ALUMINIUM CHLORIDE SOLUTION

Aluminium chloride appears on the market in three form anhydrous, crystals and aqueous solution (liquid grade).

The anhydrous material is a yellowish to grayish – white crystalline, granular or powdered solid. It reacts strongly with water and fumes in the presence of moist air to release hydrochloric acid vapours. The crystal grade is the hexahydrate, colourless form produced by crystallization from the liquid grade. The crystals are little used. The 32oBe’ (35% A;cl3) liquid grade is used for convenience. An aqueous solution (18 to 25% Alcl3) is a by-product of the use of anhydrous Alcl3 as a catalyst in the manufacture of ethyl benzene and other organics. The solution contains some free – hydrochloric acid (upto about 1.5%) and traces (0.05 to 0.2%) of hydrocarbon.

Aluminium chloride is also sometimes referred to as aluminium trichloride or aluminium (III) chloride. The compound is formed when aluminium and chlorine are reacted together. Its chemical formula is written as AlCl3. As for physical appearance, Aluminium chloride is usually white. However, due to the presence of contaminants (iron(III) chloride), it acquires a yellowish colour.

⇒ Aluminium Chloride Formula: AlCl3

Industrially, aluminium chloride is used in the production of aluminium metal, but it also has a wide number of uses in the chemical industry particularly as a Lewis acid. Solid aluminium chloride (AlCl3) is covalently bonded with low melting as well as boiling point.

PROPERTIES OF ALUMINIUM CHLORIDE

Physical Properties of AlCl3

• Aluminium chloride has a very low melting and boiling point.

• It sublimes at a temperature of 180°C.

• AlCl3 in a molten state is a poor conductor of electricity.

• The colour of aluminium chloride is white, but often it is contaminated with iron trichloride, which makes it yellow.

• It is in a liquid state only at pressures above 2.5 atm and temperature above 190°C.

Chemical Properties of AlCl3

• Aluminium chloride is a powerful Lewis acid.

• It is a major industrial catalyst.

• AlCl3 is anhydrous, non-explosive, non-flammable but a corrosive solid.

• It reacts violently when it comes in contact with water.

Aluminium Chloride Reactions

Here we will understand how AlCl3 reacts with other compounds. If we take anhydrous aluminium chloride it is a powerful Lewis acid. What this means is that it is capable of forming Lewis acid-base adducts even with bases that are weak in nature. For example, mesitylene and benzophenone. Some common reactions are:

• AlCl3 can form tetrachloroaluminate (AlCl4–) when chloride ions are present.

• Aluminium chloride can react with magnesium and calcium hydride in tetrahydrofuran to form tetrahydridoaluminate.

Aluminium Chloride Reaction with Water

Aluminium chloride is hygroscopic, where it can absorb moisture from the air. Usually, this chemical compound fumes in air containing moisture. Creates a hissing sound when it comes in contact with water. When the reaction occurs the Cl– ions are replaced with H2O molecules and forms hexahydrate [Al(H2O)6]Cl3. The anhydrous state of AlCl3 is lost and when the heat is applied HCl also dissipates and the final product that is obtained is aluminium hydroxide.

Al(H2O)6Cl3 → Al(OH)3 + 3HCl + 3H2O

When the temperature is further increased to about 400°C, aluminium oxide is formed from the hydroxide.

2Al(OH)3 → Al2O3 + 3H2O

One distinct characteristic of AlCl3 aqueous solutions is that they are ionic. Due to this, they are good conductors of electricity. They are also acidic and this can lead to partial hydrolysis in Al3+ ion. The reaction can be written as:

[Al(H2O)6]3+(aq) ⇌ [Al(OH)(H2O)5]2+(aq) + H+(aq)

Aluminium salts that contain hydrated Al3+ ion are similar to aqueous solutions of aluminium chloride. They also behave similarly. For example, it gives a thick precipitate of Al(OH)3 when it is reacted with dilute sodium hydroxide.

AlCl3 + 3NaOH → Al(OH)3 + 3NaCl