Quasi-chemical equilibrium

Here we are considering the dynamic equilibrium between molecular species in the gas phase and the adsorbed gas species on a surface. Let us consider the following quasi-chemical equilibrium between the species B in the gas, Bg, and the available sites at the surface of the adsorbate ... 

Some earlier thermodynamic studies on rutile reported expressions involving simple idealized quasi-chemical equilibrium constants for point defect equilibria (see, e.g., Kofstad 1972) by correlating the composition x in TiOx with a function of AGm (O2), which is the partial molar free energy of oxygen. However, the structural effects were not accounted for in these considerations. Careful measurements of AGm (O2) in the TiOjc system (Bursill and Hyde 1971) have indicated that complete equilibrium is rarely achieved in non-stoichiometric rutile. 

The defect structure of wiistite can be discussed from the view of a quasi-chemical equilibrium among defects, similar to the case of Nij O. Assuming that the predominant defects are iron ion vacancies, we obtain the following equations ... 

In addition, several other models have been used with method I to calculate binary or ternary phase diagrams (183, 188-201). Among these models are the quasi chemical equilibrium model (188,190), truncated Mar-gules expansions (183,191, 192), Gaussian formalism (193), orthogonal series... 

It is well known that these vapors cannot be considered as ideal gases, as the second virial coefficients are different from zero, except in the case of cesium at high temperatures (Nieto de Castro etal. 1990). However, the quasi-chemical equilibrium hypothesis can be used that states that the imperfection has its origin in the atom association, and therefore the mixture of monomers and dimers can be considered a perfect gas mixture (Ewing et al. 1967). Assuming also local chemical equilibrium, Stogryn Hirschfelder (1959) considered that the heat of reaction could affect the reactive contribution and found that the chemical reaction component is given by... 

Such an equilibrium is known as a quasi-chemical equilibrium. 

We will assume that this condition is satisfied if X takes the value and that this value results Ifom a quasi-chemical equilibrium, which would be written as ... 

Examination of the pre-exponential term in the quasi-chemical equilibrium constants... 

In this model, the water molecules occupy specific crystallographic positions, and some of those positions are not occupied, thus constituting water vacancies. In the most usual case of associated defects q and q, the quasi-chemical equilibrium is written as follows ... 

The law of mass action applied to this quasi-chemical equilibrium is written as ... 

The number of independent components is 2 (the oxide and the gas), the number of solid phase is 1, and the variance is therefore 2. The content of interstitial ions is a function of oxygen pressure and temperature. We will reconsider quasi-chemical equilibriums in the solids in the following text (see section 3.6). 

We realize that the more the number of defects taken into account in the solid, the more the number of quasi-chemical equilibriums and the more conqtlex the system becomes. Brouwer recommended a method of usually very sufficient approximation. It consists of considering the expression of electric neutrahty in oidy the two majority enhhes (one on each side of the equality). 

---