Weak Base

Definition

A weak base is a type of base that only partially dissociates in an aqueous solution, producing a relatively low concentration of hydroxide ions. This is in contrast to strong bases, which completely dissociate in water, producing a high concentration of hydroxide ions.

5 Must Know Facts For Your Next Test

1. Weak bases have a pH greater than 7, indicating a basic solution, but the pH is less than 14, which would be the pH of a strong base.
2. The degree of dissociation of a weak base is described by its equilibrium constant, known as the base dissociation constant (Kb).
3. Weak bases are commonly used in acid-base titrations to determine the concentration of an unknown acid solution.
4. The strength of a weak base is inversely proportional to the strength of its conjugate acid, as described by the relationship between Ka and Kb.
5. Examples of weak bases include ammonia (NH3), pyridine (C5H5N), and the hydroxide ions of metal ions like Fe(OH)2 and Al(OH)3.

Review Questions

• Explain how the degree of dissociation of a weak base is related to its base dissociation constant (Kb).
  • The degree of dissociation of a weak base is directly related to its base dissociation constant (Kb). Weak bases only partially dissociate in aqueous solutions, producing a relatively low concentration of hydroxide ions. The Kb value quantifies the extent of this dissociation, with a higher Kb indicating a stronger weak base that dissociates more extensively. The Kb can be used to calculate the pH of a weak base solution, which is an important factor in determining the relative strengths of acids and bases.
• Describe the role of weak bases in acid-base titrations and how their use differs from that of strong bases.
  • Weak bases are commonly used in acid-base titrations to determine the concentration of an unknown acid solution. Unlike strong bases, which completely dissociate, weak bases only partially dissociate, producing a relatively low concentration of hydroxide ions. This allows for a more gradual change in pH during the titration, providing a more precise endpoint detection. The use of weak bases in titrations is advantageous because it enables the titration of weaker acids that would be difficult to titrate using a strong base. The pH change during the titration is more gradual, allowing for better control and accuracy in determining the equivalence point.
• Analyze the relationship between the strength of a weak base and the strength of its conjugate acid, as described by the inverse relationship between Ka and Kb.
  • The strength of a weak base is inversely related to the strength of its conjugate acid, as described by the relationship between the acid dissociation constant (Ka) and the base dissociation constant (Kb). A strong weak base will have a relatively high Kb value, indicating that it dissociates extensively in aqueous solution to produce a high concentration of hydroxide ions. Conversely, the conjugate acid of a strong weak base will have a relatively low Ka value, indicating that it is a weak acid that only partially dissociates. This inverse relationship between the strength of a weak base and the strength of its conjugate acid is a fundamental principle in understanding acid-base equilibria and the relative strengths of acids and bases.

Related terms

Strong Base: A strong base is a base that completely dissociates in an aqueous solution, producing a high concentration of hydroxide ions.

Conjugate Base:

The conjugate base of an acid is the species formed when the acid loses a proton. Weak bases have relatively strong conjugate acids.

pH:

pH is a measure of the acidity or basicity of a solution, with a higher pH indicating a more basic solution.