9.6 Strong and Weak Acids

This topic explains the difference between strong and weak acids. It describes how the extent of ionization and the acid dissociation constant (K_a) are used to distinguish between them.

Definition

Strong Acid: An acid that ionizes completely in aqueous solution.

Weak Acid: An acid that ionizes only partially in aqueous solution.

Acid Dissociation Constant (K_a): The equilibrium constant for the ionization of an acid in water.

Main Idea

The extent of ionization and the acid dissociation constant K_a can be used to distinguish between strong and weak acids. The strength of an acid is generally expressed in terms of the acid ionization constant, K_a. The greater the value of K_a, the stronger is the acid. Since K_a values are usually inconvenient numbers, they are converted to pK_a values.

Core Concept

For a general acid HX in water:

HX_(aq) + H₂O_(l) ⇌ H₃O⁺_(aq) + X^-_(aq)

The equilibrium constant is:

K = [H₃O⁺][X^-][HX][H₂O]

Since water is in excess, its concentration is constant:

K[H₂O] = K_a = [H₃O⁺][X^-][HX]

• Greater K_a → stronger acid.
• Smaller K_a → weaker acid.
• pK_a = -log K_a
• Smaller pK_a → stronger acid.
• Greater pK_a → weaker acid.

Detailed Explanation

Strong and Weak Acids

The extent of ionization and the acid dissociation constant K_a can be used to distinguish between strong and weak acids.

The strength of an acid is generally expressed in terms of the acid ionization constant, K_a. Consider the case of ionization of a general acid HX in water.

In this aqueous solution, the established equilibrium may be represented as follows:

HX_(aq) + H₂O_(l) ⇌ H₃O⁺_(aq) + X^-_(aq)

The equilibrium constant K for this ionization process may be written as follows:

K = [H₃O⁺][X^-][HX][H₂O]

Since water is a solvent, it is present in excess and therefore its concentration may be regarded as constant. Thus, K[H₂O] is another constant and is designated as K_a. Thus,

K[H₂O] = K_a = [H₃O⁺][X^-][HX]

K_a is termed as the acid dissociation constant. It is a measure of the extent to which an acid is ionized or dissociated at the equilibrium state. It must be kept in mind that the acid dissociation constant, K_a, is dependent on temperature. Therefore, the value of K_a should be mentioned along with the temperature at which K_a was determined.

Dissociation constant, K_a, of acetic acid in water at 25^° C is 1.8 × 10^-5. The comparison of K_a values of different acids provides a method to compare their strengths.

"The greater the value of K_a, the stronger is the acid."

The values of K_a are usually inconvenient numbers, therefore, for convenience these values are converted to pK_a values. The relationship between K_a and pK_a is as follows:

pK_a = -log K_a

Since pK_a refers to the negative logarithm of K_a, smaller the value of pK_a, stronger shall be the acid because smaller pK_a value corresponds to a greater K_a value.

Table 9.2: Ionisation Constants and pK_a of Acids

• HCl has K_a = 1.0 × 10⁶, pK_a = -6.0
• HF has K_a = 7.2 × 10^-4, pK_a = +3.1
• Since HCl has a larger K_a (and smaller pK_a), HCl is stronger than HF.

Example 9.5: Calculating [H⁺] of a Weak Acid

Calculate concentration of H⁺ ions of a solution that contains 1.0 M HF.

Solution:

HF_(aq) ⇌ H⁺_(aq) + F^-_(aq)

K_a = [H⁺][F^-][HF]

7.2 × 10^-4 = x · x1.0 - x

Since K_a is small, x is negligible compared to 1.0, so 1.0 - x ≈ 1.0:

7.2 × 10^-4 = x²1.0

x² = 7.2 × 10^-4

x = √7.2 × 10^-4

x = 2.68 × 10^-2 M

Therefore, [H⁺] = 2.68 × 10^-2 M.

Intermediate Level

Strong vs. Weak Acids

K_a\) and \(pK_a

K_a = [H₃O⁺][X^-][HX]

pK_a = -log K_a

• Larger K_a → stronger acid.
• Smaller pK_a → stronger acid.

Table 9.2

Example 9.5

• 1.0 M HF
• K_a = 7.2 × 10^-4

Solution: 7.2 × 10^-4 = x²1.0 x = √7.2 × 10^-4 = 2.68 × 10^-2 M

Final Answer: [H⁺] = 2.68 × 10^-2 M

Important Points

• 1. Strong acid: Ionizes completely.
• 2. Weak acid: Ionizes partially.
• 3. K_a: Acid dissociation constant.
• 4. Larger K_a → stronger acid.
• 5. pK_a = -log K_a.
• 6. Smaller pK_a → stronger acid.
• 7. Table 9.2: K_a and pK_a values of common acids.
• 8. HCl is stronger than HF.
• 9. Example 9.5: [H⁺] = 2.68 × 10^-2 M.

Remember

• - Strong acid: Complete ionization
• - Weak acid: Partial ionization
• - K_a: Larger → stronger
• - pK_a: Smaller → stronger
• - HCl > HF (strength)
• - Example 9.5: [H⁺] = 2.68 × 10^-2 M

Common Mistakes

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

Quick Revision

• Strong acid: Ionizes completely.
• Weak acid: Ionizes partially.
• K_a: Acid dissociation constant.
• Larger K_a → stronger acid.
• pK_a = -log K_a.
• Smaller pK_a → stronger acid.
• Table 9.2: K_a and pK_a values of common acids.
• HCl is stronger than HF.
• Example 9.5: [H⁺] = 2.68 × 10^-2 M.

60-Second Revision

• Strong acid: Complete ionization
• Weak acid: Partial ionization
• K_a: Larger → stronger
• pK_a: Smaller → stronger
• HCl > HF (strength)
• Example 9.5: [H⁺] = 2.68 × 10^-2 M

QUALITY VALIDATION

• Topic: 9.6 Strong and Weak Acids
• Chapter: 9 (Acids-Bases Chemistry)

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