Chemical reaction and Gibbs energy

This discussion is meant to provide you some context for this chapter, where we cover a thermodynamic analysis of reacting systems the calculations we perform in this chapter do not account for rates of product formation. They are valid only at equilibrium, when the reactions are thermodynamically controlled. The fundamental question we wish to address is, What effect do temperature, pressure, and composition have on the equilibrium conversion in a chemically reacting system This analysis tells us nothing about the rates at which a chemical reaction will proceed. It does, however, tell us to what extent a reaction is possible. As in phase equilibria, we will use the Gibbs energy of the system to study chemical reaction equilibria. To illustrate the use of G, we will first consider a specific reaction (Section 9.2). We will then describe the general formalism for a single reaction (Sections 9.3-9.5) and multiple reactions (Sections 9.7-9.8). 

In this section, we will consider a specific example to illustrate how the same principle that we applied to solve phase equilibria problems also applies to chemical equilibria the minimization of Gibbs energy. As we have seen, Gibbs energy represents a trade-off between reducing the energy of a system and maximizing its entropy. 

lets consider the energetics of this chemical reaction. We can determine the relative energies of products vs. reactants by looking up the bond dissociation energies (bond strengths), Dj-j, of the three different species involved these values are reported as follows  

Inspection of these bond energies reveals that when two molecules of HCl are formed from Reaction (9.1), the energy of the molecules present will be reduced by 2 eV therefore HGl is energetically favored (more stable). Does this mean that the reaction will go 

Before we examine how to calculate the Gibbs energy, it is useful to introduce the concept of extent of reaction. This concept is based on the fact that once we specify the initial composition of the system, we are limited by Reaction (9.1) as to the possible composition of the system once the reaction has completed to equilibrium (or to any degree toward equilibrium). For illustration, let s consider a system in which we initially have 1 mole of H2 (species 1) and 1 mole ofGl2 (species 2) at 1 bar total pressure. These species can react stoichiometiicaUy to form HGl (species 3). 

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