Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

Aluminium chloride exists as dimer \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) in solid state as well as in solution of non-polar solvents such as benzene. When dissolved in water, it gives (a) \(\mathrm{Al}_{2} \mathrm{O}_{3}+6 \mathrm{HCl}\) (b) \(\mathrm{Al}^{3+}+3 \mathrm{Cl}^{-}\) (c) \(\left[\mathrm{Al}(\mathrm{OH})_{6}\right]^{3-}+3 \mathrm{HCl}\) (d) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

Option (d) is correct: \(\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right]^{3+} + 3 \mathrm{Cl}^{-}\).

Step 1: Understanding the Compound's Behavior

In non-polar solvents, aluminium chloride (\(\mathrm{AlCl}_3\)) can exist as the dimer \(\mathrm{Al}_2\mathrm{Cl}_6\). However, when it interacts with water, the compound dissociates due to its behavior as a Lewis acid.

Step 2: Reaction with Water

When \(\mathrm{AlCl}_3\) is dissolved in water, it hydrolyzes because water is a polar solvent and the Lewis acidity of \(\mathrm{AlCl}_3\) prompts the chloride ions to leave. This results in the formation of \(\text{Al}^{3+}\) ions and \(\text{Cl}^{-}\) ions.

Step 3: Identifying the Correct Dissolution

In aqueous solution, aluminium chloride dissociates completely, forming hexaaquaaluminium complex ions. \([\text{Al}(\text{H}_2\text{O})_6]^{3+}\) is formed as \(\text{Al}^{3+}\) ions attract six water molecules, while the \(\text{Cl}^-\) ions are free in solution.

Step 4: Conclusion - Choosing the Correct Option

Based on the dissolution and reaction with water, the correct dissolution would form \([\text{Al}(\text{H}_2\text{O})_6]^{3+}\) complex with 3 \(\text{Cl}^-\) ions in the solution. Hence, option (d) is the correct answer.

Key Concepts

Lewis acid

At its core, a Lewis acid is a chemical species that can accept a pair of electrons. In the context of aluminium chloride (\(\text{AlCl}_3\)), it acts as a classic Lewis acid. This property arises because aluminium in \(\text{AlCl}_3\) has an empty orbital which makes it desire additional electrons to become more stable.

• Aluminium’s Role: Aluminium can accept electron pairs due to its electron-deficient nature.
• Interaction with Bases: Lewis acids typically react with Lewis bases, which are molecules with lone pairs of electrons available for donation.

When \(\text{AlCl}_3\) is in water, it readily accepts electron pairs from water molecules, leading to hydrolysis. This characteristic of being a Lewis acid explains why \(\text{AlCl}_3\) undergoes various reactions in different environments.

Dimerization

Dimerization refers to a chemical reaction wherein two molecules join to form a larger molecule, known as a dimer. Aluminium chloride frequently exists as \(\text{Al}_2\text{Cl}_6\) when in the solid state or in non-polar solvents like benzene.

• Formation of Dimers: In \(\text{Al}_2\text{Cl}_6\), two \(\text{AlCl}_3\) units share chloride ions, which effectively links them together.
• Non-Polar Solvents: In non-polar solvents, \(\text{Al}_2\text{Cl}_6\) is stable due to the lack of competition from more polar molecules.

In these environments, the dimer is stable because it helps alleviate some of the electron deficiency of aluminium by creating additional bonds. This stability is disrupted in polar solvents, leading to the breakdown of the dimer into monomers.

Hydrolysis

Hydrolysis is the process where a compound reacts with water to form new products. Aluminium chloride undergoes hydrolysis when dissolved in water, primarily due to its Lewis acid nature.

• Polar Solvents and Hydrolysis: In polar solvents like water, the interaction with \(\text{AlCl}_3\) leads to the breakdown of its structure, resulting in the formation of \(\text{Al}^{3+}\) and \(\text{Cl}^-\) ions.
• Water's Role: Water acts as both a donor of electron pairs to aluminium and a medium for the removal of chloride ions.

The hydrolysis of \(\text{AlCl}_3\) explains why it does not exist as \(\text{Al}_2\text{Cl}_6\) in aqueous solutions. Instead, it yields free aluminium ions that are ready to form complex ions with water molecules.

Complex ion formation

When dissolved in water, aluminium ions form complex ions readily. A complex ion is a charged species consisting of a central atom surrounded by other molecules or ions.

• Structure of Hexaaquaaluminium: The \([\text{Al}(\text{H}_2\text{O})_6]^{3+}\) complex is formed in aqueous solutions. Here, six water molecules coordinate around a central \(\text{Al}^{3+}\) ion.
• Aqueous Dissolution: Upon dissolution, \(\text{AlCl}_3\) hence leads to the formation of these complex ions as a way to stabilize the aluminium ion in solution.

This aqueous environment leads to a stable arrangement of waters around aluminium, which significantly influences the chemical properties of the solution. The formation of such complexes highlights the key role of water in chemical stabilization and ionic interactions.