Difference Between Acid Ionization Constant and Base Ionization Constant

Key Difference – Acid Ionization Constant vs Base Ionization Constant
 

Acid ionization constant (Ka, also known as the acid dissociation constant) gives the quantitative measurement of the equilibrium that exists between acid molecules and their ionized forms. Similarly, base ionization constant (Kb, or base dissociation constant) gives the quantitative measurement of the equilibrium that exists between the base molecules and their ionized forms. The key difference between acid ionization constant and base ionization constant is that acid ionization constant gives a quantitative measure of the strength of an acid in a solution whereas base ionization constant gives a quantitative measure of the strength of a base in a solution.

Ionization is the separation of molecules into ionic species (cations and anions). The equilibrium constant is the relationship between the amounts of reactants and products that are in equilibrium with each other.

CONTENTS

1. Overview and Key Difference
2. What is Acid Ionization Constant
3. What is Base Ionization Constant
4. Side by Side Comparison – Acid Ionization Constant vs Base Ionization Constant in Tabular Form
5. Summary

What is Acid Ionization Constant?

Acid ionization constant is the number that expresses the relationship between acid molecules and the ionic species that exist in the same solution. The acid dissociation constant is denoted by Ka. It is a quantitative measure of the strength of an acid in a solution. The strength of an acid depends on the ionization (or dissociation) of an acid in an aqueous solution.

The ionization of an acid can be given as below,

HA + H₂O    ↔    A^–   +   H₃O⁺

In this, HA is a weak acid that dissociates partially into ions; anion is known as the conjugated base of that particular acid. Acid dissociation releases a proton (hydrogen ion; H⁺). This proton combines with a water molecule forming a hydronium ion (H₃O⁺). The acid ionization constant of this HA acid can be given as below,

Ka = [A^–][ H₃O⁺] / [HA][ H₂O]

The common form of K^_a is pKa, which is the minus log value of Ka. That is because the Ka values are very small values and are difficult to handle. The pKa gives a simple number that is easy to deal with. It can be given as below,

pKa = -log(Ka)

The Ka or pKa values can be used to express the strength of an acid.

• Weak acids have lower Ka values and higher pKa values
• Strong acids have higher Ka values and lower pKa values.

What is Base Ionization Constant?

Base ionization constant is the number that expresses the relationship between base molecules and the ionic species exist in the same solution. This is denoted by Kb. It measures the strength of a base in a solution. Higher the Kb, higher the ionization of the base. For a certain base in a solution, the base dissociation constant can be given as below,

B   +   H₂O      ↔       BH⁺   +   OH^–

Kb = [BH⁺][OH^–] / [B][ H₂O]

Since the Kb values of bases are very small values, the minus log value of Kb is used instead Kb. The minus log value of Kb is denoted by pKb. pKb gives a number that is easy to handle.

pKb = -log(Kb)

The strength of a base can be expressed by Kb values or pKb values as follows.

• Higher the value of base ionization constant, stronger the base (lower the pKb)
• Lower the value of base ionization constant, weaker the base (higher the pKb)

What is the Difference Between Acid Ionization Constant and Base Ionization Constant?

Summary – Acid Ionization Constant vs Base Ionization Constant

Acid ionization constant and the base ionization constant are measures of acid and base strengths respectively. The difference between acid ionization constant and base ionization constant is that acid ionization constant gives a quantitative measure of the strength of an acid in a solution whereas base ionization constant gives a quantitative measure of the strength of a base in a solution.

Reference:

1.“16.4: Acid Strength and the Acid Dissociation Constant (Ka).” Chemistry LibreTexts, Libretexts, 26 Feb. 2017. Available here  
2.“Strong and Weak Bases and Base Ionization Constant (Kb).” CK-12 Foundation, CK-12 Foundation, 11 Sept. 2016. Available here 
3.“Acids and Bases: Ionization Constants.” Ionization of Acids and Bases. Available here 

Image Courtesy:

1.’Acetic-acid-dissociation-2D-curly-arrows’By Ben Mills – Own work, (Public Domain) via Commons Wikimedia