Elements standard free energy

The standard free energy of formation of a substance is the standard free energy of reaction per mole of compound when it is formed from its elements in their most stable forms. The sign of AG f tells us whether a compound is stable or unstable with respect to its elements. Standard free energies of formation are used to calculate standard reaction free energies by using Eq. 21. 

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 ...

A more quantitative indication of the tendency of these elements to react with oxygen may be obtained from Fig. 6.9, where, for several metals, the standard free energies of formation of their oxides are shown. The large negative values relevant to these oxides may be noticed. 

The standard free energy of formation of an element in its most stable form is always zero, just like the standard enthalpy of formation of an element. 

In the literature (e.g., Thauer et al. 1977 Hanselmann, 1991) you find the AfG°(aq) values at 25°C for all the species involved in reaction Eq. 1. Note that by convention, the free energies of formation of the elements in their naturally occurring most stable form, as well as of the proton in aqueous solution, are set to zero. From these values, calculate the standard free energy of reaction Eq. 1 ... 

The free-energy change in forming a compound at temperature T in its standard state from its elements in their standard states (also at temperature T) is defined as AG°p the standard free energy of formation for a chemical compound. The standard state for a gaseous species is a pressure of 1 bar, denoted p°. 

The standard free energy of formation of a compound is A Gy. It is the standard-state free energy G° of the compound minus the standard-state free energy of the elements from which the compound is formed. Again, from this definition, it must be that case that AG°j(T) = 0 for the elements (in their most stable form) at every T. However, unlike H° for the elements, which are defined to be zero at one temperature Tr, in general, G°(T) 0 for any particular reference condition. 

The standard free energy of formation of HI is therefore +1.69 kj-mol-1, in good agreement with the value quoted in the text. Note that, because this value is positive, the formation of pure HI from the elements is not spontaneous. Pure HI has a tendency to decompose into its elements at 25°C. 

A thermodynamically stable compound is a compound with a negative standard free energy of formation (like water). A thermodynamically unstable compound is a compound with a positive standard free energy of formation (like benzene). Such a compound has a thermodynamic tendency to decompose into its elements. However, that tendency may not be realized in practice because the decomposition may be very slow. Benzene can, in fact, be kept indefinitely without decomposing at all. 

FIGURE 7.24 The standard free energies of formation of compounds are defined as the standard reaction free energy for their formation from the elements. They represent a thermodynamic altitude with respect to the elements at sea level. The numerical values are in kilojoules per mole. 

The standard free energy of formation, AG°f, of a substance is the free-energy change for formation of one mole of the substance in its standard state from the most stable form of its constituent elements in their standard states. For example, we found in Section 17.8 that the standard free-energy change AG° for the synthesis of 2 mol of NH3 from its constituent elements is -33.0 kj ... 

Values of AG°f at 25°C for some common substances are listed in Table 17.3, and additional values are given in Appendix B. Note that AG°f for an element in its most stable form at 25°C is defined to be zero. Thus, solid graphite has AG°f = 0 kj/mol, but diamond, a less stable form of solid carbon at 25°C, has AG°f = 2.9kJ/mol. As with standard enthalpies of formation, AH°f, a zero value of AG°f for elements in their most stable form establishes a thermochemical "sea level," or reference point, with respect to which the standard free energies of other substances are measured. We can t measure the absolute value of a substance s free energy (as we can the entropy), but that s not a problem because we are interested only in free-energy differences between reactants and products. 

The standard free energy of formation of a substance measures its thermodynamic stability with respect to its constituent elements. Substances that have a negative value of AG°f, such as carbon dioxide and water, are stable and do not decompose to their constituent elements under standard-state conditions. Substances that have a positive value of AG°f, such as ethylene and nitrogen dioxide, are thermodynamically unstable with respect to their constituent elements. Once prepared, though, such substances can exist for long periods of time if the rate of their decomposition is slow. 

A substance with a negative / V I standard free energy of formation with respect to decomposition to its constituent elements is stable. However, the substance may be unstable with respect to some other reaction under the same set of conditions. 

In plotting AG -T diagrams Ellingham made use of AG which is the standard free energy of formation of the compound, not per mole of the compound, but per mole of the gaseous element consumed. For instance,... 

Calculate the values of standard heat of formation AHj and standard free energy of formation AG j of 2-methyl propene (isobutene) from the elements at 400 K (260°F) ... 

The standard free energy of formation of a compound at a temperature T from the elements same temperature is expressed as... 

C,T = standard free energy of a compound or element at temperature T... 

Standard free energy change the change in free energy that will occur for one unit of reaction if the reactants in their standard states are converted to products in their standard states. (10.9) Standard free energy of formation the change in free energy that accompanies the formation of one mole of a substance from its constituent elements with all reactants and products in their standard states. (10.9)... 

---