Standard Gibbs free energy formation

We obtain K from the standard Gibbs free energies of formation. For reaction (9.85) we get... 

TABLE 7.6 Examples of the TABLE 7.7 Standard Gibbs Free Energies of Formation at 25°C (kJ-mol Ij ... 

Standard Gibbs free energies of formation can be determined in various ways. One straightforward way is to combine standard enthalpy and entropy data from tables such as Tables 6.5 and 7.3. A list of values for several common substances is given in Table 7.7, and a more extensive one appears in Appendix 2A. 

EXAMPLE 7.14 Calculating a standard Gibbs free energy of formation from enthalpy and entropy data... 

Calculate the standard Gibbs free energy of formation of HI(g) at 25°C from its standard molar entropy and standard enthalpy of formation. 

STRATEGY We write the chemical equation for the formation of HI(g) and calculate the standard Gibbs free energy of reaction from AG° = AH° — TAS°. It is best to write the equation with a stoichiometric coefficient of 1 for the compound of interest, because then AG° = AGf°. The standard enthalpy of formation is found in Appendix 2A. The standard reaction entropy is found as shown in Example 7.9, by using the data from Table 7.3 or Appendix 2A. 

FIGURE 7.27 The standard Gibbs free energy of formation of a compound is defined as the standard reaction Gibbs free energy per mole of formula units of the compound when the compound is formed from its elements. It represents a "thermodynamic altitude" with respect to the elements at "sea level." The numerical values are in kilojoules per mole. 

The standard Gibbs free energy of formation of HI is therefore +1.69 kj-mol in good agreement with the value of +1.70 kj-tnol-1 quoted in the text. Note that, because this value is positive, the formation of pure HI from the elements at 1 bar is not spontaneous. 

Just as we can combine standard enthalpies of formation to obtain standard reaction enthalpies, we can also combine standard Gibbs free energies of formation to obtain standard Gibbs free energies of reaction ... 

Use the standard Gibbs free energies of formation in Appendix 2A to calculate AG° for each of the following reactions at 25°C. Comment on the spontaneity of each reaction under standard conditions at 25°C. 

We are free to choose either K or Kc to report the equilibrium constant of a reaction. However, it is important to remember that calculations of an equilibrium constant from thermodynamic tables of data (standard Gibbs free energies of formation, for instance) and Eq. 8 give K, not Kc. In some cases, we need to know Kc after we have calculated K from thermodynamic data, and so we need to be able to convert between these two constants. 

The standard potential of the AI3+/A1 couple is —1.66 V. Calculate the standard Gibbs free energy of formation for Al +(aq). Account for any differences between the standard Gibbs free energy of formation of Tl,+(aq) (see Exercise 14.65) and that of Al +(aq). 

When an element enters into a reaction, its standard Gibbs free energy and standard enthalpy of formation are taken as zero if its state of aggregation is that selected as the basis for... 

As a thermodynamicist working at the Lower Slobbovian Research Institute, you have been asked to determine the standard Gibbs free energy of formation and the standard enthalpy of formation of the compounds ds-butene-2 and trans-butene-2. Your boss has informed you that the standard enthalpy of formation of butene-1 is 1.172 kJ/mole while the standard Gibbs free energy of formation is 72.10 kJ/mole where the standard state is taken as the pure component at 25 °C and 101.3 kPa. 

Just as there are tables of standard enthalpies of formation, values of the standard Gibbs free energy of formation, AfG, are listed [5—9] for very many compounds and these may be combined in a way analogous to eqn. (10)... 

In a similar manner to that employed for thermochemical AH° of chemical reactions [cf. (3.106)], the reaction free energy AG° can be expressed in terms of the standard Gibbs free energy of formation AGf [AJ for each species A, namely,... 

Given standard absolute entropies and standard enthalpies of formation, one can compute standard Gibbs free energies offormation, A Gf. Just as for standard enthalpies of formation, A Gf for elements... 

Table 6.1. Standard Gibbs free energies of formation of minerals and aqueous species considered in development of mineral compatibility diagrams in Figure 6.1.

The results of the DFT calculations for various stable C2H.V species and transitions states on Pt(lll) and Pt(211) are summarized in Table V, which also shows entropy changes for the various steps, as estimated from DFT calculations of the vibrational frequencies of the various adsorbed C2H species and transition states on 10-atom platinum clusters (55). Table V also includes estimates of the standard Gibbs free energy changes for the formation of stable C2H surface species and activated complexes responsible for C-C bond cleavage at 623 K. These estimates were made by combining... 

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 ... 

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... 

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