Acid Dissociation Constant (AQA A-Level Chemistry): Revision Notes

5.6.1 Acid Dissociation Constant

Weak Acid Dissociation

Weak acids dissociate only slightly in aqueous solution, meaning they release relatively few hydrogen ions ($\text{H}^+$) when dissolved. This limited dissociation is a key characteristic distinguishing weak acids from strong acids, which dissociate almost completely.

What is Ka?

The acid dissociation constant, $K_a$, provides a quantitative measure of the strength of a weak acid. It indicates how much the acid dissociates in solution, describing the equilibrium concentration of ions produced.

A higher $K_a$ value suggests a stronger weak acid, as it implies more dissociation into ions; a lower $K_a$ value indicates a weaker acid with less dissociation.

Expression for $K_a$

For a general weak acid, represented by $\text{HA}$, the dissociation in water can be shown as:

$$\text{HA} \leftrightharpoons \text{H}^+ + \text{A}^-$$

The expression for the acid dissociation constant, $K_a$, is:

$$K_a = \frac{{[\text{H}^+][\text{A}^-]}}{{[\text{HA}]}}$$

Where:

• $[\text{H}^+]$ is the concentration of hydrogen ions,
• $[\text{A}^-]$ is the concentration of the conjugate base,
• $[\text{HA}]$ is the concentration of the undissociated acid.

Standard Form and $K_a$

The value of $K_a$ is often very small and is therefore typically expressed in standard form.

• This practice is essential for managing very small values conveniently and accurately.
• For example, a $K_a$ value might be written as 1.8 × 10^{-5} instead of 0.000018.