## Oxygen isobars on a phase diagram

Phase diagrams are a convenient vehicle for displaying the equilibrium oxygen pressures generated by a metal and its oxides. In elements with multiple oxidation states and/or crystal structures, the equilibrium may not involve only the metal and an oxide, as in the MO2/M couple discussed in the previous section. In particular, two-phase regions separating two different oxides are represented by reactions of the following type:

The stoichiometric coefficients w and z are determined by balances on M and O:

wm = m', or w = m'/m wn + 2z = n', or z = %(n'-nm'/m)

the integers m, n, m' and n' characterize the two oxides. The equilibrium oxygen pressure is dependent on the free energy of formation of the above reaction:

po is actually a surface in the 3rd dimension of the temperature-composition plane; it is a more complex version of the equation-of-state surface of water (Fig. 2.7). As in the case with the EOS of water, projection of the po surface onto the T, O/M plane permits semi-quantitative use of the information contained the surface.

Figure 8.14 displays the po projection onto the Fe/O phase diagram. This diagram is the two-component analog of the familiar diagrams for a single-component substance such as water (Fig. 2.8). In single-phase regions, po is a function of T and O/Fe; in the two-phase zones, po is a function of temperature only (the phase rule: add a phase, lose a degree of freedom)

The 4 two-phase zones in Fig. 8.14 control the oxygen pressure by the following equilibria:

In the iron-rich region

In the oxygen-rich region

13^Fe1—xO(wustite) + J^gy O2 — Fe304(magnetite) ^2yFe3—yO4 (magnetite) + y) O2 — Fe2O3(hematite)

The stoichiometric coefficients in the above reactions were determined by the method described at the beginning of this section.

## Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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