Equilibrium expression involving pure solids

Substituting the appropriate ideal expression for the activity of gaseous or dissolved species from Equation 14.8a or 14.8b leads to the forms of the mass action law and the equilibrium constant K already derived earlier in Section 14.3 for reactions in ideal gases or in ideal solutions. We write the mass action law for reactions involving pure solids and liquids and multiple phases by substituting unity for the activity of pure liquids or solids and the appropriate ideal expression for the activity of each gaseous or dissolved species into Equation 14.9. Once a proper reference state and concentration units have been identified for each reactant and product, we use tabulated free energies based on these reference states to calculate the equilibrium constant. 

We can also write an equilibrium constant expression for this reaction, where the equilibrium concentrations of the products appear in the numerator and the equilibrium concentrations of the reactants appear in the denominator. The equilibrium constant expression does not involve pure solids or pure liquids (including water). 

Writing Equilibrium Constant Expressions for Reactions Involving Pure Solids or Liquids... 

Chemical equilibria often involve pure liquids and solids in addition to gases and solutes. The concentration of a pure liquid or solid does not vary significantly. Figure 16-4 shows that although the amount of a solid or liquid can vary, the number of moles per unit volume remains fixed. In other words, the concentrations of pure liquids or solids are always equal to their standard concentrations. Thus, division by standard concentration results in a value of 1 for any pure liquid or solid. This allows us to omit pure liquids and solids from equilibrium constant expressions. For a general reaction (2A + iBt= C D-l-. S where S is a pure solid or liquid ... 

We omit concentrations of pure solids and pure liquids from equilibrium constant expressions because their activity is taken to be 1 and the thermodynamic equilibrium constant involves activities, rather than concentrations. 

Note that the net effect of inserting an activity of 1 into the equilibrium expression for each pure solid or liquid in the reaction has the same effect as simply disregarding them. If pure solids or pure liquids are involved in a chemical reaction, their concentrations are not included in the equilibrium expression for the reaction. This simplification occurs only with pure solids or liquids, not with solutions or gases, because in these last two cases the activity cannot be assumed to be 1. 

Equilibrium constant calculations are rather straightforward, but we must keep several things in mind. (1) The values in the equilibrium constant expression are the concentrations at equilibrium. (2) The equilibrium constant expression has no addition or subtraction within it. (3) All the substances involved in the equilibrium constant expression are in the same solution (or the same gas mixture), and so have the same volume. (4) Pure solids and liquids, and the solvents for dilute solutions, do not appear in the equilibrium constant expression. (5) The equilibrium constant expression is written for a specific equilibrium equation if we reverse the equation, the value of the new K is the reciprocal of the original one. 

The concentrations of pure solids or pure liquids involved in a chemical reaction are not included in the equilibrium expression for the reaction. 

This system consists of a solid in equilibrium with two aqueous species. If we want to write the equilibrium-constant expression for this process, we encounter a problem we have not encountered previously How do we express the concentration of a solid Although we can express that concentration in moles per unit volume, it is unnecessary to do so in writing equilibrium-constant expressions. Whenever a pure solid or a pure liquid is involved in a heterogeneous equilibrium, its concentration is not included in the equilibrium-constant expression. Thus, the equilibrium-constant expression for the reaction of Equation 15.18 is... 

Many chemical processes involve heterogeneous reactions in which reactants or products are in different phases. The concentrations of pure solids and liquids do not change, and by convention are not written in the equilibrium expression. Also, in a system involving acids and bases, when a solvent such as water is in an equilibrium equation, it is not included in the equilibrium expression. In an earlier chapter, the expression for used this convention, and the concentration of water is not included in the expression. The reaction representing the self-ionization of water is... 

In this and succeeding chapters, a wide variety of different types of equilibria will be covered. They may involve gases, pure liquids or solids, and species in aqueous solution. It will always be true that in the expression for the equilibrium constant—... 

We use a different measure of concentration when writing expressions for the equilibrium constants of reactions that involve species other than gases. Thus, for a species J that forms an ideal solution in a liquid solvent, the partial pressure in the expression for K is replaced by the molarity fjl relative to the standard molarity c° = 1 mol-L 1. Although K should be written in terms of the dimensionless ratio UJ/c°, it is common practice to write K in terms of [J] alone and to interpret each [JJ as the molarity with the units struck out. It has been found empirically, and is justified by thermodynamics, that pure liquids or solids should not appear in K. So, even though CaC03(s) and CaO(s) occur in the equilibrium... 

The mass action law assumes that the reaction medium is homogeneous. In heterogeneous reactions (involving different substances in multiple phases), the densities and effective concentrations of pure condensed phases (liquids or solids) are constant. The concentrations of such species are set to unity in the equilibrium constant expression for such reactions. For example, given the following decomposition,... 

In the cases of the pure modes (or mixed) where the components of the solid solution intervene in the equilibrium conditions of the non-determining steps, concentrations of the components of the solid solution, which are not constant any more, will intervene in the expressions of the mass action law of the steps which involve one of these components. This is also true for the equilibrium of the total reaction. 

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