Problem 192 - A Complete Resource Book in Chemistry for JEE Main

Question

The electrical conductivity of the flowing aqueous solutions is highest for [2008] (a) \(0.1 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) (b) \(0.1 \mathrm{M} \mathrm{CH}_{2} \mathrm{FCOOH}\) (c) \(0.1 \mathrm{M} \mathrm{CHF}_{2} \mathrm{COOH}\) (d) \(0.1 \mathrm{M} \mathrm{CH}_{2} \mathrm{ClCOOH}\)

Step-by-Step Solution

Answer

(c) 0.1 M CHF_2COOH has the highest conductivity.

Step 1: Understanding Electrical Conductivity

Electrical conductivity in a solution is determined by the concentration and mobility of ions. The more ions present, and the more mobile these ions are, the higher the conductivity.

Step 2: Evaluating the Solutions

In all the given options, the concentration ( 0.1 M ) is the same, so the conductivity will depend on the ionization of these acids in water. Stronger acids ionize more completely in water, generating more ions and thus higher conductivity.

Step 3: Analyzing the Effect of Substituents

The presence of electronegative atoms, like F (fluorine) and Cl (chlorine), increases the acidity of carboxylic acids (inductive effect). In particular, more electronegative and more such atoms closer to the carboxyl group increase acidity. CHF_2COOH has two fluorine atoms, making it the strongest acid due to increased electron withdrawing effect, and thus it would ionize more completely compared to the others.

Key Concepts

Ionization in Solutions

Ionization in solutions refers to the process where a compound dissociates into ions when dissolved in a solvent, typically water. This process is crucial for electrical conductivity, as ions are the charged particles that carry current through a solution.
When a compound like an acid is dissolved in water, it releases hydrogen ions (H^+) and sometimes other anions as well (like CH_3COO^-  in acetic acid). The degree of ionization depends on the strength of the acid. Strong acids fully ionize in solution, producing a large number of ions, leading to high conductivity. Conversely, weak acids partially ionize, resulting in fewer free ions and lower conductivity.
The ability of a solution to conduct electricity is directly connected to the concentration and mobility of these ions. Therefore, understanding ionization helps us predict and compare the conductivity of different acid solutions.

Electronegative Substituents

Electronegative substituents are atoms or groups that can attract electrons towards themselves, affecting the acidity of compounds where they are present. Fluorine and chlorine are highly electronegative elements that can significantly alter the properties of a molecule, such as its acidity.
In carboxylic acids, electronegative substituents placed near the carboxyl group (COOH) enhance the acid's strength. This is because they withdraw electron density away from the carboxyl group via the inductive effect, stabilizing the resulting anion once the acid dissociates. This makes it easier for the acid to lose a proton (H^+ ), enhancing its ionization.
For example, in the exercise provided, two fluorine atoms in CHF_2COOH  make it a stronger acid than CH_2FCOOH, because the increased number of electronegative atoms near the carboxyl group increases electron withdrawal, enhancing ionization, and thus conductivity.

Strength of Acids

The strength of an acid is its ability to donate protons (H^+). Strong acids ionize fully in solution, while weak acids do so partially. The position and number of substituents, such as electronegative atoms, directly impact acid strength.
Stronger acids, with a greater tendency to ionize, contribute more ions to the solution, enabling higher electrical conductivity. The inductive effect of electronegative substituents plays a crucial role here. Each substituent affects the polarizability of the molecule, which influences how easily it can lose a proton.
In analyzing acids like CHF_2COOH from the exercise, it is evident that the influence of more fluorine atoms increases acid strength via electron withdrawal. Consequently, CHF_2COOH  is stronger compared to weaker acids like CH_3COOH, leading to its higher conductivity.