Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statements about anhydrous aluminium chloride is correct? [1981-1 Mark] (a) it exists as \(\mathrm{AlCl}_{3}\) molecules (b) it is not easily hydrolysed (c) it sublimes at \(100^{\circ} \mathrm{C}\) under vacuum (d) it is a strong Lewis base

Step-by-Step Solution

Answer

Option (c) is correct: anhydrous AlCl₃ sublimes at 100°C under vacuum.

Step 1: Understand Anhydrous Aluminium Chloride

Anhydrous aluminium chloride (\(\mathrm{AlCl}_3\)) is known for having distinct chemical characteristics. It typically exists as a dimer in the solid state, forming \(\mathrm{Al_2Cl_6}\), but in the gaseous phase, it can dissociate into \(\mathrm{AlCl_3}\) monomers.

Step 2: Evaluate Statement A

Statement (a) suggests that \(\mathrm{AlCl}_3\) exists as monomeric \(\mathrm{AlCl}_3\) molecules. This is partially correct; in the solid state, it exists as \(\mathrm{Al_2Cl_6}\) dimer, but it does break into \(\mathrm{AlCl_3}\) in the gaseous phase, so the statement is not wholly accurate.

Step 3: Evaluate Statement B

Statement (b) claims that \(\mathrm{AlCl}_3\) is not easily hydrolysed. In reality, \(\mathrm{AlCl}_3\) is easily hydrolysed by water to form hydrogen chloride (HCl) and aluminium hydroxide (\(\mathrm{Al(OH)_3}\)). Therefore, this statement is incorrect.

Step 4: Evaluate Statement C

Statement (c) asserts that \(\mathrm{AlCl}_3\) sublimes at \(100^{\circ} \mathrm{C}\) under vacuum. This is correct, as anhydrous aluminium chloride is known to sublime at relatively low temperatures, especially under reduced pressure.

Step 5: Evaluate Statement D

Statement (d) describes \(\mathrm{AlCl}_3\) as a strong Lewis base. However, \(\mathrm{AlCl}_3\) is actually a strong Lewis acid, meaning it can accept electron pairs, not donate them. Thus, this statement is incorrect.

Step 6: Conclusion

Review the evaluated statements and determine that only statement (c) correctly describes anhydrous aluminium chloride.

Key Concepts

Lewis Acid

Anhydrous aluminium chloride (\(\mathrm{AlCl}_3\)) plays a significant role as a Lewis acid in chemistry. A Lewis acid is a substance that can accept an electron pair from a Lewis base, filling its electron void.

This property is due to the presence of an incomplete octet in aluminium, resulting in a strong tendency to accept electron pairs.

• In reactions, \(\mathrm{AlCl}_3\) often coordinates with compounds that can donate an electron pair.
• This makes it pivotal in various industrial chemical reactions, especially in Friedel-Crafts acylation and alkylation.

The ability to accept electrons without having a full octet distinguishes \(\mathrm{AlCl}_3\) as a potent Lewis acid rather than a base. Understanding this is crucial when predicting the outcome of chemical reactions involving aluminium chloride.

Sublimation

Sublimation is a fascinating process where a substance transitions directly from a solid state to a gaseous state, bypassing the liquid stage. Anhydrous aluminium chloride showcases this property as it sublimes at relatively low temperatures, particularly under vacuum conditions.

• For \(\mathrm{AlCl}_3\), sublimation occurs at about \(100^{\circ} \mathrm{C}\) under vacuum.
• This phenomenon allows its use in certain applications where direct vaporization is more efficient or desirable than melting.

The process of sublimation also allows for the easy collection and purification of aluminium chloride since it can be directly condensed from the gaseous state back to a solid, minimizing contamination risks.

Chemical Hydrolysis

Chemical hydrolysis signifies the breakdown of a compound due to reaction with water, usually involving the cleavage of bonds. Anhydrous aluminium chloride (\(\mathrm{AlCl}_3\)) is quite prone to such hydrolysis. This reaction generally produces hydrogen chloride (\(\mathrm{HCl}\)) and aluminium hydroxide (\(\mathrm{Al(OH)_3}\)).

• The ease of hydrolysis of \(\mathrm{AlCl}_3\) is because the aluminium ion tends to attract water molecules, facilitating the attack by hydroxide ions.
• This reactivity must be considered when storing or using aluminium chloride, as it may decompose if exposed to moisture.

Understanding chemical hydrolysis is essential in predicting how \(\mathrm{AlCl}_3\) will perform in aqueous environments and in ensuring it maintains its integrity until needed in reactions.

Molecular Dimer

An interesting characteristic of anhydrous aluminium chloride is its ability to exist as a molecular dimer, specifically in its solid form. A dimer forms when two identical molecules link together.

• In the case of \(\mathrm{AlCl}_3\), it forms \(\mathrm{Al_2Cl_6}\) in solid state, where two \(\mathrm{AlCl}_3\) molecules are bonded together.
• This dimerization occurs because of the electron deficiency of aluminium, causing it to form bonds with chlorine atoms from neighboring \(\mathrm{AlCl}_3\) molecules.

However, in gaseous form, \(\mathrm{Al_2Cl_6}\) can dissociate back into monomeric \(\mathrm{AlCl}_3\) molecules. Recognizing this behavior helps in understanding its different states and how \(\mathrm{AlCl}_3\) can be utilized accordingly in various chemical processes.