Nonspontaneous dissolution

Consider the case of dissolution of MgO given by Eq. 6.22. AH and AS in this case are, respectively, — 77.67 kJ/mol and — 272.2 J/mol/K. These numbers will satisfy the third condition, so dissolution will be nonspontaneous at low temperatures, but can be spontaneous at higher temperatures. To form a good ceramic at a large scale, the temperature for Eq. 6.22 needs to be controlled such that it will not only maintain a nonspontaneous dissolution reaction but will also prevent water from boiling. As mentioned before, the whole ceramic sets at 55°C, and hence this condition is easily satisfied. 

The example of MgO represents most of the candidate oxides because they have negative ASq and exhibit an exothermic dissolution reaction. Exceptions include oxides of the monovalent alkaline metals such as Na, K, and Cs, and a few higher valent metals. Since the hydroxyl ion also has a negative ASq, the dissolution reactions of the MgO-like oxides satisfy AS < 0 and conform to the third condition and hence, these oxides will exhibit nonspontaneous dissolution at low temperatures but may be spontaneous at high temperatures. 

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