Energy requirements reaction coordinate diagram

Knowledge Required (1) The general understanding of multi-step reaction mechanisms. (2) The general shape of energy vs. reaction coordinate diagrams. (3) The meaning of the terms exothermic and endothermic. 

Graphical representation of the saddle point (here marked with an X) for the transfer of atom B as the substance A-B reacts with another species, C. Potential energy is plotted in the vertical direction. Note also that the surface resembles a horse saddle, with the horn of the saddle closest to the observer. As drawn here, the dissociation to form three discrete species (A + B J- C) requires much more energy than that needed to surmount the path that includes the saddle point. A two-dimensional "slice" through a saddle point diagram is typically called a reaction-coordinate diagram or potential-energy profile. 

The potential energy-reaction coordinate diagram for this reaction when 2 -1 is shown in Fig. 1. The greater energy required to break the C—D bond... 

In a reaction coordinate diagram, it is obvious that the potential energy content at a transition state is closer to that in the starting materials in an exothermal step and closer to the products in an endothermal step. Since potential energy is required to distort a molecule, the structure of the transition state will more closely resemble those molecules to which it is closer in potential energy that is, a small vertical difference in a reaction coordinate diagram corresponds to a small horizontal difference. Transition states are late in endothermal steps and early in exothermal steps. This is the Hammond postulate [4] and it is useful for predicting products where there is potentially close competition between two alternative steps. 

Fig. 9. Potential energy diagram for breaking chemical bonds in an energetic molecule. The specific coordinate R shown here is identified as the reaction coordinate. In ascending energy these levels are the electronic ground state, a bound excited state and a dissociative excited state. Thermal cleavage of a bond in the electronic ground state requires a minimum energy Dq. In bound electronic states the bond dissociation energy Do is usually smaller than Do, so thermochemistry often has a lower barrier electronic excited states. Chemical bonds can also be broken by electronic excitation to predissociative or dissociative electronic states.

Instability of the low-conversion branch restricts the conversion that can be achieved in an adiabatic reactor. This can have practical consequences when low conversion is desired, for selectivity reasons. Stability condition is presented in Fig. 13.26 left, where the influence of recycle purity and activation energy on the coordinates B, X) of the turning point are shown. For a given reaction (fixed B and y), one can design an adiabatic reactor to operate in a stable Reactor - Separator - Recycle system only if the required conversion is in the upper part of the diagram. Note that the performance of the separation section (Z3) has a small influence, as illustrated in Fig. 13.26 right. 

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