Reaction Gibbs energy, aqueous systems, chemical equilibrium

We need to understand the concepts of Gibbs free enei gy (G) and chemical potential i/i) in order to know the direction of spontaneous change of a reaction or system. These concepts can also be used to define or predict the most stable (equilibrium) assemblage and gas, fluid, or rock compositions expected in a system at a given pressure and temperature. Some phases and aqueous species in a system may be out of equilibrium with that system. Free-energy calculations permit us to decide which substances are out of equilibrium, and, therefore, which concentrations may be governed by chemical kinetics. 

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