Writing equilibrium constant expressions

To use equilibrium constants, we must express them in terms of reactant and product concentrations. Our only guide is the Law of Mass Action, which is the general formula for finding equilibrium concentrations. However, because the reacting species are not always in the same phase, there may be more than one way to express the equilibrium constant for the same reaction. To begin with, we will consider reactions in which the reactants and products are in the same phase.

Homogeneous equilibria

The term homogeneous equilibrium applies to reactions in which all reacting species are in the same physical state.

For a homogeneous reaction, rules followed while writing the expression for the equilibrium constant are:

• The concentrations of all the substances formed as products are written in the numerator.
• The concentrations of all the reactants are written in the denominator.
• Each concentration term is raised to the power of its respective coefficient as written in the balanced chemical reaction.

Units of the equilibrium constant

In the equilibrium expression for a particular reaction, the concentrations are given in units of moles/litre or mol/L, and the partial pressure are given in atmospheres (atm). The units of Kc and Kp, depend on the specific reaction.The unit of Kc of a reaction depends upon the number of moles of the reactants and products involved in the reaction.

Heterogeneous equilibria

A heterogeneous equilibrium results from a reversible reaction involving reactants and products that are in different phases.

Rules for Writing Expression for the Equilibrium Constant for a Heterogeneous Reaction:

• The concentration terms of products are written in the numerator and the concentration terms of reactants are written in the denominator raised to the power of their respective coefficient in the balanced chemical equation.
• Concentration terms of solids and pure liquids do not appear in the expression, as they are taken to be unity.
• While writing the expression for K_p, partial pressures of only the gaseous reactants and products are written in the expression for the equilibrium constant.

Calculating equilibrium constants

If we can measure the equilibrium concentrations of all the reactants and products in a chemical reaction, calculating the value of the equilibrium constant is straightforward. We simply insert all the equilibrium concentrations into the equilibrium-constant expression for the reaction.

The following steps outline the procedure:

1. Tabulate all known initial and equilibrium concentrations of the species that appear in the equilibrium constant expression.
2. For those species for which initial and equilibrium concentrations are known, calculate the change in concentration that occurs as the system reaches equilibrium.
3. Use initial concentrations from step 1 and changes in concentration from step 3 to calculate equilibrium concentrations not tabulated in step 1.
4. Determine the value of the equilibrium constant.