Acids and Bases: Ionization Constants


Table of Contents


Molecular Structure

Ionization Constants



Lewis Theory


  • Write ionization constant expressions for acids and bases
  • Compare strengths of acids and bases using ionization constant values
  • Calculate the ionization constant of an acid or base from the pH of an aqueous solution of known concentration
  • Calculate the expected pH of an acid or base solution using the ionization constant
  • Determine the relationship between ionization consants of conjugate acid-base pairs
  • Predict whether an acid-base reaction will be product or reactant favored

How strong is a strong acid?

An acid or base's strength refers to its degree of ionization. A strong acid will completely ionize in water while a weak acid will only partially ionize.

Since there are different degrees of ionization, there are different levels of weakness. Fortunately, there is a simple quantitative way of expressing this.

Since the ionization of a weak acid is an equilibrium, a chemical equation and an equilibrium constant expression can be written:

The equilibrium constant for the ionization of an acid is called the acid ionization constant (Ka) .

Complete the following sentence:

The stronger the acid, the

the Ka.

A stronger acid will be a better proton donor, forcing the equilibrium to the right. This produces more hydronium ions and conjugate base. How will this affect Ka?

Good! A strong acid is a better proton donor, resulting in more products. Since the concentration of the products is in the numerator of the Ka expression, the stronger the acid, the larger the Ka.

A similar expression can be written for bases:

The equilibrium constant for the ionization of a base is called the base ionization constant (Kb) .

Click on the strongest base in the list below by comparing their Kb values.



Ammonia, NH3

1.8 x 10-5

Hydroxylamine, HONH2

9.1 x 10-9

Ethylamine, C2H5NH2

4.3 x 10-4

A stronger base will be better at accepting protons from water. Hence more hydroxide ions and conjugate acid will be formed, or the equilibrium will lie further to the right. Will this result in a larger or smaller Kb?

Good! The larger the Kb, the stronger the base.