Problem 170 - A Complete Resource Book in Chemistry for JEE Main

Question

Heating an aqueous solution of aluminium chloride to dryness will give (a) \(\mathrm{AlCl}_{3}\) (b) \(\mathrm{Al}_{2} \mathrm{Cl}_{3}\) (c) \(\mathrm{Al}(\mathrm{OH}) \mathrm{Cl}_{2}\) (d) \(\mathrm{Al}_{2} \mathrm{Cl}_{6}\)

Step-by-Step Solution

Answer

The compound obtained is  Al_2Cl_6.

Step 1: Understanding the problem

We have an aqueous solution of aluminium chloride and we heat it to dryness. We need to determine the compound obtained after the water is removed.

Step 2: Recall aluminium chloride behavior

Aluminium chloride ( AlCl_3) is a compound that can exist in different forms depending on conditions. In an aqueous solution, it may form the hydrate  AlCl_3.6H_2O due to coordination with water molecules.

Step 3: Identify the form obtained when dried

When the water is removed through heating, aluminium chloride tends to revert to its anhydrous form. In its anhydrous state, aluminium chloride forms a dimer due to its electron deficiency.

Step 4: Determine the chemical formula

The anhydrous form of aluminium chloride is known to exist as a dimer,  Al_2Cl_6, due to dative bonding between two  AlCl_3 units. Therefore, the chemical formula of aluminium chloride formed after drying is  Al_2Cl_6.

Key Concepts

Chemical Reaction Steps

When an aqueous solution of aluminium chloride is heated, a chemical reaction occurs where the water molecules evaporate. This process results in the removal of any associated water, leaving behind the solid form of aluminium chloride. It's essential to understand that during this heating process, the physical state of aluminium chloride changes from a hydrated to an anhydrous form. The exact transformation can be seen as follows:

• On heating, the water in the aluminium chloride hydrate (usually \( \text{AlCl}_3 imes 6\text{H}_2\text{O} \)) evaporates.
• This leads to a change where the hydrated aluminium chloride loses water molecules and converts into its anhydrous form.
• The final product, aluminium chloride in the absence of water, reorganizes itself to form a stable dimer, \( \text{Al}_2\text{Cl}_6 \).

This chemical transition highlights the importance of the heating step in determining the final chemical structure of the compound.

Chemical Bonding in AlCl3

The molecule of aluminium chloride, \( \text{AlCl}_3 \), exhibits some interesting features in terms of its bonding. At its core, \( \text{AlCl}_3 \) is composed of one aluminium atom bonded to three chlorine atoms. This bond is primarily covalent in nature. However, due to the small size and electron deficiency of the aluminium atom, additional bonding nuances arise:

• Aluminium has an incomplete octet, seeking additional electrons for stability.
• Chlorine atoms, each sharing an electron pair with the aluminium atom, help in forming these covalent bonds.
• In an anhydrous state, \( \text{AlCl}_3 \) tends to form a dimer \( \text{Al}_2\text{Cl}_6 \), where dative bonds (or coordinate covalent bonds) occur between two \( \text{AlCl}_3 \) units, where an electron pair from a chlorine atom is donated to an aluminium atom from an adjacent \( \text{AlCl}_3 \) molecule.

These atomic interactions make the AlCl3 dimer structure not just stable but also unique.

Properties of Aluminium Chloride Anhydrous

Aluminium chloride anhydrous, commonly represented as \( \text{Al}_2\text{Cl}_6 \), possesses several notable properties:

• It is a white or yellowish solid, often appearing as powder.
• High reactivity, especially towards moisture, where it can readily absorb water to convert back into its hydrated form.
• Due to its structure as a dimer, \( \text{Al}_2\text{Cl}_6 \) easily sublimes at about 180°C, a property distinct from many other inorganic compounds.
• It serves diverse roles such as a catalyst in organic synthesis and as a crucial component in the preparation of certain aluminium compounds.

An essential property to remember is its high affinity for forming complex ions, which can act as Lewis acids, highlighting its chemical importance in various applications.