Reactions with Metals

The chemical reactions of alumina with other substances can best be explored from the thermodynamic properties of these reactions. If the Gibbs free energy AG of the reaction at a particular temperature is negative, the reaction tends to take place, and if AG this energy is positive the reaction tends not to occur. These considerations are modified by concentrations (more properly, thermodynamic activities) of the compo­nents as expressed in an equilibrium constant of the reaction. See books on thermody­namics for more details, for example [45].

The relative Gibbs free energies of the reactions of metals with oxygen tell whether or not a particular metal will displace the aluminum in alumina. The reaction of aluminum with oxygen is

4 2

4 Al + O2 = 2 Al2O3 (12)

3 2 3 2 3

All of the oxidation reactions with metals are written with one mole of O2 reacting for consistent comparison. This is the format for reactions plotted in an Ellingham dia­gram (see [46]). The Gibbs free energies of some of these oxidation reactions are given in Table 20, taken from [28, 29, 46-48]. If the Gibbs free energy of the reaction is higher than that of aluminum (-845.6 kJ mol-1 for reaction (12)), then this metal will react with alumina, displacing all the aluminum in any alumina in contact with the metal, either solid, liquid, or vapor. For example for yttrium:

4Y + O2 = 2 Y2O3 (13)

3 2 3 2 3

the Gibbs free energy is -1017 kJ mol-1, so yttrium will displace aluminum from alumina:

2Y + Al2O3 = 2Al + Y2O3 (14)

The tree energy change of reaction 14 can be deduced from the values for Eqs. (12) and (13) from Table 20 to be -256 kJ mol-1, showing the tendency for yttrium to displace aluminum. If the free energy shown in Table 20 is less than that for oxidation of aluminum, the metal will not react with alumina. Thus at 1,000°C, sodium and potassium vapors (1 atm) do not react with alumina, but 1 atm. of lithium vapor does. Liquid alkaline earths metals such as Mg, Ca, Sr, and Ba react with alumina at 1,000°C and displace aluminum metal. The relative tendency of reactions of solid and liquid metals with alumina does not change much with temperature. Of course at low temperatures (below about 500°C), the rates of reactions can be slow, even if the thermodynamics show a tendency to react. Gibbs free energies for other temperatures can be calculated from data in the thermodynamic tables in [28, 29, 46-48].