Reactant/product energy difference

The correct answer is (A). In an exothermic reaction, the potential energy of the products will be lower than that of the reactants. The energy difference is due to the loss of energy as heat. The only graph that shows a decrease in the energy of the products is choice (A). The other most common type of plot is choice (B), which represents an endothermic reaction. 

For a reaction to occur, two conditions must be met The first is that the products should be more energetically favourable than the reactants. This energy difference controls the equilibrium constant of the reaction ... 

When M is a voltmeter an indication of the energy difference between the reactants and products is obtained (see below). A current passes when M is an ammeter, and if a little potassium thiocyanate is added to the Fe (aq) a red colour is produced around the electrode, indicating the formation of iron(III) ions in solution the typical bromine colour is slowly discharged as it is converted to colourless bromide Br . 

It should be stressed that although these symmetry considerations may allow one to anticipate barriers on reaction potential energy surfaces, they have nothing to do with the thermodynamic energy differences of such reactions. Symmetry says whether there will be symmetry-imposed barriers above and beyond any thermodynamic energy differences. The enthalpies of formation of reactants and products contain the information about the reaction s overall energy balance. 

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

Preparation of enantiomerically enriched materials by use of chiral catalysts is also based on differences in transition-state energies. While the reactant is part of a complex or intermediate containing a chiral catalyst, it is in a chiral environment. The intermediates and complexes containing each enantiomeric reactant and a homochiral catalyst are diastereomeric and differ in energy. This energy difference can then control selection between the stereoisomeric products of the reaction. If the reaction creates a new stereogenic center in the reactant molecule, there can be a preference for formation of one enantiomer over the other. 

The Hammond postulate is a valuable criterion of mechanism, because it allows a reasonable transition state structure to be drawn on the basis of knowledge of the reactants and products and of energy differences between the states (i.e., AG and AG°). Throughout this chapter we have located transition states in accordance with the Hammond postulate. 

A (AE ) Change in the vibrational energy difference between 0 K and 298 K. Ae298 Difference in the rotational energies of products and reactants. 

Isodesmic reactions may be studied in themselves. For example, energy differences may be compared between the reactants and products in order to predict AH. In addition, isodesmic reactions may be used to predict the heats of formation for compounds of interest by predicting AH for the reaction and then computing the desired heat of formation by removing the known heats of formation for the other compounds from this quantity. We will look at an example of each type in this section. 

From the TST expression (12.2) it is clear that if the free energy of the reactant and TS can be calculated, the reaction rate follows trivially. Similarly, the equilibrium constant for a reaction can be calculated from the free energy difference between the reactant(s) and product(s). 

As can be seen from Tables 12.1-12.3, the electronic energy difference between the reactant/TS and reactant/product is the most important contribution to AG and AGq. The electronic energy is furthermore the most difficult to calculate accurately. Let us consider three cases. 

The BEP/Hammond/Marcus treatment only considers changes due to energy differences between the reactant and product, i.e. changes in the TS position along the reaction coordinate. It is often useful also to include changes that may occur in a direction perpendicular to the reaction coordinate. Such two-dimensional diagrams are associated with the names of More O Ferrall and Jenks (MOJ diagrams). 

AGr Gibbs free-cnergy change The energy difference between reactants and products. When AG° is negative, the reaction is exergonic, has a favorable equilibrium constant, and can occur spontaneously. When AGC is positive, the reaction is endergonic, has an unfavorable equilibrium constant, and cannot occur spontaneously. 

Figure 5.4 An energy diagram for the first step in the reaction of ethylene with HBr. The energy difference between reactants and transition state, AG, defines the reaction rate. The energy difference between reactants and carbocation product, AG°, defines the position of the equilibrium.

Each step in a multistep process can always be considered separately, bach step has its own AG and its own AG°. The overall AG° of the reaction, however, is the energy difference between initial reactants and final products. 

Figure 5. Potential-energy diagram including zero-point energy for the HCC0 + 02 reaction. Energies of reactants and products ignore differences between and Intermediates species are denoted by Roman numerals, saddle points by Arabic numerals, and reactions paths are labeled A-F. Reproduced from [47] by permission of the PCCP Owner Societies.

First, one must determine if this is an exothermic reaction. Gibbs equation states that an exothermic reaction must have a negative value of AH. This means that the heat content of the reactants is greater than the heat content of the products. The difference in heat content between the two states is released during the reaction as the system goes to a lower energy state. The opposite is true of an endothermic reaction, as is shown in Figure 6.1. 

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