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: Understand Specific Conductance

Specific conductance, also known as conductivity, measures how well a solution can conduct electricity. It typically decreases with dilution because there are fewer ions per unit volume in a more diluted solution.

Step 2: Understand Equivalent Conductance

Equivalent conductance is the conductance of all the ions in a solution, normalized to the amount of substance. It generally increases with dilution because ions are freer to move without interference, hence higher mobility.

Step 3: Understand Temperature's Effect

Conductance increases for most electrolytes with an increase in temperature. Higher temperatures provide energy to ions, increasing their mobility and, thus, the conductance.

Step 4: Analyze Each Statement

1. Specific conductance increases with dilution. This is incorrect because specific conductance typically decreases with dilution. 2. Equivalent conductance decreases with dilution. This is incorrect because equivalent conductance usually increases with dilution. 3. The conductance of all electrolytes increases with temperature. Generally correct under typical conditions.

Step 5: Select the Incorrect Statements

Based on the analysis, statements 1 and 2 are incorrect. Therefore, the correct answer is option (a) 1 and 2.

Key Concepts

Specific conductance

Specific conductance, often referred to simply as conductivity, is a fundamental concept in understanding how solutions facilitate the flow of electric current. This property measures the ability of a solution to conduct electricity, depending largely on the presence and concentration of ions. As the concentration of ions in a solution increases, specific conductance tends to increase, as more charge carriers are available to facilitate the flow of current.
However, when solutions are diluted, the specific conductance generally decreases. This decrease is because there are fewer ions per unit volume, which results in less charge being conducted. To visualize this, imagine spreading out a group of people standing in a small room into a much larger one; with fewer people per square foot, it's harder to pass messages from one side of the room to the other.
Key points about specific conductance:

• It is concentration-dependent.
• Decreases with dilution.
• Is measured in Siemens per meter (S/m).
• Integral for understanding how well a solution can carry electricity.

Specific conductance is crucial for analyzing solutions in chemistry and many industrial applications, ensuring that systems function efficiently.

Equivalent conductance

Equivalent conductance provides insight into the conductance of a solution when normalized for the specific amount of substance present. Unlike specific conductance, it considers the total conductance contributed by ions in a solution for a given equivalent of solute.
This property tends to increase upon dilution. As a solution is diluted, ions have more room to move freely, reducing interactions that might hinder their mobility. Imagine a crowded street suddenly clearing out; people (or ions) can move faster in the new, open space. Thus, even though the solution has fewer ions overall as it dilutes, the mobility of each ion actually increases, raising the equivalent conductance.
Highlights of equivalent conductance include:

• It is normalized to the quantity of solute.
• Measured usually in Siemens square meter per equivalent (S*m²/equiv).
• Increases with dilution due to reduced ion interactions and increased mobility.
• Useful in calculating the behavior of ions in solutions of varying concentrations.

Understanding equivalent conductance helps chemists and engineers create solutions with desired conductance levels for specific applications.

Effect of temperature on conductance

Temperature plays a significant role in the conductance of electrolytic solutions. Generally, conductance increases with an increase in temperature. This trend occurs because higher temperatures provide energy to ions, enhancing their movement and, consequently, their ability to conduct electricity.
When ions in a solution are heated, they gain kinetic energy. This boost in energy overcomes any attractive forces that might restrict their movement. Hence, at higher temperatures, ions move more rapidly and efficiently transfer charge, resulting in higher conductance. Imagine running a race on a hot day; the heat might tire you initially but gives the molecules in your body that initial energy boost.
Important points to remember about the effect of temperature on conductance:

• Higher temperatures generally increase conductance.
• Energy gained from heat increases ionic mobility.
• Conductance changes can vary slightly depending on the electrolyte type.
• Knowledge of temperature effects is crucial for processes requiring precise conductance levels.

This understanding aids in predicting how solutions behave under different thermal conditions, impacting fields from chemical engineering to electronics.