Endothermic reaction potential energy diagram

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

Sketch a potential energy diagram which might represent an endothermic reaction. (Label parts of curve representing activated complex, activation energy, net energy absorbed.)... 

A potential energy diagram for an endothermic reaction is shown in Figure 6.13. The reactants at the beginning of the reaction are at a lower energy level than the products. The overall difference in potential energy between reactants and products is the enthalpy change. 

The potential-energy diagram for the El reaction (Figure 6-12) is similar to that for the SnI reaction. The ionization step is strongly endothermic, with a rate-limiting... 

Both exothermic and endothermic reactions are often represented graphically with potential energy diagrams, as seen in Figure l-5b. Maybe you can see why endothermic reactions are sometimes called uphill reactions, as they need energy to be continuously added in order to continue. 

Sketch a potential energy diagram for a reaction that shows the effect of a catalyst on an endothermic reaction. 

From our earlier discussions, we know that exothermic reactions release energy and endothermic reactions consume energy. Here, we will plot the potential energy diagrams of exothermic and endothermic reactions. Before we do that, we will discuss the concept of transition state. Consider the hypothetical reaction shown below ... 

Construct a diagram like that in Figure 16-lOa. (a) Write a generic equation that would have such a potential energy diagram, (b) Is the reaction exothermic or endothermic ... 

Figure 3-6 Potential-energy diagram of the reaction of methane with a chlorine atom. Partial bonds in the transition state are depicted by dotted lines. This process, propagation step 1 in the radical chain chlorination of methane, is slightly endothermic.

Figure 3-8 Potential-energy diagrams (left) the reaction of a fluorine atom with CH4, an exothermic process with an early transition state and (right) the reaction of an iodine atom with CH4, an endothermic transformation with a late transition state. Both are thus in accord with the Hammond postulate.

The energy diagram above details endothermic reactions, where the total potential energy of the reactants is smaller than that of products. 

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. 

Representation of a reaction coordinate diagram as the sum of two parabolic potential energy surfaces for an endothermic reaction. 

Consideration of the correlation diagram (Fig. 2) for the AlHj system indicates that reactions (19,22 and 23) can occur adiabatically on the potential energy surfaces ( A or 3A"), 1z+(1a ) and 3z+( A ) respectively. For the endothermic reactions (19) and (22), the coUinear surfaces have no barrier other than that due to the endothermicity. TTie 2 surface relevant to reaction (23) is attractive and there is no activation barrier for coUinear geometries. 

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. 

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