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 [2004] (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

When aluminium chloride \(\text{Al}_2\text{Cl}_6\) is dissolved in water, it forms \([\text{Al(H}_2\text{O})_6]^{3+}\) and \(3 \text{Cl}^{-}\). Thus, option (d) is correct.

Step 1: Understand Aluminium Chloride Dimer

Aluminium chloride, in solid state and non-polar solvents, exists as a dimer \(\text{Al}_2\text{Cl}_6\). This is because individual \(\text{AlCl}_3\) molecules tend to dimerize to achieve stability.

Step 2: Dissolution in Water

When \(\text{Al}_2\text{Cl}_6\) is dissolved in water, it undergoes dissociation into its components. Since water is a polar solvent, the dimer dissociates into \( \text{Al}^{3+} \) and \( \text{Cl}^{-} \) ions.

Step 3: Formation of Complex Ions

In aqueous solution, \( \text{Al}^{3+} \) forms complex ions by coordinating with water molecules. Specifically, it forms the complex \([\text{Al(H}_2\text{O})_6]^{3+}\), which is a common behavior of \( \text{Al}^{3+} \) in water.

Step 4: Final Composition in Solution

Finally, the initial dissolution results in the ionic form: \([\text{Al(H}_2\text{O})_6]^{3+}\) and \(3 \text{Cl}^{-}\). This indicates that option (d) is the correct representation of \(\text{Al}_2\text{Cl}_6\) dissolved in water.

Key Concepts

Dimerization of Aluminium Chloride

In the study of chemistry, one often encounters the concept of dimerization, especially with compounds like aluminium chloride. Dimerization refers to the chemical process where two identical molecules combine to form a new compound, called a dimer. In the case of aluminium chloride, or \(\text{AlCl}_3\), dimerization helps achieve greater stability. This compound is known to form a dimer, \(\text{Al}_2\text{Cl}_6\), both in its solid state and when dissolved in non-polar solvents such as benzene.

But why does this happen? Well, individual \(\text{AlCl}_3\) molecules have incomplete octets, making them very reactive. By forming \(\text{Al}_2\text{Cl}_6\), each aluminium atom can achieve a stable electronic configuration through sharing chlorine atoms. This shared configuration provides greater stability by filling their valence electron shells.

• Non-polar Solvents: In environments like benzene, there is no competing polarity to break apart the dimer. This enables \(\text{Al}_2\text{Cl}_6\) to maintain its structure.
• Solid State Stability: The dimeric form also reduces the energy of the system in the solid state, contributing to its thermodynamic stability.

Understanding dimerization is crucial for grasping how aluminium chloride behaves under different solvent conditions.

Complex Ion Formation

Once aluminium chloride transitions from its dimeric form into an aqueous solution, its behaviour changes significantly. When \(\text{Al}_2\text{Cl}_6\) dissolves in water, an interesting transformation takes place. The molecules dissociate into \(\text{Al}^{3+}\) and \(\text{Cl}^{-}\) ions due to water's polar nature.

The noteworthy part of this process involves the formation of complex ions. Aluminium ions (\(\text{Al}^{3+}\)) interact with water molecules to form coordination complexes. Specifically, it binds with six water molecules to form the complex ion \[\text{[Al(H}_2\text{O})_6]^{3+}\].

• Coordination Chemistry: This complex formation is an example of coordination chemistry, where metal ions bond with several ligands that act through coordinate covalent bonds.
• Stabilization: The process leads to increased stability of the \(\text{Al}^{3+}\) ion in water, preventing it from simply staying as a solitary ion.

These complex ions are crucial in aqueous chemistry, as they dictate many properties and reactions of the solution. This is an area frequently explored in transition metal chemistry, but it is certainly relevant in the case of aluminium.

Aqueous Solutions in Chemistry

The nature of aqueous solutions significantly impacts the behaviour of compounds like aluminium chloride. When \(\text{Al}_2\text{Cl}_6\) is placed in water, the individual \(\text{AlCl}_3\) units dissociate due to water's polar characteristics. This environment is very different from non-polar solvents, impacting the chemistry notably.

Aqueous solutions are characterized by the solvent being water, a polar substance capable of stabilizing ions and facilitating various chemical reactions. Here’s how it affects the dissolution and subsequent reactions:

• Ion Dissociation: Water molecules stabilize the individual \(\text{Al}^{3+}\) and \(\text{Cl}^{-}\) ions through interactions with the charged parts of these ions.
• pH Influence: Dissolution can affect solution pH, especially with compounds that can react further with water, often resulting in acidic solutions.
• Chemical Behavior: This change in environment also makes it possible for the formation of complex ions like \[\text{[Al(H}_2\text{O})_6]^{3+}\], which would not occur in non-polar solvents.

Understanding how substances behave in aqueous solutions is pivotal to predicting their chemical behaviour and reactivity, serving as a key component of acid-base chemistry and beyond.