Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The nature of an aqueous solution of \(\mathrm{FeCl}_{2}\) is (a) basic (b) acidic (c) amphoteric (d) neutral

Step-by-Step Solution

Answer

The aqueous solution of f{FeCl}_2 is acidic.

Step 1: Identify the ions in the solution

When \(f{FeCl}_2\) is dissolved in water, it dissociates into \(f{Fe}^{2+}\) and \(f{Cl}^-\) ions.

Step 2: Analyze the effect of each ion

1. \(f{Cl}^-\) is the conjugate base of a strong acid (f{HCl}) and does not affect the f{pH} of the solution significantly.2. \(f{Fe}^{2+}\) acts as a Lewis acid and can hydrolyze with water to form f{Fe(OH)}^+ and \(f{H}^+\) ions, which increases the f{H}^+ concentration and decreases the f{pH}.

Step 3: Conclude the nature of the solution

Since \( f{Fe}^{2+} \) ions increase the concentration of \( f{H}^+ \) ions through hydrolysis, the solution becomes acidic as a result. Thus, the nature of an aqueous solution of \( f{FeCl}_2 \) is acidic.

Key Concepts

Lewis Acids and Bases

In chemistry, the concept of Lewis acids and bases revolves around the idea of electrons being shared or transferred. A Lewis acid is defined as a substance that can accept a pair of electrons, while a Lewis base is one that can donate a pair of electrons. This definition expands on the classic Brønsted-Lowry definition by focusing on the role of electron pairs rather than protons.

When we encounter an ion like \( \mathrm{Fe}^{2+} \), it acts as a Lewis acid because it has the ability to accept electrons from surrounding molecules, such as water.

• Lewis acids are often compounds with a positive charge or atoms deficient in electrons.
• Lewis bases, on the other hand, typically possess a lone pair of electrons and may carry a negative charge.

  In aqueous solutions, the interaction of Lewis acids and bases is pivotal in determining the overall pH and chemical behavior of the solution.

Hydrolysis

Hydrolysis is a crucial chemical process where water is used to break down the bonds of a particular substance. In the case of \( \mathrm{FeCl}_2 \), the \( \mathrm{Fe}^{2+} \) ion can undergo hydrolysis when it interacts with water molecules.
The hydrolysis of \( \mathrm{Fe}^{2+} \) involves a reaction that produces \( \mathrm{Fe(OH)}^+ \) and \( \mathrm{H}^+ \) ions:

• \( \mathrm{Fe}^{2+} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Fe(OH)}^+ + \mathrm{H}^+ \)

This reaction releases hydrogen ions \( (\mathrm{H}^+) \), increasing the acidity of the solution. The process of hydrolysis indicates that not only does the original compound react with water, but it also impacts the pH by altering the concentration of hydrogen ions.
Understanding hydrolysis helps explain why certain solutions are acidic or basic based on the chemicals present and their reactions with water.

Strong Acids and Their Conjugate Bases

When dealing with strong acids, such as hydrochloric acid \( (\mathrm{HCl}) \), it is important to understand their dissociation behavior and the role of their conjugate bases. A strong acid readily dissociates in water, releasing hydrogen ions \( \mathrm{H}^+ \) into the solution.
For example, \( \mathrm{HCl} \) dissociates as follows:

• \( \mathrm{HCl} \rightarrow \mathrm{H}^+ + \mathrm{Cl}^- \)

The \( \mathrm{Cl}^- \) produced is known as the conjugate base of the strong acid \( \mathrm{HCl} \). However, this conjugate base is very weak and does not significantly influence the pH of a solution because it barely recombines with hydrogen ions.

• Strong acids have very weak conjugate bases.
• The presence of a weak conjugate base means the original acid remains fully dissociated, contributing to a lower pH.

Thus, in solutions where strong acids are involved, the weak conjugate bases typically do not counteract the effect of the strong acid, leaving the solution more acidic.