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; statements 1 and 2 are incorrect.

Step 1: Understanding Specific Conductance

Specific conductance, also known as conductivity, typically decreases with dilution because the number of ions per unit volume decreases as the solution becomes more dilute. Hence, statement (1) "specific conductance increases with dilution" is incorrect.

Step 2: Analyzing Equivalent Conductance

Equivalent conductance increases with dilution because the ions become more free to move as the interactions between them get reduced in a more dilute solution. Therefore, statement (2) "equivalent conductance decreases with dilution" is also incorrect.

Step 3: Examining Electrolyte Conductance and Temperature

The conductance of electrolytes generally increases with temperature. This is because heating provides more energy for the ions to move faster, increasing the conductance. Statement (3) "the conductance of all electrolytes increases with temperature" is correct.

Step 4: Choosing the Correct Answer Based on Analysis

Based on the analysis, statements 1 and 2 are incorrect, while statement 3 is correct. Thus, the correct choice that includes only incorrect statements is option (a) 1 and 2.

Key Concepts

Specific Conductance

Specific conductance, also commonly referred to as conductivity, is a measure of a solution's ability to conduct electricity. This conductance depends on the presence and the concentration of ions within the solution. In simpler terms, the more ions you have in a solution, the better it conducts electricity.
Now, how exactly does dilution affect this? When we dilute a solution, we are effectively adding more solvent without increasing the amount of solute, which means the concentration of ions decreases. Consequently, specific conductance usually decreases with dilution because you now have fewer electrical carriers (ions) per unit volume available to contribute to the flow of electric current.
This concept is quite important in electrochemistry, particularly in understanding the behavior of ionic solutions in different concentrations. As explained, the statement "specific conductance increases with dilution" is indeed incorrect because specific conductance tends to decrease as dilution increases.

Equivalent Conductance

Equivalent conductance provides insight into the conductance contributed by one equivalent of an electrolyte at a given concentration. It differs from specific conductance by focusing on the effect of dilution on the mobility of ions rather than just their concentration.
When we dilute a solution, the volume increases, but the number of ions remains the same. In this case, ions are less crowded and experience fewer interactions with each other, such as ion pairing or cluster formation. This enables them to move more freely, enhancing their ability to conduct electricity more efficiently. Thus, equivalent conductance increases with dilution because the ions encounter less hindrance in a more diluted environment.
Therefore, the statement "equivalent conductance decreases with dilution" is incorrect, as it actually behaves in an opposite fashion with increased mobility for ions leading to increased equivalent conductance.

Temperature Effect on Conductance

Temperature has a pronounced effect on the conductance of electrolytes. As temperature increases, energy is imparted to the ions in solution, enhancing their kinetic energy. This results in ions moving more rapidly through the solution, contributing to increased conductance.
Essentially, the higher the temperature, the faster and more energetically the ions can travel within the solution. This energetic boost effectively reduces the resistance encountered by ions, facilitating more efficient current flow.
It's crucial to note that while the statement "the conductance of all electrolytes increases with temperature" is generally accurate, there can be exceptions. Some electrolytes might form complexes or precipitates at higher temperatures, potentially reducing conductance. Nonetheless, for the majority of cases, an increase in temperature does result in enhanced conductance, affirming that the conductance of electrolytes typically increases with temperature as described in the original exercise.