Problem 8 - Chemistry in 30 days

Question

The increase in equivalent conductance of a weak electrolyte with dilution is due to (a) Increase in degree of dissociation and decrease in ionic mobility (b) Decrease in degree of dissociation and decrease in ionic mobility (c) Increase in degree of dissociation and increase in ionic mobility (d) Decrease in degree of dissociation and increase in ionic mobility

Step-by-Step Solution

Answer

The answer is (c) Increase in degree of dissociation and increase in ionic mobility.

Step 1: Understanding Equivalence Conductance

The equivalent conductance of an electrolyte is the conductance of all ions produced by one equivalent of an electrolyte in a solution. For weak electrolytes, conductance increases with dilution because dissociation into ions increases.

Step 2: Analyzing Degree of Dissociation

When a weak electrolyte is diluted, it dissociates more because the increased space allows more ions to separate. This means the degree of dissociation, or fraction of the electrolyte that dissociates into ions, increases with dilution.

Step 3: Evaluating Ionic Mobility

Ionic mobility refers to the ability of ions to move through the solution. With dilution, ions have more space available, reducing intermolecular interactions and increasing their mobility.

Step 4: Correlating Conductance with Changes

The key factors that increase equivalent conductance with dilution are both the increased degree of dissociation and the increased ionic mobility. Both contribute to more ions moving freely and conducting electricity more effectively.

Step 5: Selecting the Correct Answer

Based on the understanding of dilution's impact, both increased degree of dissociation and increased ionic mobility correspond to (c) Increase in degree of dissociation and increase in ionic mobility.

Key Concepts

Degree of Dissociation

Degree of dissociation is an important concept when studying weak electrolytes. It refers to the fraction of the original substance that breaks apart into ions when dissolved in a solvent. For weak electrolytes, only a small fraction dissociates under normal conditions. However, as the solution is diluted, the degree of dissociation increases.

This occurs because more space in a diluted solution allows more ions to spread out, reducing the interaction between them. Therefore, as more ions become free, the solution becomes a better conductor of electricity. The higher the degree of dissociation, the greater the number of ions that contribute to the solution's conductance.

• Fraction of original substance dissociated into ions.
• Increases with dilution for weak electrolytes.
• Higher degree of dissociation results in increased conductance.

Ionic Mobility

Ionic mobility is another key concept essential for understanding how solutions conduct electricity. It describes how easily ions can move through a solution. When a solution is diluted, the ions have more space, which reduces the collision and attraction forces between them. This helps them move more freely and quickly.

The increased freedom of movement assists in enhancing the conductivity of the solution. With greater ionic mobility, the ions facilitate a more effective transfer of electric current. In contrast, concentrated solutions have a higher ion interaction, limiting mobility and hence reducing the conductivity.

• Ease of movement for ions in a solution.
• Increases with dilution due to reduced inter-ionic forces.
• Higher mobility enhances the solution's conductance.

Weak Electrolytes

Weak electrolytes are substances that do not completely dissociate into ions in solution. This incomplete dissociation means that only a fraction of the molecules exist as ions, contributing to the solution's overall conductivity. Examples include acetic acid (\( CH_3COOH \)) and ammonium hydroxide (\( NH_4OH \)).

When a weak electrolyte is diluted, the degree of dissociation increases. This is because weakening intermolecular forces allow more molecules to split into ions. Therefore, even though weak electrolytes initially have low conductance, their ability to conduct electricity can increase significantly through dilution.

• Substances that partially dissociate in solution.
• Dilution increases conductance by increasing dissociation.
• Common examples include organic acids and bases.