Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which one is more acidic in aqueous solution. (a) \(\mathrm{NiCl}_{2}\) (b) \(\mathrm{FeCl}_{3}\) (c) \(\mathrm{AlCl}_{3}\) (d) \(\mathrm{BeCl}_{2}\)

Step-by-Step Solution

Answer

AlCl₃ is the most acidic in aqueous solution.

Step 1: Understanding Acidity Concepts

Acidity in aqueous solutions is related to the ability of a substance to donate protons (H⁺ ions). In the case of metal chlorides, the acidity often increases with the positive charge on the metal ion and the ability of the cation to polarize water molecules, promoting the formation of H⁺ ions.

Step 2: Evaluate Metal Ions

Consider the metal ions in each compound: Ni²⁺ from NiCl₂, Fe³⁺ from FeCl₃, Al³⁺ from AlCl₃, and Be²⁺ from BeCl₂. The charge of the metal ion plays a significant role in its acidity due to charge density and its ability to hydrolyze water.

Step 3: Analyze Hydrolysis and Charge Density

Higher charged metal ions tend to have higher acidity because they can polarize water molecules more effectively, leading to increased release of H⁺ ions. Fe³⁺ and Al³⁺ both have a charge of +3, whereas Ni²⁺ and Be²⁺ have a charge of +2.

Step 4: Compare Polarizing Power

Between Fe³⁺ and Al³⁺, consider the effective nuclear charge and ionic radius. Al³⁺ generally has a smaller ionic radius than Fe³⁺, which means it can polarize water molecules more efficiently compared to other ions with similar charge.

Step 5: Determine the Most Acidic Compound

Based on the analysis, Al³⁺ in AlCl₃ has the highest polarizing power due to its small ionic radius and a high positive charge, making AlCl₃ more acidic in aqueous solutions compared to NiCl₂, FeCl₃, and BeCl₂.

Key Concepts

Metal Chlorides

Metal chlorides are a group of compounds consisting of a metal cation and a chloride anion. In aqueous solutions, these compounds dissolve in water, and the metal ions play a significant role in determining the properties of the solution. When we consider acidity, it's crucial to understand how the metal ions influence this property:

• Metal cation's charge: The charge on the metal ion affects its ability to interact with water molecules. Higher charged metal ions tend to exhibit higher acidity in solutions.
• Solubility in water: The solubility of metal chlorides in water allows the metal ions and chloride ions to dissociate and interact with water molecules, a key factor in exhibiting acidic behavior.

Focusing on the compounds from the exercise - the ability of these metal chlorides to release hydrogen ions (H⁺) can vary due to differences in the positive charge of the metals and their polarizing abilities.

Cation Polarization

Cation polarization is a critical factor when considering the acidity of metal chlorides in aqueous solutions. Simply put, it refers to the ability of a metal cation to distort or polarize its surrounding water molecules, leading to the release of H⁺ ions. When a cation has a high positive charge and a small ionic radius, it is better at polarizing water molecules. The key points to consider are:

• Charge density: A cation with a higher charge density will have more influence on surrounding water molecules, breaking the oxygen-hydrogen bond more efficiently, leading to higher acidity.
• Ionic radius: Smaller ions are typically better at polarizing than larger ions. Therefore, among ions with the same charge, an ion with a smaller size will usually have a stronger polarizing effect.

In the context of the exercise, Al³⁺ from AlCl₃ has a higher polarizing power compared to Ni²⁺, Fe³⁺, and Be²⁺, mainly due to its high charge and small ionic size.

Acid-Base Chemistry

Acid-base chemistry fundamentally revolves around the concepts of proton donors and acceptors. In aqueous solutions, metal chlorides can influence the concentration of H⁺ ions, thereby affecting the acidity of the solution. When we talk about acidity in metal chlorides, it relates to:

• Proton donation: The primary measurement of acidity is how easily a compound can donate protons to the solution.
• Water interaction: Metal ions that can efficiently interact and polarize water molecules encourage a higher release of H⁺ ions, thereby increasing acidity.

By analyzing the metal's properties, such as charge and size, in the compounds presented in the exercise, AlCl₃ stands out due to its ability to significantly increase the concentration of H⁺ ions in water, marking it as the most acidic among the given options.