Question

Which of the following methods cannot be cmployed for preparing anhydrous aluminium chloridc? (1) Heating hydrated crystals of \(\mathrm{A} \mathrm{CCl}_{3} \cdot 6 \mathrm{H}_{2} \mathrm{O}\). (2) Heating a mixture of alumina and coke in a current of dry chlorine gas. (3) Passing dry chlorine over heated aluminium powder. (4) Passing dry HCl over heated aluminium powder.

Short answer

Method 1 cannot be employed.

Step-by-step solution

Understand the nature of anhydrous aluminium chloride

Anhydrous aluminium chloride (AlCl3) is a compound without any water molecules attached. To prepare it, methods must avoid any interaction with water.

Examine method 1 (Heating hydrated crystals of AlCl3 · 6H2O)

Heating hydrated crystals of AlCl3 · 6H2O will result in the decomposition of the compound and the release of water, which would prevent the formation of anhydrous AlCl3.

Examine method 2 (Heating alumina and coke with dry chlorine gas)

Heating a mixture of alumina (Al2O3) and coke (carbon) in a current of dry chlorine gas will produce AlCl3 without the introduction of water.

Examine method 3 (Passing dry chlorine over heated aluminium)

Passing dry chlorine gas over heated aluminium powder will result in the formation of anhydrous AlCl3.

Examine method 4 (Passing dry HCl over heated aluminium)

Passing dry hydrochloric acid (HCl) gas over heated aluminium powder will also lead to the formation of anhydrous AlCl3.

Identify the unacceptable method

Based on the analysis, the only method that cannot produce anhydrous aluminium chloride is method 1, because it involves hydrated crystals, releasing water upon heating.

Key concepts

Chemical Reactions

When dealing with the preparation of anhydrous aluminium chloride, understanding the involved chemical reactions is crucial. In chemistry, a reaction describes the process where reactants transform into products.
Preparations involving *heating a mixture of alumina (Al₂O₃) and coke in a current of dry chlorine gas* or *passing dry chlorine over heated aluminium powder* leverages chemical reactions where chlorine reacts with alumina or aluminium.
These processes avoid the presence of water. Let's break down these reactions:

• Al₂O₃ + 3C + 3Cl₂ → 2AlCl₃ + 3CO
• 2Al + 3Cl₂ → 2AlCl₃

The chlorine gas plays a pivotal role in ensuring an anhydrous product. By understanding these reactions, we can see why certain methods work while others do not.

Hydrated Crystals

Hydrated crystals contain water molecules within their crystalline structure. These water molecules are integrated into the crystal lattice.
For example, the formula AlCl₃ · 6H₂O indicates each aluminium chloride molecule is bonded with six water molecules.
When hydrated crystals like AlCl₃ · 6H₂O are heated, they decompose, releasing these water molecules as steam. This results in the formation of water, preventing the existence of anhydrous aluminium chloride.
Thus, heating hydrated crystals will not yield an anhydrous product. Understanding the nature of such crystals helps in selecting the correct method for preparation.

Dry Chlorine Gas

Dry chlorine gas is simply chlorine gas that has no water content or moisture. It is exceptionally reactive and, importantly, does not introduce any water into the reaction environment.
This dryness is essential when aiming to produce anhydrous compounds, as water presence would lead to the formation of hydrated products.
Using dry chlorine in reactions like passing it over heated aluminium powder ensures that the resulting aluminium chloride remains anhydrous. By using dry gas processes, the formation of a non-water product like anhydrous AlCl₃ is guaranteed.