Reactions involving Gases and Pure Solids or Liquids

Reactions Involving Gases and Pure Solids or Liquids 

For this class of chemical reactions we will assume that all gases are ideal and that all hquids and solids have constant volumes. We first consider reactions involving only gases. For an ideal gas. 

We have disregarded the negative root to the quadratic equation, since a negative value of a is not physically possible if no NO2 is initially present. The total amount of gas is (1.000mol)(l — a + 2a) = 1.174mol, so the equilibrium total pressure is 

It is always the case that when a quadratic equation is solved in a chemistry problem, only one of the two roots corresponds to a physically possible situation. 

We now consider reactions involving pure liquids and solids as well as gases. If the pressure of the system does not differ very much from P°, then Eq. (6.3-14) gives for the activity of a pure liquid or solid 

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Section 7.2 Reactions Involving Gases and 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 ... 

In the reactions described so far, all the reactants and products have been gaseous the equilibrium systems are homogeneous. In certain reactions, at least one of the substances involved is a pure liquid or solid the others are gases. Such a system is heterogeneous, because more than one phase is present. Examples include... 

LeChateher s principle (Section 18.3) allows us to make qualitative predictions about the effects of changes of conditions on an equihbrium system but does not ahow quantitative calculations. However, at equihbrium at a given temperature, a certain ratio of concentration terms is very nearly constant for all solutes and gases involved in any given reaction. (Solids and pure liquids are not included in the ratio.) Because it is not exactly constant, we will use two or at most three significant digits in equihbrium constant calculahons. For the general reaction... 

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. 

Most chemical reactions take place not between pure solids, liquids, or gases, but among ions and molecules dissolved in water or other solvents. In Section 4.1 we noted that a solution is a homogeneous mixture of two or more substances. Because this definition places no restriction on the nature of the substances involved, we can distinguish six types of solutions, depending on the original states (solid, liquid, or gas) of the solution components. Table 13.1 gives examples of each of these types. 

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... 

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