pKa is one of the most important concepts in IB Chemistry Topic 8 (Acids and Bases) and forms the backbone of buffer calculations, equilibrium reasoning, and acid–base strength analysis. Students often memorize pKa values without truly understanding what they represent. Once you understand the meaning of pKa and how it connects to Ka, acid strength becomes far easier to evaluate.
What Is pKa?
pKa is the negative logarithm of the acid dissociation constant (Ka).
Mathematically:
pKa = −log(Ka)
This conversion turns often inconveniently small Ka values into manageable numbers that are easier to compare.
What pKa represents:
- A measure of acid strength
- How easily an acid donates a proton (H⁺)
- A value that determines equilibrium position
Lower pKa = stronger acid.
Higher pKa = weaker acid.
Relationship Between Ka and pKa
Ka measures the equilibrium strength of a weak acid:
HA ⇌ H⁺ + A⁻
Ka = [H⁺][A⁻] / [HA]
Because Ka values for weak acids are often extremely small (10⁻³ to 10⁻¹⁰), it is more convenient to use their logarithmic form:
pKa = −log(Ka)
Interpretation:
- Large Ka → small pKa → strong acid
- Small Ka → large pKa → weak acid
This inverse relationship is essential to understand.
What Does pKa Tell Us?
1. Acid Strength
pKa tells you how strongly an acid holds onto its proton.
- Low pKa (< 2) → strong acid
- Medium pKa (2–7) → weak to moderate acid
- High pKa (> 7) → very weak acid
2. Position of Acid–Base Equilibrium
Lower pKa → equilibrium lies further to the right (more dissociation).
3. Behavior in Buffer Systems
pKa determines how effectively a conjugate acid–base pair resists pH changes.
4. Predicting Proton Transfer
A proton moves from lower pKa species to higher pKa species.
This rule is highly useful in HL mechanism explanations.
pKa and the Henderson–Hasselbalch Equation (HL)
For weak acid buffers:
pH = pKa + log([A⁻]/[HA])
This equation shows:
- When [A⁻] = [HA], pH = pKa
- pKa is the pH at which a weak acid is half dissociated
This is why buffer solutions work best when pH ≈ pKa.
IB exam questions frequently rely on this relationship.
pKa and Conjugate Acid–Base Strength
Ka × Kb = Kw
Taking logs:
pKa + pKb = 14
This means:
- Lower pKa → stronger acid → weaker conjugate base
- Higher pKa → weaker acid → stronger conjugate base
Conjugate pairs always balance each other.
pKa and Molecular Structure
The pKa of an acid depends on:
1. Electronegativity
More electronegative atoms stabilize the conjugate base → lower pKa.
2. Bond strength
Weaker H–A bonds → easier to donate H⁺ → lower pKa.
3. Resonance
Resonance stabilization of A⁻ reduces pKa dramatically.
Example:
- Carboxylic acids (pKa ≈ 4–5) are stronger than alcohols (pKa ≈ 16) because their conjugate bases are resonance-stabilized.
4. Inductive effects
Electron-withdrawing groups lower pKa by stabilizing the conjugate base.
These structural ideas connect directly to organic chemistry.
Common IB Mistakes About pKa
- Confusing pKa with pH.
pH measures acidity of solutions; pKa measures intrinsic acid strength. - Thinking strong acids have pKa values.
In IB, strong acids are treated as fully dissociated, so pKa is not typically used. - Assuming high concentration means strong acid.
Strength relates to dissociation, not concentration. - Mixing up pKa and Ka.
pKa is simply the logarithmic expression of Ka.
FAQs
Does a low pKa always mean strong acid?
Yes. Lower pKa means higher Ka and greater dissociation.
How is pKa used in buffers?
pKa helps determine the optimal pH for buffering and is used in the Henderson–Hasselbalch equation.
Can pKa change with temperature?
Yes. Like any equilibrium constant, Ka (and therefore pKa) varies with temperature.
Conclusion
pKa is the negative logarithm of Ka and provides a simple, intuitive way to compare acid strengths. Lower pKa means stronger acid and more extensive dissociation. pKa is essential for understanding weak acids, conjugate pairs, buffer systems, and pH calculations in IB Chemistry. Mastering pKa gives you a powerful tool for analyzing acid–base behavior.
