Activation energy negative value

The activation parameters bring out several features. Note that the activation enthalpy and activation energy for kn, which represents a very rapid reaction, are quite small. Of course, a fast reaction can have a higher activation energy, if the value of AS is more positive, so as to compensate. The activation entropy associated with k is particularly large and negative, as is most often the case for a second-order reaction that occurs by a bimolecular step. In such cases, AS reflects the loss of entropy from the union of the two reaction partners into a single transition state. 

Table 3 shows that the small activation enthalpies of the reactions (3) and (4) are clearly affected by the zero point energy corrections. But the relative order of the activation enthalpies remains the same with or without the corrections. The activation entropies have great negative values, which is of mechanistic interest (see part 4.3.1). However, because of their similarity, when comparing the three reactions to one another they have only small importance, e.g. for estimation of copolymerization parameters (see part 4.3.2). 

Can the activation energy of a catalytic reaction have a negative value If so, under what conditions ... 

The activation energy Ed for the decomposition of the initiators commonly used is of the order of 30 kcal., while (Ep — Etl2) assumes values from 4 to 7 kcal. Hence, insofar as it is termination-controlled, the temperature coefBcient of the degree of polymerization is negative. Similarly for thermal polymerizations, the corresponding quantity... 

In Chapter 15, we learned that reactions which are thermodynamically favorable have negative AG values and occur spontaneously as written. However, thermodynamics cannot be used to determine the rate of a reaction. Kinetically favorable reactions must be thermodynamically favorable and have a low enough activation energy to occur at a reasonable rate at a certain temperature. 

All these reactions are exothermic, and the AH values are negative. All these reactions should seemingly occur equally rapidly. The question to how easily the aminyl radicals react with the H—O and H—C bonds of the peroxyl radicals can be answered by analyzing these reactions in terms of the IPM model of free radical reaction (see Chapter 6). This model gives a tool to perform the calculation of the activation energy for a thermally neutral reaction of each class. Analysis of experimental data has shown (see Chapter 15) that, when aminyl... 

Activation energies and entropies of activation are in the same region as those for Sn2 reactions of neutral A-methylaniline. Large negative AS values are again in accord with increased charge separation with attendant solvation in the transition state. The rate constants at 298 K correlate with Hammett a values with p — 1.13 (r — 0.985) in keeping with a development of benzoate character in the transition state. 

In the electrochemical case, using, for example, cyclic voltammetry, one way of driving the potential toward more negative values is to increase the scan rate. This is true whether the linearization procedure or the convolution approach is followed. In the first case, equation (3.4) shows that the activation free energy at the peak, AG, is a decreasing function of the scan rate as a result of the kinetic competition between electron transfer and diffusion. The larger the scan rate, the faster the diffusion and thus the faster the electron transfer has to be in order to compete. This implies a smaller value AG, which is achieved by a shift of the peak potential toward more negative values. 

Activation energy values are normally large, positive values. If your answer doesn t meet these two conditions, you probably made an error in your calculation. If your answer is negative, the most likely error is that you incorrectly paired k, and T,. 

---