Standard free enthalpy formation

Fig. 3-8. Energy for formation of the standard gaseous ions, S(Vnj), from the surface atoms of a semiconductor of single element S dGnbi = standard free enthalpy of the surface atom sublimation h = ionization energy of gaseous atoms aj. = unitary level of the surface ion = - (dGsM + /s) = unitary level of the surface atom referred to the standard gaseous ions and elections.

Just as the standard enthalpy change for a reaction can be calculated from the standard free enthalpies of formation of the reactants and products, the standard free energy... 

Figure 14.1 Standard free enthalpy (AG°) of formation (normal lines) of chromium oxides and carbides and standard free enthalpy (AG°) of reaction (lines with squares) of chromium with methane as a function of temperature.

The standard free enthalpy of oxygen atom formation is + 233 kj/mole (gas) and + 254 kj/ mole (aqueous solution), see Ref.1. ... 

The free enthalpy of formation of this sodium iron double oxide was estimated to be AH = —9.2 kJ mol at 600 °V. This low value indicates that the compound can only be formed in an oxygen solution in sodium, the chemical activity of which is close to unity. The standard free enthalpy of combination of the constituent oxides was measured by solution calorimetry ... 

The standard free enthalpy of formation to the compound as estimated from dissolution and thermal decomposition studies is... 

Free enthalpy of the formation of 1 mole of any substance imder standard conditions from atoms and up to the standard state is called standard free enthalpy (Gibbs energy) of formation. This potential is determined by way of subtle physicochemical experiments and measurements. Its values are continuously fine-tuned and published in articles, monographs and reference publications. In composite Tables the standard potential is usually provided for temperature of 298.15 K (25 °C) and denoted as AZp29s or AGp 298> its value, as a rule, is negative and has the dimension kcal mole or J mole Standard potential serves a measure of potential energy of inter-atomic or inter-molecular bonds in individual chemical compounds. Knowing its values, it is possible to determine free enthalpy of substances under any nonstandard conditions. 

Dissociation constant of this reaction under standard conditions (25 °C) may be calculated from values of standard free enthalpy of the formation of H, OH" and H O ... 

A pair of cation and anion is capable of forming k complex compounds, which coexist. Strength, therefore concentrations of these compounds are proportionate to value of their stability constants or standard free enthalpy of the formation of 1 mole ... 

In calculations, A G° = standard free enthalpy of formation, and AGf = free enthalpy of formation for the involved reactants and products.)... 

The apparent standard free enthalpy of formation can also be introduced ... 

Afil AfHf standard free enthalpy of formation of Cj standard enthalpy of formation of Cj standard free enthalpy of reaction i J mol l J mol l... 

Bomb-calorimetric and vapour-pressure measurements provide the standard free enthalpy of formation of diphenylcyclopropenone as 132 kcalmol . From their study the authors conclude that the strain energy is ca. 78 kcal mol , a value higher... 

The thermodynamic parameters for the double helix formation have been measured for a large number of synthetic polynucleotides and for native DNAs of different origin [77B1, 77H1, 8111, 78K1]. These data have been analyzed to provide the transition temperature T, the transition enthalpy per base pair, AH, the transition entropy, AS, and transition standard free enthalpy change AG° at room temperature. 

It is possible to expand this scheme to helix-helix displacement reactions, i.e. the mutual exchange of strands between two metastable helical complexes to form the two thermodynamically most stable double helical species out of the four single strands involved. This particular type of a displacement reaction can be illustrated by the following thermodynamic cycle [poly(rA)-poly(dU)] + [poly(dA)-poly(rU)] [poly (dA) poly(dU)] -I- [poly(rA) poly(rU)]. If all components are in their standard states the standard free energy can be listed for each of the helix-formation reactions. The difference between the standard free enthalpies of the final states and the initial states is the displacement free enthalpy AG of polyfdA) from the one and poly(rU) from the other helix. Energetics of strand-exchange between two helical complexes Cpoly(A) poly (B)]fc -I- [poly (C) poly [poly(A) poly (Q -I- [poly(B) polyfD)] ... 

CaS04 2H2O), cannot be calculated from the solubility produet (obtained from standard free enthalpy of formation) correctly, because an additional equilibrium between the dissolved ions (i.e., ion-pair formation) is present in the aqueous phase, which increases the dissolved amount of gypsum significantly [18],... 

The tri-iodide ion formation slightly decreases the apparent oxidizing power of iodine. From a practical standpoint, the value E° = 0.536 V must be principally retained since, most of the time, the used couple is 13 / for the sake of solubility. Going from the standard potential of one of these three couples to the two others is achieved according to the principle given in Chap. 2, that is, by handling the corresponding standard free enthalpies. 

The standard free enthalpy - of formation AfGAgci stands for MAgci M Mca2-... 

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. 

Key Terms enthalpy, H free energy of formation, AG standard entropy change, AS° entropy, S spontaneous process standard free energy change, AG° free energy, G... 

Standard Free Energies, Enthalpies, and Entropies of Formation of Palladium and Nickel Hydrides ... 

To find a numerical value for AHi, we need to know ArH° at one temperature, while evaluation of I requires ArG° at one temperature. The usual choice is to obtain ArH° and ArG° at T = 298.15 K from standard molar enthalpies of formation and standard molar Gibbs free energies of formation. Earlier in this chapter we referred to examples of these quantities. It is now time to define AfH° and AfG° explicitly and describe methods for their measurement. 

Standard molar enthalpies of formation, AfH°m, and standard molar Gibbs free energies of formation, Af(7, are useful, since they can be used to calculate ArH° and ArG°. The relationships are... 

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 ... 

Standard reaction enthalpy (X = H) and Gibbs free energy (X = G) from standard enthalpies of formation ... 

It is well known that such quantities as the standard free energy, enthalpy and entropy display a remarkable tendency to be additive functions of independent contributions of part-structures of the molecule. This property, on which the mathematical simplicity of many extrathermodynamic relationships is largely based, is well illustrated, for example, by the enthalpies of formation at 298°K of several homologous series of gaseous hydrocarbons Y(CH2)mH, which are expressed by the relation (28) (Stull et al., 1969). In... 

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