How conductance varies with concentration of weak electrolytes?

Increase in conductance at lower concentration 

The correct answer is option 1. Increase in conductance at lower concentration.

Conductance refers to the ability of an electrolyte solution to conduct electricity. It is influenced by the number of ions in the solution and their mobility.

Types of Conductance

1. Specific Conductance \((\kappa )\): This is the conductance of a solution containing a specific concentration of electrolyte, measured in siemens per meter (S/m).

2. Molar Conductance \((\Lambda _m)\): This is the conductance of a solution containing one mole of the electrolyte per liter of solution, measured in siemens meter squared per mole \((S·m^2/mol)\).

Behavior of Weak Electrolytes

Weak electrolytes only partially dissociate into ions in solution. Examples include acetic acid \((CH_3COOH)\) and ammonia \((NH_3)\).

Conductance and Concentration

1. At Higher Concentrations:

Specific Conductance \((\kappa )\): Increases because there are more ions in the solution, but this is a relatively small effect for weak electrolytes

Molar Conductance \((\Lambda _m)\): Decreases because:

Incomplete Ionization: At higher concentrations, weak electrolytes are less ionized. For example, in a concentrated solution of acetic acid, most of the molecules remain undissociated.

Interionic Interactions: The ions present are closer together, leading to stronger electrostatic interactions. This reduces their mobility and thus the conductance.

2. At Lower Concentrations:

Specific Conductance \((\kappa )\): Decreases because there are fewer ions in the solution.

Molar Conductance \((\Lambda _m)\): Increases because:

Increased Ionization: As the solution is diluted, the degree of ionization of the weak electrolyte increases. More molecules dissociate into ions.

Reduced Interionic Interactions: The ions are more spread out in a dilute solution, experiencing less electrostatic attraction and moving more freely. This increases their mobility and thus the conductance.

Example: Acetic Acid \((CH_3COOH)\)

High Concentration: In a concentrated solution of acetic acid, most \(CH_3COOH\) molecules remain intact, and only a small fraction ionizes into \(CH_3COO^-\) and \(H^+\). The specific conductance might increase slightly due to more ions, but the molar conductance decreases because the ions are less mobile.

Low Concentration: As the solution is diluted, more \(CH_3COOH\) molecules ionize. The molar conductance increases significantly because the ions are now more mobile due to reduced interionic attractions.

If we plot molar conductance \((\Lambda _m)\) against the concentration of a weak electrolyte, we typically observe that:

\((\Lambda _m)\) increases sharply at lower concentrations, indicating increased ionization and ion mobility.

\((\Lambda _m)\) levels off at higher concentrations because ionization is limited and ion mobility is reduced due to interionic interactions.

Conclusion

For weak electrolytes, as the concentration decreases (dilution increases), the degree of ionization increases, leading to more ions and less interionic attraction. This results in an increase in molar conductance \((\Lambda _m)\) at lower concentrations.

Therefore, the correct explanation is: Increase in conductance at lower concentration.