Gibbs free energy change of formation

Calculate AG° and Kfor each independent reaction. This may be done as in the relevant examples earlier in this section, with determination of AG° as a function of temperature. An easier route, however, is to use the standard Gibbs free-energy change of formation A Gy for each compound at the temperature of interest in the relationship... 

Both the enthalpy and the Gibbs free energy changes of formation are affected by the temperature. The temperature should be indicated when the theoretical efficiency is calculated. 

The Gibbs free energy change of formation of an element is zero. 

A solution is a single-phase mixture of more than one compound, and the driving force for its spontaneous formation from the pure compounds at constant T and p is the negative Gibbs free energy change of the mixing process, —AG, as... 

AG° is the standard Gibbs free energy change of reaction obtained from the standard energy of formation of pure components AG at 25 °C, P = 1 atm and a suitable aggregation state. Once having determined IQq in standard conditions, the van t Hoff equation may be used to calculate fQq at other temperatures, as follows ... 

FIGURE 10.10 Gibbs free energy change for formation of a droplet of radius Rp from a vapor with saturation ratio S. 

Even for the most favorable cases, however, the error bars that have to be accepted are larger than one would wish. This is illustrated in Table 1.2, adapted from Cohen and Benson [64] who give references to the archival literature. Here one sees that the best available standard enthalpy of formation values for the small hydrocarbons come with error ranges that imply significant uncertainty in equilibrium constants (a 1 kJ/mol uncertainty in the enthalpy or Gibbs free energy change of a reaction at lOOOK implies an uncertainty of 12% in its equilibrium constant). 

Scheme 2 Gibbs free energy change of the formation of differently sized nanocrystals (nucleation)...

Strategy To calculate the standard Gibbs free energy change of a reaction, we look up the standard free energies of formation of reactants and products in Appendix 2 and use these values in Equation 8.40. Remember that AGf for elements such as 02(g) and Mg(i) is zero because they are stable allotropes of their respective elements at 1 bar and 25°C. Check that the chemical equations are balanced so that you use the correct stoichiometric coefficients in Equation 8.40. Finally, aU stochiometric coefficients are unitless, so AG xn is expressed in units of kJ moP. ... 

FIGURE 13.1 Standard Gibbs free energy change of Equations 13.1 through 13.4 as a function of the temperature. Below the interception point at =1000 K formation of COj is thermodynamically favored. 

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