Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In the following reactions, \(\mathrm{ZnO}\) is respectively acting as a/an: (i) \(\mathrm{ZnO}+\mathrm{Na}_{2} \mathrm{O} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2}\) (ii) \(\mathrm{ZnO}+\mathrm{CO}_{2} \rightarrow \mathrm{ZnCO}_{3}\) (a) base and acid (b) base and base (c) acid and acid (d) acid and base

Step-by-Step Solution

Answer

\( \mathrm{ZnO} \) acts as an acid in (i) and a base in (ii), choice (d).

Step 1: Analyze Reaction (i)

In the reaction \( \mathrm{ZnO} + \mathrm{Na}_{2} \mathrm{O} \rightarrow \mathrm{Na}_{2} \mathrm{ZnO}_{2} \), we see that \( \mathrm{ZnO} \) is reacting with \( \mathrm{Na}_{2} \mathrm{O} \), which is a basic oxide. \( \mathrm{ZnO} \) combines with this base to form a salt \( \mathrm{Na}_{2} \mathrm{ZnO}_{2} \). This indicates that \( \mathrm{ZnO} \) is acting as an acid in this reaction, since acids react with bases to form salts.

Step 2: Analyze Reaction (ii)

In the reaction \( \mathrm{ZnO} + \mathrm{CO}_{2} \rightarrow \mathrm{ZnCO}_{3} \), \( \mathrm{ZnO} \) is reacting with \( \mathrm{CO}_{2} \), which is an acidic oxide. \( \mathrm{ZnO} \) combines with the acidic oxide to form a carbonate. This means \( \mathrm{ZnO} \) is acting as a base here, since bases react with acids to form salts.

Step 3: Determine the Pattern

Now that we have analyzed both reactions, we recognize that \( \mathrm{ZnO} \) behaves as an acid in the first reaction (i) and as a base in the second reaction (ii).

Key Concepts

Acid-Base Reactions

When we talk about acid-base reactions, we're referring to a process where an acid and a base interact to form a salt and often water. But how do acids and bases identify each other? In simple terms, acids are substances that have the ability to donate a proton (hydrogen ion, H⁺) or accept an electron pair in reactions. Conversely, bases can accept a proton or donate an electron pair.

Amphoteric substances, like zinc oxide (ZnO), can uniquely behave as either an acid or a base depending on the environment they are in. In reaction (i) from the exercise, ZnO acts as an acid by reacting with the basic oxide, sodium oxide ( ext{Na}_{2} ext{O}), to form a salt. Here, ZnO shows its acidic side because it chemically combines with a base. In contrast, in reaction (ii), ZnO demonstrates its basic characteristics by reacting with carbon dioxide ( ext{CO}_{2}), an acidic oxide, to form zinc carbonate ( ext{ZnCO}_{3}). Through this interaction, ZnO accepts the acidic oxide, confirming its role as a base. This dual behavior underscores the amphoteric nature of zinc oxide, a hallmark of complex acid-base chemistry.

Zinc Oxide

Zinc oxide (ZnO) is an interesting compound due to its amphoteric nature. Amphoteric compounds can behave like acids or bases depending on their chemical surroundings. For students exploring its nature, understanding this dual behavior is crucial.

In the presence of basic oxides like sodium oxide ( ext{Na}_{2} ext{O}), ZnO acts as an acid. It undergoes a reaction that typically involves donating electrons or forming bonds with the base to yield a new salt compound, sodium zincate ( ext{Na}_{2} ext{ZnO}_{2}). In this instance, zinc oxide showcases its acidic behavior.

On the other hand, when ZnO encounters acidic oxides such as carbon dioxide ( ext{CO}_{2}), it exhibits its basic nature. Here, ZnO engages in a chemical reaction that effectively neutralizes the acid and results in the formation of a carbonate salt ( ext{ZnCO}_{3}). This adaptable character of ZnO makes it a unique study in chemistry. It's not just about recognizing reactions but also predicting how ZnO might respond in various chemical environments.

Salt Formation

Salt formation is a fundamental aspect of chemistry that occurs when acids react with bases. In simple terms, a salt is the product of this neutralization reaction, which typically includes the ion from the acid and the ion from the base.

In the example of zinc oxide (ZnO), its ability to act as both an acid and a base allows for diverse salt formation. Consider reaction (i), where ZnO reacts with sodium oxide ( ext{Na}_{2} ext{O}). The result is the formation of sodium zincate ( ext{Na}_{2} ext{ZnO}_{2}), a classic example of a salt produced from the reaction between an acid (ZnO in this case) and a base. Similarly, in reaction (ii), ZnO acts as a base when it reacts with carbon dioxide ( ext{CO}_{2}). This interaction results in the formation of zinc carbonate ( ext{ZnCO}_{3}), demonstrating salt formation from a base reacting with an acid.

This concept illuminates the broader principle of neutralization, teaching how oppositely reactive compounds can create stable, often solid, compounds that we categorize as salts.