalpies. As the reaction proceeds in the particle, the rate of reaction is impeded hy resistance to diffusion of the water through the already formed product. A particular substance may have sever hydrates. Which one is present will depend on the... 

Activation energy values for the recombination of the products of carbonate decompositions are generally low and so it is expected that values of E will be close to the dissociation enthalpy. Such correlations are not always readily discerned, however, since there is ambiguity in what is to be regarded as a mole of activated complex . If the reaction is shown experimentally to be readily reversible, the assumption may be made that Et = ntAH and the value of nt may be an indication of the number of reactant molecules participating in activated complex formation. Kinetic parameters for dissociation reactions of a number of carbonates have been shown to be consistent with the predictions of the Polanyi—Wigner equation [eqn. (19)]. 

The apparent activation energy is evidently substantially reduced. The same explanation, i.e. a decrease in AG+, was also proposed for the decomposition of sodium hydrogen carbonate in aqueous solution [23],... 

The thermal decomposition is first-order and the rate coefficient is given by k = 1.7 x 1014 exp(—51,000/i T) sec-1. The mean metal-carbon bond dissociation energy in this alkyl is 58.0 kcal.mole-1. In view of the normal frequency factor, it might seem reasonable to relate the observed activation energy to... 

The reason for this difference is that if 16 were to undergo a concerted elimination, it would have to follow the forbidden (high-energy) [2ns + 2ns] pathway. For 17, the elimination can take place by the allowed [2ns + 4rcv] pathway. Thus, these reactions are the reverse of, respectively, the [2 + 2] and [4 + 2] cycloadditions, and only the latter is an allowed concerted process. The temperature at which 16 decomposes is fairly typical for strained azo compounds, and the decomposition presumably proceeds by a noncon-certed diradical mechanism. Because a C—N bond must be broken without concomitant compensation by carbon-carbon bond formation, the activation energy is much higher than for a concerted process. 

Carbon-based catalysts have also been considered for the methane decomposition reaction. Yoon and co-workers have recently investigated the kinetics of methane decomposition on activated carbons as well as on carbon blacks.In case of activated carbons the authors observed mass transport effects in the catalyst particles and also significant pore mouth plugging. The reaction order was found to be 0.5 and the activation energy was found to be 200 kJ/mol for the different activated carbon samples. On the other hand, for... 

Thermal decompositions are sensitive to the nature of pretreatment given to solids. Perhaps the earliest observation in this respect is that of Michael Faraday who noticed that the efflorescence of sodium carbonate was promoted by scratching the crystal with a pin. Modern interpretation of this observation is that dislocations produced by scratching provide sites for the reaction. Enhanced reactivity at dislocations has been demonstrated in several decomposition reactions (Thomas Williams, 1971 Parasnis, 1970). Preirradiation of solids with neutrons, protons, or with UV-, X- or y-radiation decreases the induction period and increases the rate of the decomposition without affecting the activation energy significantly. This observation has been taken to... 

In cold carbon tetrachloride, a number of ketimines have been nitrosated with nitrosyl chloride. The A-nitrosoketimines exhibit varying degrees of stability. This stability is enhanced in the case of A-nitrosobenzophenonimines by electron-withdrawing substituents on the benzene ring and by the presence of bulky substituents on the ketimine carbon atom. Unhindered A-nitrosoketimines may decompose at room temperature or even below. This observation is considered to be consistent with a low activation energy for the decomposition and a substantial contribution of the 1,4-dipolar structure (III), in Eq. (16), to the ground state. Structures III, IV, and V represent possible resonance structures of the A-nitrosoketimines [64a]. 

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