Determining whether a base is strong or weak is an important concept in chemistry. The strength of a base depends on how completely it dissociates in water. Strong bases dissociate completely, while weak bases only partially dissociate. There are several ways to tell if a base is strong or weak.
Definition of Strong and Weak Bases
A strong base is a base that completely dissociates into its ions when dissolved in water. This means that all of the base particles separate into their constituent cations and anions. For example, when sodium hydroxide (NaOH) dissolves in water, virtually 100% of the NaOH molecules dissociate into sodium ions (Na+) and hydroxide ions (OH-).
In contrast, a weak base only partially dissociates in water. This means that only some of the base particles dissociate into ions, while the rest remain intact as molecules. For example, when ammonia (NH3) dissolves in water, only about 1% of the NH3 molecules dissociate into ammonium ions (NH4+) and hydroxide ions (OH-). The remaining 99% exist as NH3 molecules.
Strong Bases Completely Dissociate
When determining base strength, the most fundamental property to examine is the degree of dissociation in water. Strong bases completely dissociate, while weak bases only partially dissociate. Some examples of strong bases include:
- Group 1 hydroxides: LiOH, NaOH, KOH, RbOH, CsOH
- Group 2 hydroxides: Ca(OH)2, Sr(OH)2, Ba(OH)2
- Hydrazine: N2H4
When any of these strong bases dissolve in water, 100% of the base particles dissociate into ions. For example:
NaOH(aq) → Na+(aq) + OH-(aq)
In this reaction, there is a complete transfer of Na+ ions and OH- ions into solution. None of the NaOH compound remains intact.
Weak Bases Partially Dissociate
Weak bases, in contrast, only partially break apart into ions when dissolved in water. Here are some examples of weak bases:
- Ammonia: NH3
- Amines: RNH2, R2NH, R3N
- Pyridine: C5H5N
- Ions of weak acids: CH3COO-, HCO3-
When these weak bases dissolve, only a small fraction of the base particles dissociate. For ammonia:
NH3 + H2O ⇌ NH4+ + OH-
In this equilibrium, only about 1% of the original NH3 dissociates. The amount that dissociates depends on the particular weak base. But for all weak bases, the degree of dissociation is less than 100%.
pH of Strong vs. Weak Base Solutions
The strength of a base can also be determined by measuring the pH of its solutions. Strong bases form very alkaline solutions with very high pH values. Weak bases form solutions with pH values closer to neutral.
Here is a table comparing the pH values of 0.1 M solutions of some strong and weak bases:
|Base||Strength||pH of 0.1 M Solution|
As shown, the strong base solutions have very high pH values of 13, indicating complete dissociation into hydroxide ions. The weak base solutions have lower pH values closer to neutral, indicating only partial dissociation.
Concentrations of Ions
The concentrations of hydroxide ions [OH-] and hydronium ions [H3O+] in solution also indicate a base’s strength. Strong bases produce high [OH-] and low [H3O+]. Weak bases result in lower [OH-] and higher [H3O+].
For example, in a 0.1 M NaOH solution:
- [OH-] = 0.1 M
- [H3O+] = 1 x 10-13 M
Whereas in a 0.1 M NH3 solution:
- [OH-] = 5 x 10-4 M
- [H3O+] = 1 x 10-11 M
The strong base NaOH yields complete dissociation into OH- ions, reflected in the high hydroxide concentration. NH3 as a weak base results in far lower [OH-].
Strength Constant Values
The strength constant, Kb, also indicates base strength. Kb quantifies the extent to which a base dissociates in water. Strong bases have large Kb values, while weak bases have small Kb values.
- Kb of NaOH = Very large, essentially infinite
- Kb of NH3 = 1.8 x 10-5
NaOH as a strong base has an extremely large Kb, reflecting its complete dissociation. NH3 as a weak base has a Kb of only 1.8 x 10-5, indicating its slight dissociation.
Titration curves provide graphical representations of a base’s strength. The shape of the curve differs substantially between strong and weak bases.
Titrating a strong base with a strong acid leads to an extremely sharp, vertical curve due to the neutralization occurring all at once. With weak bases, the curve is more gradual as the base is only partially neutralized.
Here are sample titration curves for strong vs. weak bases:
The steep curve indicates the strong base NaOH, while the gradual curve represents the weak base NH3. The sharp change for the strong base reflects its complete dissociation.
A strong base solution conducts electricity better than a weak base solution of the same concentration. This is because strong bases dissociate completely into many free-moving ions that can conduct electricity.
For example, in 0.1 M solutions at 25°C:
- NaOH has a conductivity of around 1.2 S/m
- NH3 has a conductivity of 0.08 S/m
The high conductivity of NaOH reflects the complete dissociation into Na+ and OH- ions. NH3 has far lower conductivity as few ions are present.
Reaction with Indicators
Certain acid-base indicators can also reveal a base’s strength. Indicators change color depending on the pH of the solution. Strong bases quickly change the color of an indicator, while weak bases induce slower color changes.
For example, phenolphthalein is colorless below pH 8.6 and pink above pH 10. Adding a few drops of 0.1 M NaOH instantly turns phenolphthalein pink, reflecting the solution’s high pH. Adding 0.1 M NH3 causes a slower, more gradual color change as the pH rises more slowly.
Finally, the rates of acid-base neutralization reactions indicate the strength of a base. Strong base solutions react instantly and vigorously with acids. Weak base solutions react more slowly and gently as fewer OH- ions are available.
For example, mixing HCl with NaOH causes immediate, effervescent fizzing as the reaction proceeds rapidly. Mixing acetic acid with NH3 results in slower, more gradual bubbling as the weak base reacts more slowly.
In summary, strong bases completely dissociate in water, while weak bases only partially dissociate. Several major properties indicate base strength:
- Degree of dissociation
- Ion concentrations
- Strength constant (Kb)
- Titration curves
- Indicator color change
- Reaction rates
By analyzing these factors, chemists can definitively categorize a base as strong or weak. Understanding base strength is critical for predicting reactivity, calculating pH, determining solubility, and more. Proper identification of strong vs. weak bases is an essential skill in chemistry.