Writing Equilibrium Expressions

In order to write the equilibrium expression for a system in a state of equilibrium you need to know:

Writing Expressions for Kc

The general equilibrium expression for a reaction:

aA + bB -->cC + dD

is written as:

General Equilibrium Expression

The brackets "[ ]" represent the concentration of the species (moles per liter or molarity). "a, b, c, and d" represent the coefficients used to balance the equation. The "c" in Kc indicates that the value of K is determined using the concentrations of each species.

There are two cases when a species is not shown in the equilibrium expression:

Writing Expressions for Kp

When one or more of the species in a system exists in the gaseous phase, the partial pressure of that species can be used in the equilibrium expression  Dissolved species are still expressed as moles per liter (molarity).



Examples of equilibrium expressions Kc for a variety of equilibrium systems follow. When one or more gaseous substances are involved, the Kp expression is also given.

Equation and equilibrium expressions Kc and Kp for water evaporation


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