The treatment of chemical equilibrium in the preceding sections of this chapter gives the impression that all that is required is searching the available database for AGo of the reaction, using this information to calculate the equilibrium constant and then applying the law of mass action. This approach may also require estimation of activity coefficients if solid or liquid solutions are involved, and specification of the total pressure if gas mixtures are part of the reaction. This equilibrium condition always needs to be supplemented by specification of the ratios of the elements involved in the reaction, irrespective of their molecular forms.
The equilibrium aspects of the above method are less obvious when the process involves a solution in which one of the components exists in a form different from its normal pure state. Such a situation occurs when gases dissolve in condensed phases. The dissolved gas species behave either like a liquid component if incorporated in a liquid solvent or as a solute in a crystal when dissolved in a solid. In either case, there is no pure condensed phase of the diatomic gas to serve as a standard state. However, the normal gaseous state can still be used as a standard state. The consequence in the equilibrium equation is an activity coefficient of the dissolved gas that is very far from unity (see below).
There are two distinct mechanisms by which gases dissolve in condensed phases: dissociative and nondissociative. The latter refers to physical dissolution without change in the molecular structure of the gas. Equilibrium of O2(g) between air and water is an example of nondissociative dissolution of a gas. The thermodynamics of this type of solution process is treated in Sect. 8.3.5.
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