Problem 4 - General Chemistry

Question

Describe how the degree of ionization of a weak acid changes as the concentration increases.

Step-by-Step Solution

Answer

As concentration increases, the degree of ionization of a weak acid decreases.

Step 1: Understand the Concept of Degree of Ionization

The degree of ionization refers to the fraction of acid molecules that donate protons to form ions in a solution. For a weak acid, this means a small percentage of its molecules dissociate into ions.

Step 2: Recall Le Châtelier's Principle

Le Châtelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change. This is crucial to understanding the behavior of weak acids as concentration changes.

Step 3: Examine the Expression for Ionization Equilibrium

The ionization of a weak acid (HA) in water can be represented by the equation: \[ HA \rightleftharpoons H^+ + A^- \]. The equilibrium constant expression is \( K_a = \frac{[H^+][A^-]}{[HA]} \).

Step 4: Consider the Effect of Concentration on Ionization

As concentration of acid increases, according to Le Châtelier's principle, the system will shift to reduce the concentration of acid. However, because \( K_a \) is a constant, this results in a smaller degree of ionization as the concentration of HA at equilibrium increases.

Step 5: Conclusion on Concentration and Degree of Ionization

Although more acid is present, a smaller fraction is ionized because the equilibrium constant does not change with concentration. Therefore, the degree of ionization actually decreases as concentration increases.

Key Concepts

Degree of Ionization

The degree of ionization is an essential concept when studying weak acids, as it describes how completely an acid dissociates into ions in an aqueous solution. For weak acids, this degree is typically low because they do not fully dissociate.
To understand the degree of ionization, imagine an acidity scale. A higher degree of ionization indicates more ion formation. Conversely, a lower degree indicates fewer ions.
When the concentration of a weak acid is increased, fewer molecules tend to donate protons. This is because the system attempts to minimize change according to Le Châtelier's Principle, hence the degree of ionization decreases with concentration increases.

• Low Degree of Ionization: Weak acids, like acetic acid, ionize less than strong acids.
• Influence of Concentration: As concentration rises, there is a smaller fraction of dissociated ions.

Le Châtelier's Principle

Understanding Le Châtelier's Principle helps us predict how a change in conditions affects chemical equilibrium. This principle asserts that any change in the conditions of a reaction prompts the equilibrium to shift in a way that counteracts the change.
For weak acids, consider an increase in concentration. According to this principle, adding more acid disrupts the equilibrium. The system adjusts by favoring the reverse reaction, where ions recombine to form the non-dissociated acid.
This decrease in ionization helps maintain the equilibrium constant over varying concentrations.

• Equilibrium Shift: Adding reactants or products causes shifts to maintain balance.
• Response to Concentration Changes: More acid means less ionization to restore equilibrium.

Equilibrium Constant

The equilibrium constant, specifically for ionization of weak acids, is denoted as \( K_a \). This constant is crucial because it defines the ratio of the concentration of products (ions) to reactants (the non-dissociated acid) at equilibrium.
The equilibrium constant is unaffected by concentration changes or alteration of other conditions like pressure or temperature. This stability means that even when quantities of weak acid are added, the actual reaction balance, expressed via \( K_a \), remains constant.
Thus, as concentration increases, the proportion of acid that ionizes decreases to maintain the equilibrium constant.

• Consistency Across Conditions: \( K_a \) remains unchanged by concentration shifts, ensuring consistent reaction proportions.
• Importance in Reactions: Helps predict the extent of ionization under various conditions, significant for understanding and calculating reaction progress.