Bedded Down - Thermodynamic Stability

The process we are seeing in the copper(II)-ammonia solution is a ligand substitution process, where one ligand is replacing another. In general, we can represent this, for reaction in aqueous solution with a neutral monodentate ligand at this stage, by Equation (5.1)  

Introduction to Coordination Chemistry Geoffrey A. Lawrance 2010 John Wiley Sons, Ltd 

Since we have written this reaction as an equilibrium, it is possible to write an equilibrium 

Because water has a concentration of approximately 55 M, it varies by only trivial amounts for reactions of dilute species. As a result, it is convenient, and traditional, to leave out the solvent term and usually also to ignore coordinated water molecules. We shall adopt this representation henceforth - but do not assume we are dealing with bare metal ions This reduces Equation (5.1) to Equation (5.4)  

The equilibrium constant A in this case is called the formation constant or stability constant, since it is measuring formation of a metal complex, and defining its thermodynamic stability. Experimentally, because we measure K values under non-ideal (in a thermodynamic sense) conditions, the term constant here is not absolutely correct, as discussed above. Remember that it is defined only under the particular experimental conditions employed in reality, although it is fairly true to say that the value varies in only a limited way across the range of conditions that we are most likely to apply. 

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