HETEROGENEOUS EQUILIBRIA

The equilibrium reactions in which the reactants and the products are not present in the same phase are called heterogeneous equilibrium reactions. Such equilibria in which the reactants and products are not all in the same phase are called heterogeneous equilibria. Heterogeneous equilibrium is demonstrated by the breakdown of calcium carbonate into calcium oxide and carbon dioxide when heated. The following equilibrium is obtained if the reaction is carried out in a closed vessel.

CaCO3(s)CaO(s) + CO2(g)CaCO3(s)CaO(s) + CO2(g)

 

Some other examples of heterogeneous equilibria are:

NH4Cl(s)NH3(g) + HCl(g)

3Fe(s) + 4H2O(g)Fe3O4(s) + 4H2(g)

●Quantitative Characteristics of Heterogeneous Equilibria

An important point to note is that in the case of heterogeneous equilibria, the expression for equilibrium constant does not include the concentrations of pure solids and pure liquids as their concentrations remain constant. In other words, the concentration (moles per unit volume) of a pure solid (or liquid) is fixed and cannot vary. Thus, the concentrations of pure solids or liquids are not included in the equilibrium constant expression. For example, consider the following heterogeneous reaction,

CaCO3(s)CaO(s) + CO2(g)

The equilibrium constant expression for this reaction is:

Kc = [CO2][CaO][CaCO3] CO2

But CaCO3 and CaO are pure solids. So, their concentrations may be assumed to be constant and can be ignored in the equilibrium constant expression. So the equilibrium constant expression for this reaction can be written as

Kc = [CO2]

Since the partial pressure of a gaseous component is proportional to its concentration,

Kp = pco2

As a result, we can deduce that when CaCO3 decomposes at a specific temperature, the equilibrium partial pressure of carbon dioxide gas equals its equilibrium constant in terms of partial pressure. In other words, there is constant pressure of carbon dioxide gas in equilibrium with calcium carbonate and calcium oxide at a particular temperature. So, it gives us a very easy method to find the value of Kp. For this, we need to measure the pressure of carbon dioxide gas in equilibrium with calcium carbonate and calcium oxide at a particular temperature.

Similarly, consider the following reaction:

2H2O(l)2H2(g) + O2(g)

After ignoring the concentration for liquid water, its equilibrium constant expression becomes:

Kc = [H2]2[O2]

Kp = (pH2)2(pO2)

●Some Important Points Regarding Heterogeneous Equilibria

For heterogeneous chemical reactions, the concentration of pure solids and pure liquids are not included in the expression for the equilibrium constant. By convention

[solid]=1 and [liquid]=1

When the substance occurs in the liquid solutions, their concentrations are variable, and therefore their concentrations are given in the expression for the equilibrium constant.

It must be noted that pure substances must be present (however small it may be) for the equilibrium to exist, but they do not appear in the expression for the equilibrium constant.

The concentration of the solvent is considered constant because it is present in large quantities and will not change appreciably.