Problem 72 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statements are incorrect? (1) specific conductance increases with dilution (2) equivalent conductance decreases with dilution (3) the conductance of all electrolytes increase with temperature Select the correct answer using the codes given below (a) 1 and 2 (b) 1 and 3 (c) 2 and 3 (d) 1,2 and 3

Step-by-Step Solution

Answer

Option (a) 1 and 2 are incorrect.

Step 1: Understanding Specific Conductance

Specific conductance, also known as conductivity, is the ability of a solution to conduct electric current. It is directly related to the concentration of ions in the solution. As the solution is diluted, the concentration of ions decreases, thus specific conductance decreases. Therefore, statement (1) is incorrect.

Step 2: Evaluating Equivalent Conductance

Equivalent conductance is defined as the conductance of one equivalent of an electrolyte dissolved in a solution. As dilution increases, the ions are further apart but more freely moveable, leading to an increase in equivalent conductance. Therefore, statement (2) is incorrect.

Step 3: Considering Temperature Effects on Conductance

For most electrolytes, increasing the temperature increases the kinetic energy of ions, which usually results in greater movement and hence higher conductance. However, while this is generally true, it may not apply universally to all electrolytes in all conditions. Statement (3) is generally correct but not absolute.

Step 4: Summarizing Incorrect Statements

Combining our evaluation, both statement (1) and (2) are incorrect, while statement (3) tends to be correct. Hence, the incorrect statements using the given codes include (a) 1 and 2.

Key Concepts

Specific Conductance

Specific conductance is a fundamental concept in electrochemistry that describes a solution's capacity to transmit electric current. It's essential to remember that specific conductance is dependent on the concentration of ions in the solution.

• Ion Concentration: Higher ion concentration results in greater specific conductance. This is because more ions are available to carry charge.
• Effect of Dilution: When a solution is diluted, the concentration of ions decreases. As a result, specific conductance typically decreases because there are fewer ions to carry the current.

This means that contrary to statement (1) in the exercise, specific conductance does not increase with dilution. Instead, it decreases due to the reduced number of charge carriers in the solution. It's crucial to grasp this point when working with solutions in electrochemical processes.

Equivalent Conductance

Equivalent conductance provides insight into how effectively ions in a solution can conduct electricity. This measurement is particularly useful for comparing the conductances of different electrolytes.

• Definition: It refers to the conductance of an electrolyte solution containing one equivalent (gram equivalent weight) of the electrolyte.
• Effect of Dilution: As you dilute a solution, equivalent conductance actually increases. This might seem counterintuitive at first, but it becomes clearer when you consider the nature of ion movement.

When a solution is highly diluted, ions have more freedom to move without being restricted by other ions. This means that each ion can carry charge more effectively despite being further apart. That’s why, unlike specific conductance, equivalent conductance increases as a solution becomes more diluted. Statement (2) from the exercise, suggesting that it decreases with dilution, is incorrect.

Temperature Effects on Conductance

Temperature plays a crucial role in electrolytic conductance. Generally, as temperature increases, so does the conductance of electrolytic solutions.

• Kinetic Energy: Warmer solutions mean increased kinetic energy of the ions, making them move more rapidly. This enhanced movement increases conductance.
• Common Trend, Not Absolute: While most solutions behave this way, it's important to remember that exceptions might exist depending on the specific electrolyte and conditions.

In most cases, elevated temperatures will cause an increase in conductance due to faster ion movement. However, students should be cautious and avoid thinking of this effect as universal. Although statement (3) is generally correct, certain electrolytes or external factors may lead to different outcomes.