Solid Solutions and Open Systems

When reaction 3.37 reverses during respiration, 1 mol of glucose liberates —2879 kJ of available energy to fuel the life processes of living organisms. 

A third important facet of the Gibbs free energy is its relation to the equilibrium constant of a reaction. A reaction proceeds until the components are at their lowest energy level, the most stable state. This state is defined by the equilibrium constant K  

Equation 3.41 requires that the standard states of the products and reactants be known, that the components can be defined quantitively and in a thermodynamic sense. In soils and much of nature these definitions are rarely possible. The states of ions or molecules in soil systems, and in probably all colloidal systems, are ill-defined thermodynamically. In rigorous thermodynamic terms even ions are undefined. Soil reactions, because of the nonequilibrium in soils and the lack of defined standard states, yield reaction coefficients, rather than reaction constants, and their values vary with soil conditions. 

Strictly speaking, thermodynamics applies only to total equilibrium and to closed systems. A closed system gains and loses no matter during the reaction. Soils steadily lose matter during weathering and gain matter by aerial deposition, rain, and tectonic movements. Soils also are not at total equilibrium the world would be sterile if nature were at total equilibrium. 

Soil chemistry cannot afford the luxury of rigorous thermodynamics and instead has to stretch and bend the rules into what is called extrathermodynamics, but the bending and stretching must be done wisely. Cation exchange, solubility, Donnan, and adsorption studies find use in soils and assume equilibrium and a closed system during the experiment. Applying the thermodynamics of solid solutions to the reaction between the soil solution and soil particles requires the same rule bending as the other studies. 

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