Gibbs energy spontaneity determination

The recognition that AH - T AS = 0 for a system at equilibrium led J. W. Gibbs to realize that the proper thermodynamic function for determining the spontaneity of a reaction is what is now known as the Gibbs energy or Gibbs function G (Eq. 6-14). 

The use of this Legendre transform has introduced the intensive property T as an independent variable. It can be shown that the criterion for spontaneous change and equilibrium is given by (dG)rp 0. The Gibbs energy is so useful because T and P are convenient intensive variables to hold constant and because, as we will see shortly, if G can be determined as a function of T and P, then S, V, H, and U can all be calculated. 

A principal possibility for one or another chemical reaction to occur is determined by the sign of AG. According to the laws of thermodynamics, the process occurs spontaneously in the direction of decreasing the free Gibbs energy. A reaction between the substances proceeds with the formation of products, if... 

Explain how a comprehensive table of standard Gibbs energies of formation can be used to determine the spontaneity... 

As in the case of fluid interfaces, the question of whether the adsorption of proteins onto solids is reversible or irreversible is very important for correct estimation of physicochemical characteristics of the process. In a reversible process, dilution of sorbate in the bulk phase should lead to spontaneous desorption of some portion of adsorbed molecules up to elimination of a transient difference in the chemical potential of the sorbate at the interface and in the solution the ascending and descending branches of the isotherm must overlap at all values of Cb. Only in this case the isotherm represents thermodynamic equilibrium, and the equilibrium constant Kads and the standard Gibbs energy of adsorption AG°ads = A/7°ads - rAS°ads can be determined. 

An important feature of this model is the invocation of the Gibbs-Thompson equation to describe the dependence of the Gibbs energy of LiOH (s) in the pores of the outer layer on the curvature of the internal pore surface. This analysis shows that the pore radius cannot go below a critical value, because below this critical radius LiOH precipitation is no longer spontaneous. It is for this reason that the pores remain open, but, because the concentration of LL in the pores is determined by the rates of Reactions (3) and (8), the critical radius is potential dependent, such that the radius decreases as the voltage is made more positive (Pensado-Rodriguez [1999]). 

The value of the Gibbs energy of copol)unerization, Aj G, determines whether copolymerization can be a spontaneous process (Ajj G < 0) or it can be the reverse reaction, depolymerization (Ajq,G>0). When A,q,G = 0, both processes are thermodynamically possible and copolymerization can be in equilibrium. 

EXAMPLE 6.1 We have seen that the Gibbs energy determines whether a process can occur spontaneously. 

Now, let s return to the Gibbs free energy equation to determine if hydrogen will react spontaneously with oxygen to form water. The equation for the reaction may be written as... 

Which of the following is NOT a variable in the Gibbs free energy equation, which determines reaction spontaneity ... 

Table 2 illustrates the effect of the Gibbs free energy on the spontaneity of a chemical/biochemical reaction and the resulting release of energy. Thus, it is useful to use AG values for any biochemical reaction mediated by microbes to determine whether energy is liberated for work, and how much energy is liberated. 

The Gibbs free energy G is a central thermodynamic quantity in understanding chemistry. The Gibbs free energy determines whether a reaction, or perhaps its reverse reaction, will proceed spontaneously. It provides for the location of chemical equilibrium, at which there is no net forward or reverse reaction. The free-energy change of a reaction determines the equilibrium constant, which also determines the reverse rate constant for a reaction, if the forward rate constant is known. 

Josiah Willard Gibbs studied thermodynamics and statistical mechanics in the 1870s. He formulated the concept now called Gibbs free energy that will determine whether or not a chemical process at constant pressure will spontaneously occur. 

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