Endothermic 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. 

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 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. 

A schematic energy profile for the endothermic elementary reaction in Equation 2.5 is given in Figure Q.l. The difference in potential energy between products and reactants is positive, which is consistent with the endothermic nature of the reaction. 

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... 

The potential and free-energy profiles along the reaction coordinate calculated both by the standard ab initio MO and the RISM-SCF in the Hartree-Fock level are shown in Fig. 2.8. Although this reaction is endothermic in the gas phase by 106.3 kcal/mol with the HF method, it becomes exothermic in aqueous solution by 27.8 kcal/mol at the RISM-HF level. The barrier height was calculated to be 17.7 kcal/mol. As seen in Fig. 2.8, there is a very shallow potential well around 1.9 A, which corresponds to a contact ion pair of NH3CH3+ and Cl formed in aqueous solution. The Cl-H RDF depicted in Fig. 2.9 clearly demon-... 

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