9.7 Strong and Weak Bases

This topic explains the strength of bases — what it means and how it is measured using the base ionization constant K_b and pK_b.

Definition

Strength of a Base: The ability of a base to accept a proton from a solvent.

Base Ionization Constant (K_b): The equilibrium constant for the ionization of a base in water.

Main Idea

The strength of a base is the ability to accept a proton from a solvent. Hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide are strong bases and ionize completely in aqueous solution. The ability of a base to accept a proton from an acid, usually water, is termed as strength of the base.

Core Concept

For a base B, an equilibrium reaction with water can be represented by the following equation:

B + H₂O_(aq) ⇌ BH⁺_(aq) + OH^-_(aq)

The equilibrium constant K_b is referred to as the base ionization constant:

K_b = [BH⁺][OH^-][B]

• Large K_b → strong base (high degree of ionization).
• Small K_b → weak base (low degree of ionization).
• pK_b = -log K_b
• Smaller pK_b → stronger base.

Detailed Explanation

What is a Strong Base?

- Hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide are strong bases and ionize completely in aqueous solution.

NaOH_(aq) → Na⁺_(aq) + OH^-_(aq)

KOH_(aq) → K⁺_(aq) + OH^-_(aq)

- The OH^- ion thus formed is a Brønsted base because it can accept a proton H⁺.

OH^- + H⁺ → H₂O

What is Base Strength?

• The ability of a base to accept a proton from an acid, usually water, is termed as strength of the base.
• For a base B, an equilibrium reaction with water can be represented by the following equation:

B + H₂O_(aq) ⇌ BH⁺_(aq) + OH^-_(aq)

- The equilibrium constant K_b is referred to as the base ionization constant and can be derived in the same way as K_a for acids. Thus:

K_b = [BH⁺][OH^-][B]

Interpreting K_b

• K_b value will be large if the degree of ionization of the base B is high, i.e., if the base B is strong.
• The value of K_b will be small for a weak base B.
• Again, for convenience, a parameter pK_b has been devised to express K_b value in convenient numbers. Thus, pK_b is defined as the negative logarithm of K_b:

pK_b = -log K_b

• Smaller pK_b → stronger base.
• Larger pK_b → weaker base.

Intermediate Level

Strong vs. Weak Bases

K_b Expression

For base B:

B + H₂O ⇌ BH⁺ + OH^-

K_b = [BH⁺][OH^-][B]

pK_b

pK_b = -log K_b

• Smaller pK_b → stronger base.
• Larger pK_b → weaker base.

Example

• NaOH: Strong base, completely ionizes.
• KOH: Strong base, completely ionizes.
• OH^-: Brønsted base (accepts H⁺).

Important Points

• 1. Strength of base: Ability to accept a proton.
• 2. Strong base: Ionizes completely (NaOH, KOH).
• 3. Weak base: Ionizes partially (NH₃).
• 4. K_b: Base ionization constant.
• 5. Larger K_b → stronger base.
• 6. pK_b = -log K_b.
• 7. Smaller pK_b → stronger base.
• 8. OH^- is a Brønsted base.

Remember

• - Strong base: Complete ionization
• - Weak base: Partial ionization
• - K_b: Larger → stronger
• - pK_b: Smaller → stronger
• - Examples: NaOH, KOH (strong); NH₃ (weak)
• - OH^-: Brønsted base

Common Mistakes

• 1. Confusing strong with concentrated: Strong = complete ionization; Concentrated = high amount.
• 2. Forgetting the relationship between K_b and pK_b: pK_b = -log K_b.
• 3. Thinking all bases are strong: Some are weak (e.g., NH₃).
• 4. Mixing up K_a and K_b: K_a for acids; K_b for bases.

Quick Revision

• Strength of base: Ability to accept a proton.
• Strong base: Ionizes completely (NaOH, KOH).
• Weak base: Ionizes partially (NH₃).
• K_b: Base ionization constant.
• Larger K_b → stronger base.
• pK_b = -log K_b.
• Smaller pK_b → stronger base.
• OH^- is a Brønsted base.

60-Second Revision

• Strong base: Complete ionization
• Weak base: Partial ionization
• K_b: Larger → stronger
• pK_b: Smaller → stronger
• Examples: NaOH, KOH (strong); NH₃ (weak)
• OH^-: Brønsted base

QUALITY VALIDATION

• Topic: 9.7 Strong and Weak Bases
• Chapter: 9 (Acids-Bases Chemistry)

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