Temperature Dependence of Solubility Product Constant

In addition to the direction of reaction, AG also indicates whether the solubility of a given oxide will be adequate. Consider, for example, the case of dissolution of alumina. Its dissolution reaction in a neutral region is given by Eq. S.lOd 

Calculations show that AG for this reaction is 188.02 J/mol, which is considerably larger than that for the dissolution of MgO. This calculation implies that a significant amount of heat will be required to dissolve alumina hence, it will not dissolve on its own. Therefore, it may not be possible to develop a ceramic with alumina unless its solubility is enhanced by providing heat. A method of solubility enhancement at warm temperatures ( 200°C) is discussed in Chapter 11. 

The conditions under which spontaneous reactions can occur are listed below  

2) AH 0, A5 0, spontaneous at high temperatures, where AG 0 otherwise, nonspontaneous. 

The first condition implies that for a dissolution reaction to be useful, AH cannot be negative, and at the same time AS cannot be positive, and the fourth condition should be satisfied. 

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