Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

Action of caustic soda on \(\mathrm{Al}(\mathrm{OH})_{3}\) gives a compound having formula (a) \(\mathrm{Al}_{2}(\mathrm{OH})_{4}\) (b) \(\mathrm{Na}_{3} \mathrm{AlO}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{Al}(\mathrm{OH})_{4}\) (d) \(\mathrm{NaAlO}_{2}\)

Step-by-Step Solution

Answer

(c) \( \mathrm{Na}_{2} \mathrm{Al}(\mathrm{OH})_{4} \)

Step 1: Understand the Reactants

Caustic soda is the common name for sodium hydroxide (aOH]). The compound l(OH)_{3}] represents aluminum hydroxide.

Step 2: Analyze the Reaction

When l(OH)_{3}] reacts with aOH], it forms a compound known as sodium aluminate. Aluminum hydroxide, being amphoteric, reacts with a base like sodium hydroxide.

Step 3: Determine the Product Formula

The reaction between l(OH)_{3}] and excess aOH] forms a_{2}Al(OH)_{4}]. This is because the hydroxide ions from aOH] convert l(OH)_{3}] into aluminate ion, resulting in sodium aluminate.

Step 4: Match with Provided Options

Among the options provided, a_{2}Al(OH)_{4}] corresponds to option (c). This is consistent with the reaction of aluminum hydroxide with an excess of caustic soda.

Key Concepts

Sodium Hydroxide

Sodium hydroxide, often referred to as caustic soda, is a widely used chemical compound. Its chemical formula is \( \text{NaOH} \), and it appears as a white solid. This compound is known for its strong alkaline properties. It easily dissolves in water to form a strongly basic solution. This makes it a powerful cleaning agent and a necessary reagent in several chemical processes.

Here are a few key points about sodium hydroxide:

• It is a strong base and fully dissociates in water to form hydroxide ions \( \text{(OH}^- \text{)} \).
• It has a high solubility in water, meaning that it can readily produce a solution when mixed.
• Common uses include cleaning, manufacturing of soap, and various chemical preparations.

Sodium Aluminate

When sodium hydroxide reacts with aluminum hydroxide, a compound called sodium aluminate is formed. Its common formula is \( \text{Na}_2\text{Al(OH)}_4 \). This particular reaction highlights the nature of aluminum hydroxide reacting with bases, a process known as forming aluminates.

Here are crucial aspects of sodium aluminate:

• It typically appears as a crystalline solid.
• It involves the displacement of the hydroxide from sodium hydroxide with the aluminate ion.
• This formation is due to the reaction of a strong base with an amphoteric compound.

These factors make sodium aluminate a valuable compound in water treatment and the paper industry. It acts as a coagulant, helping in the removal of impurities from water.

Amphoteric Nature

The term "amphoteric" refers to a substance's ability to react with both acids and bases. Aluminum hydroxide, \( \text{Al(OH)}_3 \), is a classic example of an amphoteric compound. This means it can react with both acids and bases, forming different products depending on the nature of the reactant.

Important points about amphoteric substances include:

• They have dual chemical behavior, characterized by their ability to act either as acids or bases depending on the surrounding environment.
• When reacting with an acid, \( \text{Al(OH)}_3 \) will typically form a salt and water.
• When reacting with a base, it forms compounds like sodium aluminate, as in the discussed reaction with sodium hydroxide.

Understanding this nature helps recognize the unique place these substances hold in chemistry, highlighting their versatility in different chemical reactions.