Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 14.8 Ka expression-Study Notes - New Syllabus

14.8 Acid Dissociation Constant (\( \mathrm{K_a} \)) for a Weak Acid

The acid dissociation constant, \( \mathrm{K_a} \), is an equilibrium constant that measures the extent to which a weak acid dissociates in aqueous solution. It is derived directly from the equilibrium expression for the ionisation of a weak acid.

Step 1: Write the Dissociation Equation

For a weak acid \( \mathrm{HA} \):

\( \mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)} \)

(Water is often omitted for simplicity):

\( \mathrm{HA(aq) \rightleftharpoons H^+(aq) + A^-(aq)} \)

Step 2: Write the Equilibrium Expression

Applying the general expression for equilibrium constants:

\( \mathrm{K_c = \frac{[H^+][A^-]}{[HA]}} \)

Step 3: Define \( \mathrm{K_a} \)

Since water is a pure liquid and its concentration is constant, it is omitted. The equilibrium constant is therefore called the acid dissociation constant, \( \mathrm{K_a} \):

\( \mathrm{K_a = \frac{[H^+][A^-]}{[HA]}} \)

Key Features of \( \mathrm{K_a} \)

• Applies only to weak acids.
• Derived from an equilibrium expression.
• Does not include water.
• Units depend on concentration powers (typically \( \mathrm{mol\ dm^{-3}} \)).

Interpretation of \( \mathrm{K_a} \)

• Large \( \mathrm{K_a} \) → greater dissociation → stronger acid.
• Small \( \mathrm{K_a} \) → less dissociation → weaker acid.

Alternative Form (using \( \mathrm{H_3O^+} \))

\( \mathrm{K_a = \frac{[H_3O^+][A^-]}{[HA]}} \)

Both forms are acceptable.

Key Features

• \( \mathrm{K_a} \) is derived from equilibrium of weak acid dissociation.
• Water is omitted from the expression.
• Indicates strength of a weak acid.
• Larger \( \mathrm{K_a} \) means stronger acid.