Dissociation Constant

What is the dissociation constant?

The dissociation constant is a type of equilibrium constant that is used in chemical equilibrium systems of the dissociation of weak acids and bases.

When a weak acid or a weak base is dissolved in water, they don’t dissociate completely like in strong acids or bases. Only partial dissociation occurs when weak acid or weak base is dissolved in water. Because of that partial dissociation, forms an equilibrium system in the solution. The equilibrium constant associated with that equilibrium is known as the dissociation constant.

Because strong acids and bases completely dissociate in water, their equilibrium constants are larger.

Properties of the dissociation constant

Dissociation constants (K_a and K_b) are types of equilibrium constants. So, like other equilibrium constants, K_a and K_b are temperature-dependent constants. These constants behave according to the laws of thermodynamics.

They can be used to find information such as,

• The completeness of the dissociation
• Determine the strength of the acid or base
• K_a and K_b are quantitative measurements of the strength of an acid or a base
• The yield of the products (e.g.: pH calculation)
• The direction of the reaction (By comparing K_a and K_b)

But they don’t provide any information about the rate of the reaction

Using Acid/ base dissociation constants

Symbols of dissociation constants

• K_a – Acid dissociation constant
• K_b – Base dissociation constant (base hydrolysis constant)

Examples and dissociation constant expressions

Acid dissociation constant expression

Examples:

Base dissociation constant expression

Or it can be represented like this

Examples:

In the above examples, H₂O is in every equilibrium reaction. However, it doesn’t appear in any equilibrium constant expression because [H₂O] is already incorporated into the K_a and K_b.

The dissociation constant for conjugated acids and bases

What is a conjugated acid/base pair?

According to the ‘Bronsted-Lowry concept’ by definition, acids donate/ release protons (H⁺) (proton donors) and bases accept protons (H⁺) (proton acceptors).

If a proton donor donates protons and becomes a proton acceptor or a proton acceptor accepts protons and becomes a proton donor, that type of species is known as a conjugated acid/base pair (conjugated pair).

When the above two processes are combined, that forms a neutralization reaction

Example for conjugated acid/base pairs

In the above system of ammonia aqueous solution, NH₃ acts as a base, and water act as an acid. So NH₄⁺ is the conjugated acid of NH₃ and OH^- is the conjugated base of water.

In the above system of the acetic acid aqueous solution, CH₃COOH acts as an acid, and water acts as the base. So CH₃COO^- is the conjugated base of CH₃COOH and H₃O⁺ is the conjugated acid of water.

Dissociation Constants for Conjugate Acid/Base Pairs

Let's consider an aqueous solution of ammonia (NH₃) and ammonium (NH₄⁺) ions.

In a situation (question) you are only provided the K_a, but you need the K_b you can use the above equation to find the K_b using the above equation.

Dissociation Constants for Polyprotic Acids and Bases

Polyprotic acids are compounds that donate more than one proton.

• Examples:- H₂SO₄, H₂SO₃, H₃PO₄

Polyprotic bases are compounds that accept more than one proton.

• Examples:- Mg(OH)₂, Al(OH)₃

Polyprotic acids and bases have multiple Ka and Kb values that are responsible for every proton they donate or accept.

We will discuss calculations of polyprotic acids and bases (pH, K_a, K_b ...) in another article.

Useful equations for solving questions related to acid/ base dissociation

You can find out about solving problems related to pH, acid-base equilibrium system, K_a, and K_b using the article this link.