Acid and Base Dissociation Constants (Ka and Kb)

HSC Chemistry Syllabus

• Explore the use of K_eq for different types of chemical reactions, including but not limited to:

What are Acid and Base Dissociation Constants?

This video will explore acid and base dissociation constants, what they are, and how they are calculated. 

Introduction to Acids

• Acids, by various definitions, are hydrogen ion (H⁺) producers (Arrhenius theory) or proton donors (Brønsted-Lowry theory). 

• Not all hydrogen–containing molecules are acids. Molecules are only considered acids if the hydrogen atoms can be ionised or given away. Hydrogen atoms that can be ionised are described as acidic.

• For example, hydrogen atoms in methane, CH₄, are not easily ionisable therefore methane is not considered an acid.

You can review Arrhenius and Brønsted-Lowry acid/base models here

Acid Dissociation/Ionisation

• Dissociation is a chemical process whereby a compound splits into ions. Ionic compounds (e.g. NaCl), acids and bases can dissociate in water.

 

 

• Weak acids are ones which do not ionise fully when dissolved in water. In other words, molecules of weak acids only partially produce hydrogen ions or donate protons. Therefore, weak acids exist in dynamic equilibria when in solution. 

• Organic acids (acids that contain carbon) are weak acids. Examples of this include methanoic acid (CH₃OH), acetic acid (CH₃COOH) and citric acid (C₆H₈O₇).

 

• Some inorganic acids (acids that do not contain carbon) are weak acids. An example of this is hydrofluoric acid (HF).

• Ethanoic acid (acetic acid, found in vinegar) is a typical weak acid which reacts with water to produce hydronium ions (H₃O⁺) and ethanoate ions (CH₃COO^–)

 

 $$CH_3COOH(aq)+H_2O(l) \rightleftharpoons CH_3COO^–(aq) + H_3O^+(aq)$$

 

Under standard conditions, only about 1% of ethanoic acid molecules are ionised and converted into ethanoate ions. The rest 99% remain as simple ethanoic acid molecules.

Acid Dissociation Constant (K_a)

The strength of weak acids can be measured and quantified by the equilibrium constant of their dissociation reaction. For a generic weak acid, HA, dissolved in water:

 

$$HA(aq) + H_2O(l) \rightleftharpoons A^–(aq) + H_3O^+(aq)$$

 

The equilibrium constant or acid dissociation constant of this reaction is:

 

$$K_{eq} = \frac{[A^–][H_3O^+]}{[HA]}$$

 

Since the concentration of water is large and remains relatively constant compared to a weak acid, it is not included in the equilibrium constant expression.

Acid strength is proportional to the value of K_a. Higher the value indicates higher concentration of hydrogen ions and more acids ionised. This corresponds to a stronger acid.

Let's apply this concept to a weak acid. The dissociation constant for ethanoic acid is: