Chemical reactions energy changes associated with

Estimation of the free-energy change associated with a reaction permits the calcula-aon of the equilibrium position for a reaction and indicates the feasibility of a given chemical process. A positive AG° imposes a limit on the extent to which a reaction can x cur. For example, as can be calculated using Eq. (4.2), a AG° of 1.0 kcal/mol limits conversion to product at equilibrium to 15%. An appreciably negative AG° indicates that e reaction is thermodynamically favorable. 

It is usual to encounter in studies of energy changes associated with reactions of chemical interest, a great variety of chemical materials and transformations. There are many ways by which transformations are implemented but it is convenient to consider two conditions that are special and occur frequently (i) the volume of the system is kept constant and (ii) the pressure on the system is held constant. The second scenario, for example, is that occurring for reactions or other processes carried out in containers that are open to atmosphere. 

The basic criterion for equilibrium with respect to a given chemical reaction is that the Gibbs free energy change associated with the progress of the reaction be zero. 

The following Concept Organizer summarizes what you learned about the energy changes associated with physical changes, chemical reactions, and nuclear reactions. 

The energy changes associated with chemical reactions are determined solely by the state of the reactants and the state of the products, and are totally independent of the path or method of preparation. As a result, if a reaction can be considered to be the sum of two or more other reactions, AH for that reaction must be the sum of the AH values for the other reactions this is known as Hess s law. For example, C02 may be made directly from the elements, or indirectly by first making CO which is subsequently burned to C02 ... 

It is rare that a catalyst can be chosen for a reaction such that it is entirely specific or unique in its behaviour. More often than not products additional to the main desired product are generated concomitantly. The ratio of the specific chemical rate constant of a desired reaction to that for an undesired reaction is termed the kinetic selectivity factor (which we shall designate by 5) and is of central importance in catalysis. Its magnitude is determined by the relative rates at which adsorption, surface reaction and desorption occur in the overall process and, for consecutive reactions, whether or not the intermediate product forms a localised or mobile adsorbed complex with the surface. In the case of two parallel competing catalytic reactions a second factor, the thermodynamic factor, is also of importance. This latter factor depends exponentially on the difference in free energy changes associated with the adsorption-desorption equilibria of the two competing reactants. The thermodynamic factor also influences the course of a consecutive reaction where it is enhanced by the ability of the intermediate product to desorb rapidly and also the reluctance of the catalyst to re-adsorb the intermediate product after it has vacated the surface. 

The energy changes associated with normal chemical reactions are small enough that the corresponding mass changes are not detectable. 

Let us consider the energy change associated with a chemical reaction (Figure 2.1). Transformation from a reactant to a product often gives rise to the release or absorption of energy. If equilibrium exists between the reactant and the product, the amount of the reactant and that of the product in the system are determined by the energy difference between them as well as temperature. If two products are formed in the equilibrium. 

Figure 2.1 Energy change associated with a chemical reaction. S Substrate, P, PI, P2 Product, AG Gibbs free energy change...

In order to analyze energy changes associated with chemical reactions we must first define the system, or the specific part of the universe that is of interest to us. For chemists, systems usually include substances involved in chemical and physical changes. For example, in an acid-base neutralization experiment, the system may be a beaker containing 50 mL of FlCl to which 50 mL of NaOFl are added. The surroundings are the rest of the universe outside the system. 

Thermodynamic data give us a means of quantitatively expressing stability. Now we need to explore the relationship between structure and reactivity. The quantitative description of reactivity is called chemical kinetics. A fundamental thermodynamic equation relates the equilibrium constant for a reaction to the free-energy change associated with the reaction ... 

To relate the concentrations of point and electronic defects to temperature and externally imposed thermodynamic conditions such as oxygen partial pressures, the defects are treated as chemical species and their equilibrium concentrations are calculated from mass action expressions. If the free-energy changes associated with all defect reactions were known, then in principle diagrams, known as Kroger-Vink diagrams, relating the defect concentrations to the externally imposed thermodynamic parameters, impurity levels, etc., can be constructed. 

Free energy changes associated with a biochemical reaction are determined at a standard state, with all reactants and products at 1M. Many biomolecules are unstable in acid, so the biochemical standard state is set at pH = 7.0 rather than at pH = 0 (IM acid), the standard state for chemical reactions. Biochemical standard free energies of reaction are given as AG° to where the indicates this change in standard conditions. 

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