learnbin.net nav logo

More results...

Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors

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.

Dissociation constant eq 01
Dissociation of strong acid and a base

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

Dissociation constant eq 02
Dissociation of a weak acid and a base

Properties of the dissociation constant

Dissociation constants (Ka and Kb) are types of equilibrium constants. So, like other equilibrium constants, Ka and Kb 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
  • Ka and Kb 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 Ka and Kb)

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

Using Acid/ base dissociation constants

Symbols of dissociation constants

  • Ka – Acid dissociation constant
  • Kb – Base dissociation constant (base hydrolysis constant)

Examples and dissociation constant expressions

Acid dissociation constant expression

Representation of dissociation of a weak acid and Acid dissociation constant expression
Representation of dissociation of a weak acid and Ka expression

Examples:

Dissociation of acetic acid and Acid dissociation constant expression of acetic acid

Base dissociation constant expression

Representation of dissociation of a weak base and Base dissociation constant expression
Representation of dissociation of a weak base and Kb expression

Or it can be represented like this

Representation of dissociation of a weak base and Base dissociation constant expression
Representation of dissociation of a weak base and Kb expression

Examples:

Dissociation constant eq 07

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

The dissociation constant for conjugated acids and bases

What is 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).

Dissociation constant eq 08

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

Dissociation constant eq 09

Example for conjugated acid/base pairs

Dissociation constant eq 10

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.

Dissociation constant eq 11

In the above system of the acetic acid aqueous solution, CH3COOH acts as an acid, and water act 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.

Dissociation constant eq 12

In a situation (question) you are only provided the Ka, but you need the Kb you can use the above equation to find the Kb 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, Ka, Kb ...) in another article.

Useful equations for solving questions related to acid/ base dissociation

Dissociation constant eq 13

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


Buy me a coffee

References and Attributes

Daniel C. Harris, Quantitative Chemical Analysis; Ninth edition

Douglas A. Skoog, Donald M. West, F. James Holler, Stanley R. Crouch, Fundamentals of Analytical Chemistry; Ninth edition

Figures:

The cover image was created using an image by OsloMetX from Pixabay


Express your thoughts below!

Leave a Reply

Your email address will not be published. Required fields are marked *

0 Comments

© 2024 learnbin.net. All rights reserved.
linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram