Free energy of formation standard

More generally the heat capacities of the components can be expressed in the form 

Standard Gibbs free energies of formation of compounds from their elements may be defined in the same way as enthalpies. 

As we have seen (Section 4.11), AG° defines the equilibrium position for a chemical reaction as AG° = — RTIn KP, For a reaction such as 

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A2.1.6.7 STANDARD STATES AND STANDARD FREE ENERGIES OF FORMATION... 

Thermodynamic analysis of this reaction shows favorable energy relations (18). The standard free energy of formation of DPA is 310.5 kj /mol (74.2 kcal/mol) (19). 

The standard heats of formation AH of gaseous HX diminish rapidly with increase in molecular weight and HI is endothermic. The very small (and positive) value for the standard free energy of formation AGj of HI indicates that (under equilibrium conditions) this species is substantially dissociated at room temperature and pressure. However, dissociation is slow in the absence of a catalyst. The bond dissociation energies of HX show a similar trend from the very large value of 574kJmol for HF to little more than half this (295kJmol ) for HI. 

In its general corrosion behaviour, beryllium exhibits characteristics very similar to those of aluminium. Like aluminium, the film-free metal is highly active and readily attacked in many environments. Beryllium oxide, however, like alumina, is, a very stable compound (standard free energy of formation = —579kJ/mol), with a bulk density of 3-025g/cm as compared with 1 -85 g/cm for the pure metal, and with a high electronic resistivity of about 10 flcm at 0°C. In fact, when formed, the oxide confers the same type of spurious nobility on beryllium as is found, for example, with aluminium, titanium and zirconium. 

Figure 7.59 shows the standard free energies of formation of metal chlorides as a function of temperature . 

Fig. 18.4 The standard free energy of formation of various oxides as a function of...

The Gibbs-Helmholtz equation can be used to calculate the standard free energy of formation of a compound. This quantity, AGf, is analogous to the enthalpy of formation, AH . It is defined as the free energy change per mole when a compound is formed from the elements in their stable states at 1 atm. 

Tables of standard free energies of formation at 25°C of compounds and ions in solution are given in Appendix 1 (along with standard heats of formation and standard entropies). Notice that, for most compounds, AG is a negative quantity, which means that the compound can be formed spontaneously from the elements. This is true for water ...

Values of the standard free energies of formation at T— 298.15 K for selected substances are summarized in Table 9.1. 

Equations for the Standard Free Energy of Formation and Partial Molar Free Energies of Atomic Oxygen for Plutonium Oxides (1600-2150 K), cal/mol... 

Plot a graph of the standard free energy of formation of the hydrogen halides against the period number of the halogens. What conclusions can be drawn from the graph ... 

There are two ways to calculate A CJreaction ttiethod uses standard free energies of formation and... 

In 1944 Ellingham compiled, for the first time, diagrams depicting the temperature dependence of the standard free energies of formation of numerous oxides and sulphides. In the discussion presented here, attention will be confined to the oxide reaction... 

In Figure 4.10 are shown the lines corresponding to the standard free energies of formation of a metal oxide and of carbon monoxide also shown is the line for the free energy change associated with the reaction... 

The chlorination of titanium dioxide (titania) is thus entirely feasible at 900 °C. Similarly, many other metal oxides can be converted to metal chlorides by reaction with chlorine in the presence of carbon. It should be noted that carbon itself is not easily chlorinated as the standard free energy of formation of carbon tetrachloride is positive at temperatures above 500 °C. 

The copper remaining in lead after the above operation is removed by matte formation. Finely divided sulfur is added to molten lead at temperatures slightly above its melting point, and the melt is stirred continuously. Copper sulfide forms and floats on the surface, leaving the bullion substantially free of copper (less than 0.005%). The standard free energies of formation of cuprous sulfide and lead sulfide are about the same the observed separation must, therefore, be due to kinetic factors or to the influence of certain minor impurities that are present in the lead. 

Here, P represents the partial pressure, a the activity, and AG° the standard free energy of formation of the species indicated by the subscripts. The activity of oxygen in both above the relationships is the same. Hence, it follows that... 

Table 6.1 Standard free energy of formation data for ammonia synthesis.

Tabulated data are available for standard free energy of formation at different temperatures. 

In a later publication,96 the standard free energy of formation of the products, AG in V, was used instead of AH in Eq. (23) so that comparisons could be made with the commonly reported efficiencies of solid state solar cells. For the reduction of carbon dioxide to organic compounds, the optical conversion efficiency of the system is the sum of the efficiencies for each product. Thus, it can be given as... 

A Since the reaction is taking place at 298.15 K, we can use standard free energies of formation to calculate the standard free energy change for the reaction ... 

If there is no agreement in calculated and observed solid-solution properties we can only conclude that equilibrium was not established. The validity of the provisional activity coefficients depends on the validity of the original assumption that stoichiometric saturation was established. If independent data for the standard free energy of formation of the solid... 

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