Exothermic reactions potential energy profile, 57

The potential energy profile and also the location of saddle point appears different for exo- and endothermic reactions. In general, the saddle point is located in the entry channel for an exothermic reaction as shown in Fig. 9.16. For example, reaction F + H2 —> FH + H. The shift in location of the saddle point from symmetric position has also been related to the magnitude of the exo (endo) thermicity. 

The forward reaction is exothermic, and so the back reaction must be endothermic, as is shown on the potential energy profile. 

The potential energy profile is smooth without excessive barriers and too stable intermediates which would break the sequence of steps. The rate-determining step is found to be olefin insertion followed by isomerization, supporting the Halpern mechanism. Isomerization of the ethyl hydride complex is an important part of the rate-determining step. These two reactions, exothermic overall, has an overall barrier height of about 20 kcal/mol. The trans ethyl hydride complex, the product of ethylene insertion, may not be a local minimum (per MP2 calculation) and these two steps may well be a combined single step. 

Potential energy profiles for the elementary reaction A + B endothermic reaction and (b) an exothermic reaction. 

The difference in energies of the reactants and products is related to the heat of reaction—a thermodynamic quantity. Figure 2.3.1 shows potential energy profiles for endothermic and exothermic elementary reactions. 

Fig. 31a and b. Potential energy profiles along the reaction coordinate a) exothermic b) endothermic processes... 

The reaction of G2 with E2 to form 2EG is exothermic, and the reaction of G2 with X2 to form 2XG is endothermic. The activation energy of the exothermic reaction is greater than that of the endothermic reaction. Sketch the potential energy profile diagrams for these two reactions on the same graph. 

Figure 4 Potential energy profiles along the path of minimum energy shown for reactions proceeding in the exothermic direction. On an attractive surface, as in (a), most of the energy is released as the reactants approach, i.e. before rAB/r.,AB = rBc/r ,Bc In case (b), the surface is more repulsive, the bulk of the energy being released as the products separate...

Figure 13.17 Potential energy profiles for (a) exothermic and (b) endothermic reactions. These plots show the change in potential energy as reactants A and B are converted to products C and D. The activated complex (AB ) is a highly unstable species with a high potential energy. The activation energy is defined for the forward reaction in both (a) and (b). Note that the products C and D are more stable than the reactants In (a) and less stable than those In p).

Consider the potential energy profiles for the following three reactions (from left to right), (a) Rank the rates of the reactions from slowest to fastest, (b) Calculate A77 for each reaction and determine which reaction(s) are exothermic and which reaction(s) are endothermic. Assume the reactions have roughly the same frequency factors. 

Potential energy profiles for (a) exothermic and (b) endothermic reactions. These plots show the change in potential energy as reactants A and B are converted to products C and D. 

It is useful to illustrate how the dynamics problem is solved by referring to Figure 1. This illustrates the typical potential energy profile for a fast reaction dominated by long-range forces. It is essential that the reaction is exothermic. Superimposed on the electronic potential is the centrifugal term... 

Fig. 6.18 A potential energy profile for an exothermic reaction. The graph depicts schematically the changing potential energy of two species that approach, collide, and then go on to form products. The activation energy is the height of the barrier above the potential energy of the reactants.

A concerted reaction is one in which the conversion of reactants (R) into the products (P) occurs directly by way of a single transition state (T.S.). An exothermic concerted reaction is represented by the potential energy profile of Fig. 3.1(a). When the conversion of reactants into products proceeds by way of more than one transition state, such that one or more intermediates (I) are formed, the processes are accordingly non-concerted. A two-step process involving one (metastable) intermediate is represented by Fig. 3.1(b). However, since each elementary step of any chemical reaction must be concerted, by definition, then case (b) may be divided into the two concerted sequences ... 

FIGURE 13.30 A reaction profile for an exothermic reaction. In the activated complex theory of reaction rates, it is supposed that the potential energy (the energy due to position) increases as the reactant molecules approach each other and reaches a maximum as they form an activated complex. It then decreases as the atoms rearrange into the bonding pattern characteristic of the products and these products separate. Only molecules with enough energy can cross the activation barrier and react to form products. 

The next reaction to be studied in a similar manner was the addition of hydroxide ion to formaldehyde. Ab initio 6-31 -I- G(d) calculations provided the gas-phase energy profile and solute-water potential functions. Conversion of the reactants to the tetrahedral intermediate 3 was computed to be exothermic by 35 kcal/mol. 

The free-energy profile with solvent interaction taken into account is shown in Fig. 27. The feature of the entire potential surface is dramatically changed. The large barriers in the reaction from 40 to 42 and from 48 to 40 have disappeared because of the stabilization of the four-coordinate intermediates 41 and 47 by the solvation. The endothermic ethylene and CO insertion reactions became exothermic and the exothermic H2 oxidative addition became endothermic, because the four-coordinate intermediate 43 and... 

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