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

Question

Heating an aqueous solution of aluminium chloride to dryness will give \(\quad\) [2005] (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

Heating aluminium chloride solution results in \(\text{Al}_2\text{Cl}_6\).

Step 1: Understand the Dehydration Process

When an aqueous solution containing aluminium chloride \((\text{AlCl}_3\cdot x\text{H}_2\text{O})\) is heated, the water molecules are removed in the form of steam, leaving behind the anhydrous form of the substance.

Step 2: Identify the Anhydrous Form

Since aluminium chloride is a hygroscopic compound, when heated to dryness, it will convert from its hydrated form \((\text{AlCl}_3\cdot x\text{H}_2\text{O})\) to anhydrous\(\text{AlCl}_3\). However, \(\text{AlCl}_3\) is commonly found in the dimeric form as \(\text{Al}_2\text{Cl}_6\) due to its tendency to dimerize.

Step 3: Conclude with the Dimeric Form

Since the problem asks for the result after heating to dryness, which implies removal of any water molecules, the correct answer is the dimeric form of aluminium chloride, which is \(\text{Al}_2\text{Cl}_6\).

Key Concepts

Dehydration Process

The dehydration process involves removing water from a chemical compound. When an aqueous solution of aluminium chloride (\(\text{AlCl}_3 \cdot x\text{H}_2\text{O}\)) is heated, water molecules evaporate as steam. This leaves behind the substance in its dry or anhydrous state.
In chemistry, dehydration typically means losing or transferring water by heating or chemically reacting a substance.
This temperature-induced removal of water alters the compound's physical and sometimes chemical properties. It often converts hydrated compounds into anhydrous ones, which are the same in composition but devoid of water.
For students, it's important to remember:

• Water molecules are released as gas during dehydration.
• This process can significantly change the substance's behavior and appearance.
• Understanding dehydration helps in predicting the resultant form of compounds when heated.

Anhydrous Form

Anhydrous compounds are substances that lack water molecules, either in crystal form or combined in any manner. The term 'anhydrous' simply means 'without water.'
For aluminium chloride, the anhydrous form is referred to as AlCl₃.
This state is usually achieved by removing the water from its hydrated form through heating, as seen in the dehydration process.
When in anhydrous form, compounds often exhibit different properties:

• AlCl₃ without water becomes more reactive and therefore needs careful handling.
• Anhydrous aluminium chloride is deliquescent, meaning it can absorb moisture from the air.
• It's vital in industrial processes like Friedel-Crafts reactions due to its catalytic properties.

Understanding the anhydrous forms of compounds helps predict their reactivity and role in various chemical processes.

Dimerization

Dimerization is a chemical process where two molecules join to form a single entity, known as a dimer.
In the case of aluminium chloride, it tends to dimerize when anhydrous.
This dimeric form of aluminium chloride is represented as \(\text{Al}_2\text{Cl}_6\).
This transformation occurs because AlCl₃ molecules bond through chlorine bridges to achieve a more stable electronic configuration. Let's break this down further:

• Dimerization reduces the molecule's reactivity, enhancing stability.
• AlCl₃ forms \(\text{Al}_2\text{Cl}_6\) to stabilize by sharing electrons between two aluminum atoms.
• Knowing the tendency to dimerize helps predict molecular behavior in different conditions.

This characteristic of aluminium chloride is crucial in understanding its behavior in reactions and when heated.