Question

Account for the following:
Conductivity of CH\(_3\)COOH decreases on dilution.

Hint:

Weak electrolytes like acetic acid dissociate partially, and dilution reduces the number of ions in solution, causing a decrease in conductivity.

Answer 1

To solve the problem, we need to explain why the conductivity of acetic acid (CH₃COOH) decreases on dilution.

1. Understanding Conductivity:
Conductivity is the ability of a solution to conduct electricity, which depends on the presence of charged particles (ions) in the solution. The more ions present, the higher the conductivity of the solution.

2. Nature of Acetic Acid (CH₃COOH):
Acetic acid (CH₃COOH) is a weak acid, meaning it does not fully dissociate into ions in water. In its dissociated form, acetic acid produces hydrogen ions (H⁺) and acetate ions (CH₃COO⁻) according to the following equilibrium reaction: \[ \text{CH₃COOH} \rightleftharpoons \text{CH₃COO}⁻ + \text{H}⁺ \] However, this dissociation is not complete, and only a small fraction of acetic acid molecules dissociate into ions at any given time.

3. Effect of Dilution on Conductivity:
When acetic acid is diluted, the concentration of the acetic acid molecules decreases, which means fewer molecules are available to dissociate into ions. The dissociation of acetic acid into H⁺ and CH₃COO⁻ is governed by its dissociation constant (Kₐ). On dilution, the ionization of acetic acid increases to some extent because the equilibrium shifts to the right, but this increase is relatively small compared to the decrease in the concentration of the acid molecules themselves.

4. Decrease in Conductivity with Dilution:
Although dilution increases the dissociation of acetic acid slightly, the overall effect is a decrease in conductivity. This happens because: - The number of acetic acid molecules (and thus the number of ions) decreases as the solution is diluted. - Even though the dissociation slightly increases, it is not enough to compensate for the large reduction in ion concentration caused by dilution. Thus, the overall number of ions in the solution decreases as acetic acid is diluted, leading to a decrease in conductivity.

5. Final Answer:
The conductivity of acetic acid (CH₃COOH) decreases on dilution because the concentration of ions in the solution decreases as the number of acetic acid molecules (and dissociated ions) decreases, despite a small increase in dissociation. The reduction in ion concentration outweighs the slight increase in dissociation, leading to lower conductivity.