Question

What are homogeneous equilibria? Heterogeneous equilibria? What is the difference in writing K expressions for homogeneous versus heterogeneous reactions? Summarize which species are included in K expressions and which species are not included.

Short answer

A homogeneous equilibrium occurs when all species are in the same phase simultaneously.

A heterogeneous equilibrium is an equilibrium in which species can exist in more than one phase.

All reactants and products are written explicitly in the equilibrium constant, the K formula for a homogeneous equilibrium. All reactants and products that are either gas phase or aqueous phase are written in the K formula for a heterogeneous equilibrium because equilibrium is independent of the amounts of pure solids or liquids present.

Solutes (in the aqueous phase) and gases are written in K expressions, while pure liquids and solids are not included in K expressions due to the unit activity of pure liquids and solids.

Step-by-step solution

Homogenous and heterogenous equilibrium

A homogeneous equilibrium occurs when all species are in the same phase at the same time. For examples,

${\text{N}}_{\text{2}}\left(\text{g}\right){\text{+ 3H}}_{\text{2}}\left(\text{g}\right)\stackrel{}{\rightleftharpoons {\text{2NH}}_{\text{3}}\left(\text{g}\right)}$ …(1)

All species are in the gas phase.

A heterogeneous equilibrium is an equilibrium in which species can exist in more than one phase. For example,

${\text{CaCO}}_{\text{3}}\left(\text{s}\right)\stackrel{}{\rightleftharpoons \text{CaO}\left(\text{s}\right){\text{+ CO}}_{\text{2}}\left(\text{g}\right)}$ …(2)

Calcium carbonate and calcium oxide are present in solid states, while carbon dioxide is present in the gas phase.

Difference in K expression for homogenous and heterogeneous equilibria

All reactants and products are written explicitly in the equilibrium constant, the K formula for a homogeneous equilibrium. All reactants and products that are either gas phase or aqueous phase are written in the K formula for a heterogeneous equilibrium because equilibrium is independent of the amounts of pure solids or liquids present.

Solutes (in the aqueous phase) and gases are written in K expressions, while pure liquids and solids are not included in K expressions due to the unit activity of pure liquids and solids.