What are some equilibrium expressions that chemists use for weak acid equilibrium?
Chemists use equilibrium expressions to describe the balance between reactants and products in a chemical reaction. Specifically, for weak acid equilibria, we use the dissociation constant (Ka) to express the equilibrium.
The dissociation of a weak acid (HA) in water can be represented by the following equation:
HA ⇌ H+ + A-
To write the equilibrium expression using the concentration of reactants and products, we raise the concentration of the products (H+ and A-) to the power of their stoichiometric coefficients (generally 1 for weak acids) and divide it by the concentration of the reactant (HA):
Ka = [H+][A-] / [HA]
Note that the square brackets denote the concentration of the species in moles per liter (M). The Ka value represents the relative strength of the acid; higher Ka values indicate a stronger acid.
It's important to mention that the concentrations used in the equilibrium expression are typically taken at equilibrium, which means that the reaction has reached its balance point and the concentrations are constant.