Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction \(\mathrm{Zn}+\mathrm{NaOH} \stackrel{\Delta}{\longrightarrow} \mathrm{A}\), the product \(\mathrm{A}\) is (a) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (b) \(2 \mathrm{NaZnO}_{-}\) (c) \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\) (d) none of these

Step-by-Step Solution

Answer

The product A is \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\) (option c).

Step 1: Recall the Reactants

The problem includes zinc (Zn) and NaOH (sodium hydroxide) as the reactants. Under the influence of heat (denoted by \( \Delta \)), zinc reacts with sodium hydroxide.

Step 2: Understand Reaction with Sodium Hydroxide

Zinc reacts with sodium hydroxide in the presence of water to form a salt and hydrogen gas. This is because zinc is amphoteric and can react with hydroxides to form complex salts.

Step 3: Identify the Zinc Complex Formed

Instead of forming zinc hydroxide \(\mathrm{Zn(OH)}_{2}\), in a more basic environment, zinc will form a soluble complex salt. The correct salt is sodium zincate, \(\mathrm{Na}_2\mathrm{ZnO}_2\), in the presence of excess base.

Step 4: Choose the Correct Product Option

The compound \(\mathrm{Na}_{2} \mathrm{ZnO}_{2}\) (sodium zincate) corresponds to option (c), based on the chemical behavior when zinc and sodium hydroxide react under heat.

Key Concepts

Amphoteric Metal

Zinc is described as an amphoteric metal. This means it can react with both acids and bases to form different products. Amphoteric behavior is a fascinating chemical property. It allows a metal to exhibit flexibility in reactions. Most metals react with acids, releasing hydrogen gas and forming salts. However, amphoteric metals like zinc also react with bases.

In acidic solutions, zinc dissolves to form \[\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow\]producing zinc chloride and hydrogen gas. In contrast, in basic solutions such as NaOH, zinc undergoes different reactions. This unique property is vital in understanding various industrial and laboratory processes involving zinc.

Sodium Zincate

Sodium zincate is a complex salt formed when zinc reacts with sodium hydroxide. This reaction requires heating and occurs in an alkaline environment, leading to the dissolution of zinc. The process can be depicted by the following equation:\[\text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{Zn(OH)}_4\]However, this intermediate often dehydrates under high heat to form \(\text{Na}_2\text{ZnO}_2 \),sodium zincate, while releasing hydrogen gas. Sodium zincate is an important compound in the preparation of other zinc salts and in qualitative chemical analysis.

This compound is highly soluble in water, distinguishing it from other zinc compounds like zinc hydroxide. Its formation exemplifies zinc's amphoteric nature, as it forms differently depending on whether the environment is acidic or basic.

Chemistry Reactions

In chemistry, reactions occur when substances known as reactants transform into different substances called products. These transformations depend on the chemical properties of the reactants and the conditions under which the reaction takes place.
A classic example is the reaction of zinc with sodium hydroxide. Here, zinc undergoes a reaction in an alkaline medium:\[\text{Zn} + \text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \uparrow\]In this process, the amphoteric nature of zinc is highlighted.

• It reacts with the strong base (NaOH).
• Sodium zincate, a complex salt, is formed.
• Hydrogen gas is released as a byproduct.

These types of reactions demonstrate how metals can interact with their environment. Understanding these reactions is crucial for chemists in practical applications and theoretical studies.

Complex Salts

Complex salts are fascinating chemicals formed when highly charged metal ions in solution bond with surrounding ligands. These ligands can be either water molecules or more complex anions.
Zinc, being amphoteric, forms complex salts when reacting with bases, leading to compounds like sodium zincate. This reaction involves the formation of coordinate bonds, creating a stable complex by accommodating hydroxide ions around the zinc ion.\[\text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \uparrow\]Here, the sodium zincate exemplifies such a complex salt. Understanding these salts is crucial for applications in electrochemistry, metallurgy, and various chemical syntheses.

• These salts often show unique solubility properties.
• They play essential roles in creating stable solution-based reactions.
• Their structural complexity makes them a significant topic in advanced chemistry studies.

Through this lens, we see a broader view of how complex salts deepen our understanding of chemical interactions and compound stability.