Understanding Acid Strength: Ka, pKa, and Their Relationship to Strong and Weak Acids
Dot-Point 4: calculate and apply the dissociation constant (Ka) and pKa (pKa = -log10 (Ka)) to determine the difference between strong and weak acids (ACSCH098)
Acids and bases are central to chemistry, biology, and many industrial processes. But what makes an acid strong or weak? The dissociation constant (Ka) and its logarithmic counterpart (pKa) are key to understanding and measuring acid strength. Here's a breakdown of these concepts.
What Are Ka and Kb?
The Ka value, or acid dissociation constant, measures how much a weak acid dissociates into ions in water. For a generic weak acid, HA:
HA + H₂O ⇌ A⁻ + H₃O⁺
The formula for Ka is:
Ka = [A⁻][H₃O⁺] / [HA]
For a weak base, BOH:
BOH ⇌ B⁺ + OH⁻
The formula for Kb is:
Kb = [B⁺][OH⁻] / [BOH]
How Does Ka Relate to Acid Strength?
A larger Ka value means the acid dissociates more, making it stronger.
A smaller Ka value means the acid dissociates less, making it weaker.
The same concept applies to bases, with larger Kb values indicating stronger bases and smaller values indicating weaker ones.
What Is pKa?
Since Ka values can span a vast range, we use a logarithmic scale to simplify comparisons. The formula for pKa is:
pKa = -log₁₀(Ka)
Stronger acids have lower pKa values (often negative).
Weaker acids have higher pKa values (positive numbers).
For example:
Hydrochloric acid (HCl): Ka = 10⁴, pKa = -4 (strong acid)
Acetic acid (CH₃COOH): Ka = 1.7 × 10⁻⁵, pKa = 4.8 (weak acid)
Comparing Ka and pKa
In polyprotic acids (those that can lose more than one proton), each additional ionization has a smaller Ka value and a correspondingly larger pKa.
The Relationship Between Ka, Kb, pKa, and pKb
For conjugate acid-base pairs:
Ka × Kb = Kw
where Kw = 1 × 10⁻¹⁴
pKa + pKb = 14
This means if you know the Ka or pKa of an acid, you can easily find the Kb or pKb of its conjugate base.
Why Use pKa Instead of pH?
pKa is a better measure of acid strength because it does not depend on the concentration of the acid. While pH changes with dilution, pKa remains constant for a given acid. This makes pKa ideal for comparing acids.
