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Stability diagrams

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Equations (9.33a) and (9.33b) are plotted in Fig. 9.6. These plots are called stability diagrams because the lines separate regions in which only one of the two phases is present. The line represents the p - T combinations where both the metal and its oxide coexist. The oxide-metal stability diagram is similar to the p-T phase diagram of a single substance such as water, where lines separate existence regions of solid, liquid, and vapor phases (see Figs. 5.1 and 5.3). The zones above and below the lines in Fig. 9.6 represent regions in which Eq (9.31) is not in equilibrium. If a gas phase containing an O2 pressure above the line is imposed on an initial M+MO2 mixture, all M will be oxidized and only MO2 will remain. Conversely, p0 - T points below the lines represent conditions where only M is stable.

Note that the slope of the line in Fig. 9.2b is positive, which from Eq (9.33b), implies that AS° is negative. This is consistent with the rule of thumb for AS° discussed in Sect. 9.3; the reactant side of the metal oxidation equation has one gaseous molecule and the product side

intercept = AHo