Activation energy determining, from experimental

The epimerization equilibria for the diastereomers of 2,4,6,8-tetramethylnonane and 2,4,6,8,10-pentamethylundecane are determined and the results are interpreted in terms of a RIS model. The results yield correct values for the optical activity and are consistent with conformational energies calculated from experimental values of the unperturbed dimensions of PP. 

To reduce the number of parameters in the kinetic equations that are to be determined from experimental data, we used the following considerations. The values klt k2, and k4 that enter into the definition of the constant L, (236), are of analogous nature they indicate the fraction of the number of impacts of gas molecules upon a surface site resulting in the reaction. So the corresponding preexponential factors should be approximately the same (if these elementary reactions are adiabatic). Then, since k1, k2, and k4 are of the same order of magnitude, their activation energies should be almost identical. It follows that L can be considered temperature independent. 

An excellent illustration of the LHHW theory is catalytic cracking of n-alkanes over ZSM-5 [8]. For this reaction, the observed activation energy decreases from 140 to -50 ( ) kj/mol when the carbon number increases from 3 to 20. The decrease appeared to linearly depend on the carbon number as shown in Fig. 3.11. This dependence can be interpreted from a kinetic analysis that showed that the hydrocarbons (A) are adsorbed weakly under the experimental conditions. The initial rate expression for a rate-determining surface reaction applies (3.30), which in the limiting case of weak adsorption of A reduces to Eqn. (3.52). The activation energy is then represented by equation (3.53). 

Hence, the total excess Gibbs energy ean be easily determined from experimental values of activity coefficients, as it will be shown in the next section. Because In /, is a partial property, the following relation may be written ... 

To be able to compare thermal stabUity between polymers of different structures, it is necessary to rely on some standardized system, sucdi as the temperature of half-decomposition Tyi). The temperature of halfdecomposition is defined as the temperature at whicJi the polymer loses half of its weight when heated in vacuo for 30 min. Experimentally, Ty2 can be conveniently determined by thermal gravimetry (TG). From the TG curves obtained at different scan speeds, an Arrhenius plot at constant weight-loss ratio is derived. The pre-exponential factor and activation energy determined are then used to calculate Ty2. 

Note that the concentration Cus has been assumed to be small in deference to the observation that the aromatics affect the rate only modestly. With this formulation, the activation energy of = 30 kcal/mol is that of desorption of the strongly bound toluene. From the rate constant ratio (ki/ks), it is seen that the activation energy for the chemisorption of methylcyclohexane is 11 kcal/mol. They went on to show that the ratio of preexponential factors determined from experimental data compared reasonably well with a value calculated from transition state theory. 

---