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illustrates the application of Table 9.4

Example: Calculate the free energy of formation of Al2O3(s) (per mole of O2) at 1000 K from the graphical, analytic, and tabular data sources.

The formation reaction is: 4/3Al(L) +O2(g) = 2/3 Al2O3(s).

From Fig. 9.8: AG" = -220 x 4.184 = -920 kJ/mole.

AGo = 2/3(-1,678,000-7x1000ln1000+386x1000)x10-3 = -907 kJ/mole

From Table 9.4:

AGo = 2/3gA^2o - 4/3gAl(L) - go — [2/3(-424.8) - 4/3(-10.2) - (-52.8)]x4.184=-908kJ/mole All three sources are in good agreement.

The databases represented by Fig. 9.8, tables 9.3 and 9.4 assume that the total pressure is low enough not to influence AG of a reaction. However, like other thermodynamic properties, AG of a reaction is a function of both T and the total pressure p. the effect of the latter is small, so that AG is very nearly equal to AGo (i.e., for p = 1 atm) even if the total pressure is very high (see problem 9.19).

When a metal reacts with a gas to form a compound, the thermodynamic property of the reaction (AGo) is dependent on the phase of the metal. Problems 9.16 and 9.22 analyze the effect on AGo of changing the metal reactant from solid to liquid.

Table 9.4

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