Constant expression for heterogeneous

Example 16.4 - Writing Equilibrium Constant Expressions for Heterogeneous Equilibria... 

Explain why pure solids and liquids are left out of equilibrium constant expressions for heterogeneous equilibria. 

Strategy Because they are constant quantities, the concentrations of solids and liquids do not appear in the equihbrium constant expressions for heterogeneous systems. The total pressure at equilibrium that is given is due to both NH3 and CO2. Note that for every 1 atm of CO2 produced, 2 atm of NH3 will be produced due to the stoichiometry of the balanced equation. Using this ratio, we can calcrrlate the partial presstrres of NH3 and CO2 at eqtrilibritrm. 

We then learn how to write equilibrium-constant expressions for heterogeneous reactions. 

The equilibrium-constant expression for the equilibrium between a solid and an aqueous solution of its component ions is written according to the rules that apply to any other equilibrium-constant expression. Remember, however, that solids do not appear in the equilibrium-constant expressions for heterogeneous equilibrium, j- (Section 15.4) Thus, the solubility-product expression for BaS04, which is based on Equation 17.15, is... 

As with any other equilibrium, the extent to which this dissolution reaction occurs is expressed by the magnitude of the equilibrium constant. Because this equilibrium equation describes the dissolution of a solid, the equilibrium constant indicates how soluble the solid is in water and is referred to as the solubility-product constant (or simply the solubility product). It is denoted K p, where sp stands for solubility product The equilibrium-constant expression for the equilibrium between a solid and an aqueous solution of its component ions (Ksp) is written according to the rules that apply to any other equilibrium-constant expression. Remember, however, that soUds do not appear in the equihbrium-constant expressions for heterogeneous equihbrium. ooo (Section 15.4) Thus, the solubihty-product expression for BaS04, which is based on Equation 17.15, is... 

For a heterogeneous equilibrium (a chemical equilibrium with components in different phases), reactants or products may be pure liquids or solids. The concentration of a pure liquid or solid in moles/liter cannot change. It is a constant property of the material, and these constants are incorporated into the equilibrium constant. Therefore the concentrations of pure liquids and solids are absent from equilibrium expressions for heterogeneous equilibria. 

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,... 

This equilibrium is known as a heterogeneous equilibrium, which is to say that it consists of substances that are in different states (phases). If you recall from Chapter 13, the equilibrium constant expressions for such equilibria do not contain the concentrations of liquids or solids. The equilibrium expression for this reaction will describe the degree to which the solid dissolves in solution, which is another way to say the degree to which it is soluble. And since the solid is not shown in the equilibrium expression, the equilibrium constant will express the product of the concentrations of the dissolved solute ions. For this reason, the equilibrium constant is referred to as the solubility-product constant. For this equilibrium, the solubility-product constant, Ksp is ... 

You are given a heterogeneous equilibrium involving gases and solids. The general form of the equilibrium constant expression for this reaction is... 

Write equilibrium constant expressions for these heterogeneous equilibria. 

Heterogeneous Equilibrium). Hence, the concentration of the solid is not included in the equilibrium constant expression. For a saturated solution of BaSO in contact with solid BaSO, we write... 

As a prelude to the development of kinetic rate expressions for heterogeneous chemical reactions, if A reacts with B, for example, then the next step in the mechanism is ha + Ba, forming an activated complex on the snrface. Each reversible step in the seqnence above is characterized by a forward rate constant adsoiption for adsoiption, with units of mol/area time atm, and a backward rate constant A ,desoiption for desorption, with units of mol/area time. The ratio of these rate constants adsorption/ h, desoiption defines the adsorption/desorption equi-... 

Many reactions occur in a series of steps, and the overall rate may not be described by a simple equation with a constant reaction order. Some of the many types of complex rate expressions for heterogeneous catalysts are discussed in Chapter 2. For homogeneous reactions, two examples of complex kinetics are enzyme reactions and chain reactions. 

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... 

Write the equilibrium-constant expression for a heterogeneous reaction (Section 15.4). 

We then learn to write the equihbrium constant expression for homogeneous and heterogeneous equhibria. We see how to express equilibrium constants for multiple equhibria. (14.2)... 

In writing the equilibrium constant expression for a heterogeneous equilibrium, the concentrations of pure liquids and solids are considered to be one. So the equilibrium constant expression for the equilibrium above is the following ... 

In writing the equiHbrinm-constant expression for a heterogeneous equilibrium, you omit concentration terms for pure solids and liquids. For the reaction of iron with steam, you would write... 

In contrast to gases, the concentrations of pure solids and pure liquids are constant they do not change. Therefore, pure solids and liquids are not included in the equilibrium constant expression. For this heterogeneous equilibrium, the expression does not include the concentration of CaCOs (x) or CaO (x). It is written as = [CO2 ]. 

An equilibrium in which the reactants and products of a reaction exist in more than one physical state is called a heterogeneous equilibrium. The equiUbrium constant expression for a heterogeneous equilibrium is similar to that for a homogeneous equilibrium, except that the concentrations of pure solids and pure liquids are eliminated... 

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