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 FeCl₂ is acidic (option b).

Step 1: Identify the components

FeCl₂ is composed of Fe²⁺ (ferrous ion) and Cl⁻ (chloride ion). In an aqueous solution, the compound dissociates into these ions.

Step 2: Analyze the ions

The chloride ion (Cl⁻) comes from a strong acid (HCl), and it is a neutral spectator ion in solution. The ferrous ion (Fe²⁺) can potentially hydrolyze in water.

Step 3: Consider hydrolysis

Fe²⁺ can hydrolyze in water to form Fe(OH)⁺ and H⁺ ions: \[\mathrm{Fe}^{2+} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{Fe(OH)}^{+} + \mathrm{H}^{+}\]This reaction releases H⁺ ions, which can make the solution acidic.

Step 4: Determine the final nature of the solution

Since the hydrolysis of Fe²⁺ releases H⁺ ions, it causes the solution to have a higher concentration of H⁺ ions than OH⁻ ions, making the solution acidic.

Key Concepts

Aqueous Solutions

An aqueous solution is a type of mixture where the solvent is water. In chemistry, when substances are dissolved in water, they form ions, which allows them to interact with one another.
When we talk about an aqueous solution, we often refer to both the solute (the substance being dissolved) and the solvent (water, in this case).

• Water is known as a 'universal solvent' due to its excellent capability to dissolve numerous substances.
• In an aqueous solution, the dissolved substance can either be ionic or molecular.

The key feature of an aqueous solution is the presence of dissolved substances, which are often completely ionized, meaning the substances break apart into their respective ions.
For example, in a solution of FeCl₂ in water, the compound dissociates into Fe²⁺ and Cl⁻ ions, as the exercise above describes.
Understanding the nature of these solutions helps us predict how different substances will behave when mixed with water.

Hydrolysis

Hydrolysis is a chemical process that involves the reaction of a compound with water. In the context of acid-base chemistry, it refers to the reaction of ions with water, often resulting in the formation of an acid or a base.
When Fe²⁺ ions are placed in water, they tend to undergo hydrolysis, a reaction where they interact with water molecules.
This happens as follows where Fe²⁺ reacts with water to form Fe(OH)⁺ and releases H⁺ ions: \[\mathrm{Fe}^{2+} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{Fe(OH)}^{+} + \mathrm{H}^{+}\]This reaction shows that hydrolysis can change the pH of the solution, making it more acidic due to the release of hydrogen ions.

• Hydrolysis is significant as it can alter the chemical environment in a solution, affecting its acidity or basicity.
• Not all ions undergo hydrolysis; it depends on their nature and the strength of their parent acids or bases.

This process is particularly important in solutions like FeCl₂, as it explains why the solution may turn acidic due to the behavior of the Fe²⁺ ions.

pH and Acidity

pH is a measurement of the acidity or basicity of a solution, reflecting the concentration of hydrogen ions (H⁺) present. The pH scale ranges from 0 to 14, with values below 7 indicating an acidic solution and those above 7 indicating a basic one.
Acidic solutions have higher concentrations of H⁺ ions. In contrast, basic solutions have fewer H⁺ ions, usually compensated by higher OH⁻ ion concentrations.
For example:

• A neutral pH is around 7, meaning the concentrations of H⁺ and OH⁻ ions are equal.
• In the presence of Fe²⁺ ions, hydrolysis increases the H⁺ ion concentration, thereby decreasing the pH.

The acidic behavior of an aqueous solution like FeCl₂ arises from the Fe²⁺ ions' capability to undergo hydrolysis, releasing H⁺ ions. This means that even without an initial strong acid in the solution, the interaction between the ions and water can significantly influence the pH.
Understanding how pH works and what influences it is crucial for predicting the behavior of various chemical reactions in aqueous solutions.