The amino acid example again

The only commonly used relation between pressure and the equilibrium constant is the case where all reactants and products are solids. Assuming the solids are incompressible, applying relation (5.32) to each of the terms in A,.G results in 

The value of A,G° of —20120 Jmol means that reaction (9.39) will also go to the right. But this reaction will continue to go (strictly, it should continue to go, according to our model) until either NaAlSi04(s ) or Si02(s ) is used up. Thus NaAlSi04(5) and Si02(s ) are not stable together - one of them must disappear. 

This is not the case with leucylglycine. We cannot say that leucylglycine is not stable in water-what happens to it depends entirely on its concentration and on the concentrations of other things in solution such as leucine and glycine. The unit activities are only a starting point, and a very unrealistic one at that. The next step is to calculate the equilibrium constant for (9.37) 

You cannot reliably tell which way the reaction will go by looking at A G° 

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