Gibbs Free-Energy Change, AG

According to the second law of thermodynamics, AS unjv 0 for a spontaneous process. What we are usually concerned with and usually measure, however, are the properties of the system rather than those of the surroundings or those of the universe overall. Therefore, it is convenient to have a thermodynamic function that enables us to determine whether or not a process is spontaneous by considering the system alone. 

Now we have an equation that expresses the second law of thermodynamics (and predicts whether or not a process is spontaneous) in terms of only the system. We no longer need to consider the surroundings. For convenience, we can rearrange the preceding equation, multiply through by -1, and replace the sign with a sign  

According to this equation, a process carried out at constant pressure and temperature is spontaneous if the changes in enthalpy and entropy of the system are such that AH ys TASsys is less than zero. 

To express the spontaneity of a process more directly, we introduce another thermodynamic function called the Gibbs free energy (G), or simply free energy. 

Each of the terms in Equation 18.9 pertains to the system. G has units of energy just as H and TS do. Furthermore, like enthalpy and entropy, free energy is a state function. The change in free energy, AG, of a system for a process that occurs at constant temperature is 

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It is known that thermodynamic and structural studies are mutually complimentary and both are necessary for a complete elucidation of the molecular details of any binding process for the delineation of the molecular interaction involved at the interaction site. The Gibbs free energy change (AG) may be determined from the binding constant from the relation ... 

As equation 2.4.8 indicates, the equilibrium constant for a reaction is determined by the temperature and the standard Gibbs free energy change (AG°) for the process. The latter quantity in turn depends on temperature, the definitions of the standard states of the various components, and the stoichiometric coefficients of these species. Consequently, in assigning a numerical value to an equilibrium constant, one must be careful to specify the three parameters mentioned above in order to give meaning to this value. Once one has thus specified the point of reference, this value may be used to calculate the equilibrium composition of the mixture in the manner described in Sections 2.6 to 2.9. 

Gibbs free energy change, AG°, for methane transformation reactions (10)... 

The partial equilibria of equations 8.165 8.168 reveal the usefulness of standard potentials the Gibbs free energy change AG of the redox equilibrium is always given by applying Faraday s equation to the algebraic sum of the standard potentials of the redox couples in question. For equation 8.163, the bulk potential is thus... 

Under conditions of constant T, P, the criterion for general Gibbs free energy changes AG can be stated as... 

Similar relationships can be derived for the Gibbs free energy changes (AG = AGsoin) of solution formation. Starting from (6.22c), we obtain in a similar manner... 

How do we weigh the contributions of changes in both heat (AH) and randomness (AS) to the overall spontaneity of a process To take both factors into account when deciding the spontaneity of a chemical reaction or other process, we define a quantity called the Gibbs free-energy change (AG), AG = AH — TAS. 

The standard electrode potential E° of a redox reaction is a measure of the potential that would be developed if both reductants and oxidants were in their standard states at equal concentrations and with unit activities. The units of E° are volts and ° can be calculated from the Gibbs free energy change (AG ) of the redox reaction from the relationships... 

In this section, we will perform the stability analysis for PFPE Zdol and Ztetraol films via the Gibbs free-energy change (AG) for the PFPE system [7] to obtain criteria for uniform, stable thin films. 

Figure 1.27. (a) The second derivative of Gibbs free-energy change (AG) of Zdol film versus... 

It emerges from thermodynamics that the overall thermodynamic criterion for spontaneity is vested in a quantity defined as the Gibbs (free) energy change, AG which, for an isothermal (Frame 1) process, is defined by the equation ... 

The standard Gibbs free energy change, AG°, is most commonly used. The symbol0 designates a reaction involving reactants and products in their standard states (pure substances in their most stable states at 25 °C and 1 atm pressure). The relationship between A G° and Kcq is given by the expression... 

Calculate the standard Gibbs free-energy change AG° for the reaction CH3CH2OH —> CH2=CH2 + H20 at 443 K (338°F). 

Thermodynamics is the area of science that relates to the interplay of heat and other forms of energy. At the molecular level, this science describes the balance between two generally opposing thermodynamic forces the natural tendency for mechanical systems to move toward lower energies and the equally natural tendency for thermal Brownian motion to perturb this mechanical order. For open systems at constant pressure, this balance is expressed by the classic Gibbs free energy change (AG) ... 

The overall Gibbs free energy change AG for the reaction is the sum of the free energy changes for the three steps in the path ... 

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