Ionization of acids and bases
Understanding the dissociation of strong and weak acids/bases, and calculations involving their ionization constants (Ka, Kb) and ° of ionization.
What is Ionization of acids and bases?
The process by which a molecule or atom splits into ions.
Key formula / rule: Acid Dissociation Constant (Ka)
Key points
- Differentiate between strong and weak acids/bases.
- Explain the concept of ionization equilibrium.
- Define and calculate Ka and Kb values.
- Determine the ° of ionization.
Common exam trap
Confusing strong and weak acids/bases.
Definitions
- Term
Ionization
- Meaning
The process by which a molecule or atom splits into ions.
- Term
Strong Acid/Base
- Meaning
An acid or base that dissociates almost completely in aqueous solution.
- Term
Weak Acid/Base
- Meaning
An acid or base that dissociates only partially in aqueous solution, establishing an equilibrium.
- Term
Acid Dissociation Constant (Ka)
- Meaning
The equilibrium constant for the dissociation of a weak acid.
- Term
Base Dissociation Constant (Kb)
- Meaning
The equilibrium constant for the dissociation of a weak base.
- Term
Degree of Ionization (α)
- Meaning
The fraction of a molecule of an electrolyte that is ionized in solution.
Learning objectives
Differentiate between strong and weak acids/bases.
Explain the concept of ionization equilibrium.
Define and calculate Ka and Kb values.
Determine the ° of ionization.
Relate ionization constants to the strength of acids and bases.
Formulae
- Name
Acid Dissociation Constant (Ka)
- Note
For a weak acid HA ⇌ H+ + A-
- Expression
Ka = [H+][A-] / [HA]
- Name
Base Dissociation Constant (Kb)
- Note
For a weak base B + H2O ⇌ BH+ + OH-
- Expression
Kb = [B+][OH-] / [B]
- Name
Ionic Product of Water (Kw)
- Note
Kw = 1.0 x 10-14 at 25°C
- Expression
Kw = [H+][OH-]
- Name
Relationship between Ka and Kb
- Note
For a conjugate acid-base pair
- Expression
Ka × Kb = Kw
- Name
Degree of Ionization (α)
- Note
Often expressed as a fraction or percentage.
- Expression
α = Amount of substance ionized / Initial amount of substance
- Name
Approximate Ka for weak acid
- Note
Valid when α is very small (<< 1), i.e., C is large or Ka is small.
- Expression
Ka ≈ Cα²
- Name
Approximate Kb for weak base
- Note
Valid when α is very small (<< 1), i.e., C is large or Kb is small.
- Expression
Kb ≈ Cα²
Prerequisites
Chemical Equilibrium
Ionic Product of Water (Kw)
pH and pOH Scales
Concentration terms (Molarity)
Common mistakes
Confusing strong and weak acids/bases.
Assuming complete ionization for all acids and bases.
Incorrectly applying the equilibrium expression for ionization.
Forgetting the relationship between Ka, Kb, and Kw.
Miscalculating pH/pOH from concentrations without considering ionization.
Keywords
Ionization
Dissociation
Acids
Bases
Strong Electrolytes
Weak Electrolytes
Equilibrium
Ka
Kb
Kw
Degree of Ionization
pH
pOH
Practice preview
What is the degree of ionization of a 0.01 M acetic acid solution if its Ka is 1.8 x 10^-5?…
medium
For a monoprotic acid HA, the pH of a 0.01 M solution is 3. What is the ionization constant (Ka) of this acid?…
medium
A weak base will have:…
easy