Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is obtained when \(\mathrm{NH}_{4} \mathrm{OH}\) is added to \(\mathrm{AlCl}_{3}\) solution? (a) \(\mathrm{NH}_{3}\) (b) \(\mathrm{NH}_{4} \mathrm{Cl}\) (c) \(\mathrm{NH}_{4}\) (d) \(\mathrm{Al}(\mathrm{OH})_{3}\)

Step-by-Step Solution

Answer

(d) \(\mathrm{Al(OH)}_{3}\) is formed.

Step 1: Identify the compounds in the reaction

The two main compounds involved in this reaction are ammonium hydroxide (\(\mathrm{NH}_{4} \mathrm{OH}\)) and aluminum chloride (\(\mathrm{AlCl}_{3}\)). We want to find the product formed when these two react.

Step 2: Determine the possible reaction products

Ammonium hydroxide can release ammonia gas (\(\mathrm{NH}_3\)), and aluminum chloride can react with hydroxide ions to form aluminum hydroxide (\(\mathrm{Al(OH)}_3\)).

Step 3: Write the chemical reaction

The reaction of \(\mathrm{NH}_{4} \mathrm{OH}\) with \(\mathrm{AlCl}_{3}\) can be represented as follows:\[\mathrm{AlCl}_{3} + 3\mathrm{NH}_{4} \mathrm{OH} \rightarrow \mathrm{Al(OH)}_{3} + 3\mathrm{NH}_{4} \mathrm{Cl}\]This equation shows that aluminum hydroxide \(\mathrm{Al(OH)}_{3}\) and ammonium chloride \(\mathrm{NH}_{4} \mathrm{Cl}\) are formed.

Step 4: Analyze the reaction products

Among the options given, \(\mathrm{Al(OH)}_{3}\) is formed, which corresponds to option (d). The other options, \(\mathrm{NH}_{3}\), \(\mathrm{NH}_{4} \mathrm{Cl}\), and \(\mathrm{NH}_{4}\), are either not formed primarily or do not adequately describe the main substance formed in the reaction context.

Key Concepts

Ammonium Hydroxide

Ammonium hydroxide is an aqueous solution of ammonia in water. This solution, often simply referred to as

• NH₄OH,
• is weakly basic,.
• It releases hydroxide ions (OH^-) in the solution.

This basic characteristic allows it to react with acids and certain metallic compounds.

In reactions, it is important to note that ammonium hydroxide can release ammonia gas (NH₃) when heated or under certain conditions.

In the context of the reaction with aluminum chloride (

• AlCl₃

), ammonium hydroxide serves as the source of hydroxide ions which are crucial for forming precipitates like aluminum hydroxide.

Aluminum Chloride

Aluminum chloride, with the chemical formula AlCl₃, is a compound of aluminum and chlorine. It is often found in its hydrated form and is

• commonly used in chemical processes like the Friedel-Crafts alkylation.

It plays an important role in reacting with bases or hydroxides.

In aqueous solutions, it dissociates into aluminum ions (Al³⁺) and chloride ions (Cl^-). These aluminum ions are highly reactive with hydroxide ions to form aluminum hydroxide.

During the reaction with ammonium hydroxide, the presence of these aluminum ions is key to producing aluminum hydroxide precipitate, which is a solid that forms in the solution.

• This process is an example of a classic precipitation reaction in chemistry.

Aluminum Hydroxide

Aluminum hydroxide, represented as Al(OH)₃, is an inorganic compound and carries significant importance in chemical reactions involving hydroxide ions. It is typically encountered as a gelatinous, white precipitate when aluminum salts come into contact with a base like ammonium hydroxide.

In the given reaction with AlCl₃, the aluminum ions in solution react with hydroxide ions to form this precipitate.

• Its formation is indicated by a visible white solid forming in the mixture.
• It is insoluble in water, which is why it precipitates out of the solution.

Aluminum hydroxide has multiple uses, including as an antacid in medicine and as a flame retardant. Understanding how it forms through basic reactions helps in grasping wider concepts of chemistry like solubility, precipitation, and reactions between acids and bases.