Problem 136 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among the species will be soluble in excess of \(\mathrm{NaOH} ?\) (a) \(\mathrm{ZnCl}_{2}\) (b) \(\mathrm{Sr}_{2} \mathrm{~S}_{3}\) (c) \(\mathrm{CuS}\) (d) \(\mathrm{AlCl}_{2}\)

Step-by-Step Solution

Answer

(a) ZnCl2 and (d) AlCl2 are soluble in excess NaOH.

Step 1: Identify the Reaction Type

We are looking at solubility of compounds in a base, specifically NaOH. The question asks which species will dissolve in excess NaOH.

Step 2: Recognize Amphoteric Behavior

Amphoteric substances can react with both acids and bases. Certain metal hydroxides, such as zinc and aluminum, are amphoteric. They can dissolve in excess NaOH by forming complex ions.

Step 3: Analyze the Options

Examine each species: (a) ZnCl2: Zinc forms Zn(OH)2, which is amphoteric and can dissolve in excess NaOH to form [Zn(OH)4]^{2-}. (b) Sr2S3: Does not form an amphoteric hydroxide. (c) CuS: Copper sulfide is not soluble in NaOH. (d) AlCl2: Aluminum forms Al(OH)3, which is amphoteric and can dissolve in excess NaOH to form [Al(OH)4]^{-}.

Step 4: Conclusion on Solubility

From the analysis, both ZnCl2 and AlCl2 are soluble in excess NaOH due to their amphoteric nature, allowing them to form complex ions.

Key Concepts

Solubility in NaOH

Understanding the solubility of a substance in NaOH revolves around knowing whether the substance is capable of reacting with a base. In this context, we are talking about specific metal hydroxides that can dissolve when there is an excess of sodium hydroxide. Here's how it works:

• Solubility is not just about physical dissolution but involves a chemical reaction.
• Some metal hydroxides are amphoteric, meaning they can behave like both an acid and a base.
• In the presence of excess NaOH, amphoteric hydroxides are able to dissolve by forming soluble complex ions.

A classic example is zinc chloride ( ZnCl_2 ) that forms zinc hydroxide ( Zn(OH)_2 ). In the presence of excess NaOH , it transforms into a soluble complex ion, [Zn(OH)_4]^{2-} . This happens because these metal ions can coordinate with hydroxide ions to form larger complexes that can dissolve in solutions. Similarly, aluminum chloride ( AlCl_2 ) forms aluminum hydroxide ( Al(OH)_3 ), which is amphoteric. This substance can dissolve in excess NaOH by forming [Al(OH)_4]^{-} . Thus, the key point is that metals like zinc and aluminum can form amphoteric hydroxides, which enables them to react and dissolve in an alkaline environment.

Complex Ion Formation

Complex ion formation is a fascinating process where smaller ions or molecules called ligands bind to a central metal ion, forming a larger, stable entity known as a complex ion. This process plays a crucial role in the solubility of amphoteric hydroxides in NaOH.

• As the metal hydroxide dissolves, hydroxide ions wrap around the metal center, creating a coordinated ion or complex.
• These complex ions are usually more stable and soluble than the original metal hydroxide.

For instance, when Zn(OH)_2 is treated with an excess of NaOH , it forms [Zn(OH)_4]^{2-} . The zinc ion in this new complex is surrounded by four hydroxide ions, increasing its solubility. A similar process occurs with Al(OH)_3 , which forms the soluble complex [Al(OH)_4]^{-} . These reactions are a hallmark of coordination chemistry and help to explain why certain hydroxides dissolve in an excess base, despite their initial limited solubility.

Amphoteric Hydroxides

Amphoteric hydroxides have the unique ability to react with both acids and bases, a trait which significantly affects their solubility behavior in solutions like NaOH. Recognizing and predicting the behavior of amphoteric hydroxides is pivotal in grasping their role in complex ion formation and solubility.

• Amphoteric hydroxides react with acids to form soluble salts and water.
• When reacting with bases, they often form complex ions, enhancing their solubility.

Zinc hydroxide Zn(OH)_2 and aluminum hydroxide Al(OH)_3 are prime examples of amphoteric substances. When exposed to strong bases like NaOH , these compounds undergo transformation into complex ions that are soluble in water. This dualistic nature allows them to play versatile roles in chemical reactions and solutions. Understanding this concept is crucial, as it provides insights into the reactive nature of certain elements and helps in predicting the outcomes of chemical reactions in various contexts. Recognizing amphoterism is essential for those studying chemistry, particularly when analyzing the solubility of compounds in different environments.