Strong and weak bases appear throughout IB Chemistry in acid–base theory, equilibrium, titrations, and pH calculations. Although the distinction seems simple, many students confuse concentration with strength. Understanding the real difference—degree of dissociation—helps explain conductivity, equilibrium constants, titration curves, and the behavior of bases in solution.
What Is a Strong Base?
A strong base is a base that completely dissociates into ions in aqueous solution.
This means:
- Every unit of the base separates into its ions
- No significant amount of the original molecule remains
- The reaction goes essentially to completion
Examples of strong bases:
- NaOH → Na⁺ + OH⁻
- KOH → K⁺ + OH⁻
- Ba(OH)₂ → Ba²⁺ + 2OH⁻
Key characteristics:
- High conductivity (lots of ions)
- High pH
- Fully ionized in solution
- No equilibrium between base and its ions
Strong bases are straightforward to handle in pH calculations because they produce predictable amounts of OH⁻.
What Is a Weak Base?
A weak base only partially dissociates in aqueous solution.
This means:
- Only a small fraction of molecules form ions
- Most of the base remains in molecular form
- An equilibrium exists between the base and its ions
Example:
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
Other weak bases include:
- Amines (CH₃NH₂, (CH₃)₂NH)
- Weak metal hydroxides
- Many organic nitrogen-containing compounds
Key characteristics:
- Lower conductivity
- Lower pH compared to strong bases of the same concentration
- Presence of an equilibrium
- Governed by Kb (base dissociation constant)
Weak bases require equilibrium calculations, not simple stoichiometry.
The Core Difference: Degree of Dissociation
Strong bases: ~100% dissociated
Weak bases: small percentage dissociated
This is independent of concentration.
Example:
- A 0.10 mol dm⁻³ strong base may have higher pH than a 1.0 mol dm⁻³ weak base
- Strength refers to intrinsic ionization behavior, not amount present
IB exams often test this misconception.
Conductivity Differences
Because strong bases fully dissociate:
- They conduct electricity extremely well
- They produce many ions in solution
Weak bases:
- Produce fewer ions
- Have noticeably lower conductivity
Conductivity experiments in IB lab work directly demonstrate this difference.
pH Differences
Strong bases:
[OH⁻] equals the base’s concentration (for monoprotic bases).
Example:
0.10 M NaOH → [OH⁻] = 0.10 M
Weak bases:
[OH⁻] must be calculated using Kb.
The smaller the Kb, the weaker the base.
Weak bases produce much less OH⁻, giving a lower pH even at the same concentration.
Equilibrium Considerations (Weak Bases Only)
Weak bases establish an equilibrium:
B + H₂O ⇌ BH⁺ + OH⁻
The equilibrium constant Kb is given by:
Kb = [BH⁺][OH⁻] / [B]
Strong bases do not use Kb because they do not establish equilibrium.
Behavior in Titration Curves
Strong base titrations produce:
- A sharp, steep vertical region
- Higher initial pH
Weak bases produce:
- A more gradual curve
- Lower initial pH
- A buffer region when titrated with strong acids
Recognizing titration curves is a common Paper 2 task.
Real-World Examples
Strong bases:
- Sodium hydroxide in drain cleaners
- Potassium hydroxide in batteries
- Barium hydroxide in analytical chemistry
Weak bases:
- Ammonia used in fertilizers and cleaning products
- Amines found in pharmaceuticals
- Organic bases used in biological systems
Weak bases are common in nature; strong bases are more common in industrial chemistry.
FAQs
Does a concentrated weak base have a high pH?
It can, but not as high as a strong base of equal concentration. Strength is not the same as concentration.
Why do weak bases have lower conductivity?
Because only a small fraction of molecules form ions.
Can a weak base ever fully dissociate?
No. Its chemical structure prevents it from ionizing completely, regardless of concentration.
Conclusion
The difference between strong and weak bases lies in the degree of dissociation. Strong bases fully dissociate and produce high concentrations of OH⁻, while weak bases partially dissociate and establish an equilibrium. This affects conductivity, pH, titration curves, and how calculations are performed. Understanding this distinction is essential for mastering acid–base reactions in IB Chemistry.
