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) \([\mathrm{Al}(\mathrm{OH})]^{3-}+3 \mathrm{HCl}\) (d) \(\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{Cl}^{-}\)

Step-by-Step Solution

Answer

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

Step 1: Analyze given compound

Aluminium chloride exists as a dimer, \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\), both in the solid state and in solution when in non-polar solvents like benzene. However, this form changes when dissolved in water.

Step 2: Identify possible interactions with water

Aluminium chloride is a Lewis acid, and when it is dissolved in water, it undergoes hydrolysis. This means it reacts with water to form different species than when it is in a non-polar solvent or solid-state.

Step 3: Expect resulting species in water

In water, the \(\mathrm{AlCl}_3\) part of \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) will dissociate and undergo hydrolysis. As a result, \(\mathrm{Al}^{3+}\) generally coordinates with water molecules to form the hexaaqua complex \(\left[\mathrm{Al}\left(\mathrm{H}_2\mathrm{O}\right)_6\right]^{3+}\).

Step 4: Identify release of other ions

When \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\) dissociates in water and hydrolyzes, chloride ions \(\mathrm{Cl}^-\) are released into solution as the counter ion to the positively charged complex.

Step 5: Choose the correct option based on reaction

The process ultimately leads to the formation of \(\left[\mathrm{Al}\left(\mathrm{H}_2\mathrm{O}\right)_6\right]^{3+}\) and \(3 \mathrm{Cl}^-\) ions per aluminum chloride unit. This corresponds to option (d) \(\left[\mathrm{Al}\left(\mathrm{H}_2\mathrm{O}\right)_6\right]^{3+}+3 \mathrm{Cl}^-\).

Key Concepts

Dimerization

Dimerization is the process through which two identical compounds, or monomers, chemically bond to form a dimer. In the case of aluminum chloride, \(\mathrm{AlCl}_3\) units link together to create the dimer \(\mathrm{Al}_{2}\mathrm{Cl}_{6}\). This occurs because aluminum, a small atom with a high charge density, needs to satisfy its coordination number.A few reasons why dimerization happens include:

• Stabilization: Forming a dimer can lower the system's energy, making it more stable in non-polar solvents or solid states.
• Coordination Satisfaction: The aluminum in \(\mathrm{AlCl}_3\) aims to achieve six coordinated chloride ions, which the dimer can accommodate.

Dimerization doesn't occur when \(\mathrm{AlCl}_3\) is dissolved in water because water's polar nature can effectively solvate both aluminum and chloride independently.

Hydrolysis

Hydrolysis is a chemical reaction that involves breaking down a compound by reacting it with water. For aluminum chloride \(\mathrm{AlCl}_3\), which behaves differently in water, hydrolysis plays a crucial role.When \(\mathrm{AlCl}_3\) is dissolved in water:

• It reacts with water because of its basic and acidic balance disturbance.
• The \(\mathrm{Al}^{3+}\) ions undergo hydrolysis, leading to the formation of acidic species, such as \([\mathrm{Al}(\mathrm{OH})]^{3-}\).
• Hydrolysis results in the evolution of \(\mathrm{Cl}^{-}\) ions.

This complex reaction effectively turns aluminum chloride into a new form that can coexist in aqueous solutions, such as the hexaaqua complex \(\left[\mathrm{Al}(\mathrm{H}_{2}\mathrm{O})_{6}\right]^{3+}\).

Lewis Acids

A Lewis acid is a compound or element that can accept an electron pair. In the realm of aluminum chloride chemistry, \(\mathrm{AlCl}_3\) is a classic example of a Lewis acid.Key attributes of \(\mathrm{AlCl}_3\) as a Lewis acid include:

• Electron Deficiency: Aluminum in \(\mathrm{AlCl}_3\) has an empty orbital, making it thirsty for electron pairs.
• Bond Formation: Its electron-accepting ability allows it to form bonds with electron donors—referred to as Lewis bases.
• Hydrolysis: When \(\mathrm{AlCl}_3\) encounters water, its Lewis acid characteristic promotes the uptake of hydroxide ions, leading to hydrolysis.

Understanding this property of \(\mathrm{AlCl}_3\) helps explain its behavior in reactions and its role in larger structural changes such as dimerization and complex formation.

Coordination Complexes

Coordination complexes involve a central atom that binds to surrounding molecules or ions, known as ligands. When aluminum chloride reacts in water, it forms a well-known coordination complex.Here's how it works with \(\mathrm{AlCl}_3\):

• The \(\mathrm{Al}^{3+}\) ion coordinates with water molecules due to their polar nature, forming the hexaaqua complex \(\left[\mathrm{Al}(\mathrm{H}_2\mathrm{O})_6\right]^{3+}\).
• The coordination number refers to the number of ligand bonds with the central atom. Here, aluminum coordinates with six water molecules.
• This forms a stable complex in solution, vital to the hydrolysis process.

Understanding coordination complexes is essential for comprehending how products are formed and dissolved into solutions, especially for metal compounds such as \(\mathrm{AlCl}_3\).