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) \(\mathrm{AlCl}_{3}\)

Step-by-Step Solution

Answer

Both \(\mathrm{ZnCl}_{2}\) and \(\mathrm{AlCl}_{3}\) are soluble in excess \(\mathrm{NaOH}\).

Step 1: Understanding Solubility in NaOH

To determine solubility in excess \(\mathrm{NaOH}\), we must understand which metal hydroxides are amphoteric. Amphoteric hydroxides will dissolve in excess \(\mathrm{NaOH}\).

Step 2: Identify the Hydroxides

Convert the given compounds to their hydroxides: \(\mathrm{ZnCl}_{2}\) to \(\mathrm{Zn(OH)}_{2}\), \(\mathrm{Sr}_{2}\mathrm{S}_{3}\) to \(\mathrm{Sr(OH)}_{2}\), \(\mathrm{CuS}\) to \(\mathrm{Cu(OH)}_{2}\), and \(\mathrm{AlCl}_{3}\) to \(\mathrm{Al(OH)}_{3}\).

Step 3: Determine Amphotericity

Among these hydroxides, \(\mathrm{Zn(OH)}_{2}\) and \(\mathrm{Al(OH)}_{3}\) are known to be amphoteric. Amphotericity means they can dissolve in both acids and bases, including excess \(\mathrm{NaOH}\).

Step 4: Conclusion on Solubility in Excess NaOH

Thus, \(\mathrm{ZnCl}_{2}\) and \(\mathrm{AlCl}_{3}\) will dissolve in excess \(\mathrm{NaOH}\) due to their amphoteric hydroxides.

Key Concepts

Solubility in NaOH

When talking about solubility in \( \text{NaOH} \), it's important to consider whether a compound can dissolve in a sodium hydroxide solution. Some metal hydroxides, when treated with excess \( \text{NaOH} \), will dissolve, indicating that these metal hydroxides are amphoteric.

The process starts with considering the nature of metals forming the hydroxides. Metals that form amphoteric hydroxides will often dissolve in both acids and bases. This dual capability provides a clue that when such hydroxides encounter a basic solution like \( \text{NaOH} \), they may become soluble.

In practice, this means identifying which compounds, when switched to their hydroxide form, show this behavior. For instance, zinc chloride, when it forms zinc hydroxide, can dissolve in sodium hydroxide due to its amphoteric characteristics. It's essential to apply this understanding as you analyze reactions for solubility patterns.

Amphotericity

Amphotericity is an exciting concept that describes a substance's ability to react with both acids and bases. In keeping with its dual nature, some metal hydroxides can form soluble complexes in strong bases like \( \text{NaOH} \).

Key points to remember about amphotericity:

• Amphoteric substances act as acids in the presence of strong bases.
• They can also act as bases when acids are present.

These characteristics make amphoteric hydroxides versatile in reactions. Zinc hydroxide \( \left( \text{Zn(OH)}_2 \right) \) and aluminum hydroxide \( \left( \text{Al(OH)}_3 \right) \) are prime examples.

When in excess \( \text{NaOH} \), these amphoteric hydroxides dissolve and can form complex ions. This ability to form complex compounds contributes significantly to their solubility in strong bases, not only following the typical dissolution reactions but also broadens their chemical behavior.

Metal Hydroxides Solubility

The solubility of metal hydroxides varies greatly among different metals and is largely determined by their chemical properties. This factor is crucial when predicting their behavior in aqueous solutions like \( \text{NaOH} \).

Metal hydroxides typically fall into three categories:

• Insoluble Hydroxides: Hydroxides like copper hydroxide \( \left( \text{Cu(OH)}_2 \right) \) usually do not dissolve in either acids or bases under standard conditions.
• Soluble Hydroxides: These hydroxides dissolve in water, such as sodium hydroxide itself.
• Amphoteric Hydroxides: As noted in amphotericity, have special roles due to their reactivity with both acids and bases.

To solve questions about solubility in basic solutions, identify the type of hydroxide. This identification helps predict which will dissolve.

Practicing this differentiation allows clearer understanding of chemical reactions, particularly when predicting and explaining phenomena in chemical experiments involving amphoteric substances.