Problem 199 - A Complete Resource Book in Chemistry for JEE Main

Question

The main oxides formed on comustion of \(\mathrm{Li}, \mathrm{Na}\) and \(\mathrm{K}\) in excess of air are, respectively: (a) \(\mathrm{LiO}_{2}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{K}_{2} \mathrm{O}\) (b) \(\mathrm{Li}_{2} \mathrm{O}_{2}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\) (c) \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\) (d) \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}\) and \(\mathrm{KO}_{2}\)

Step-by-Step Solution

Answer

The correct answer is (c): \(\mathrm{Li}_2\mathrm{O}, \mathrm{Na}_2\mathrm{O}_2\), and \(\mathrm{KO}_2\).

Step 1: Determine Lithium Oxide

Lithium \((\mathrm{Li})\) reacts with oxygen \((\mathrm{O}_2)\) in excess air to primarily form lithium oxide \((\mathrm{Li}_2\mathrm{O})\). The balanced equation for this reaction is: \(4\mathrm{Li} + \mathrm{O}_2 \rightarrow 2\mathrm{Li}_2\mathrm{O}\).

Step 2: Determine Sodium Oxide

Sodium \((\mathrm{Na})\), when burned in excess air, forms sodium peroxide \((\mathrm{Na}_2\mathrm{O}_2)\) as the primary oxide. The balanced equation is: \(2\mathrm{Na} + \mathrm{O}_2 \rightarrow \mathrm{Na}_2\mathrm{O}_2\).

Step 3: Determine Potassium Oxide

Potassium \((\mathrm{K})\) reacts with excess oxygen to form potassium superoxide \((\mathrm{KO}_2)\). The balanced reaction is: \(\mathrm{K} + \mathrm{O}_2 \rightarrow \mathrm{KO}_2\).

Step 4: Match Oxides to Choices

From our analysis: Lithium forms \(\mathrm{Li}_2\mathrm{O}\), Sodium forms \(\mathrm{Na}_2\mathrm{O}_2\), and Potassium forms \(\mathrm{KO}_2\). Reviewing the given choices, the pattern matches option (c): \(\mathrm{Li}_2\mathrm{O}, \mathrm{Na}_2\mathrm{O}_2\), and \(\mathrm{KO}_2\).

Key Concepts

Lithium Oxide

Lithium oxide, written chemically as \( \mathrm{Li}_2\mathrm{O} \), is the primary oxide that forms when lithium reacts with oxygen. To create lithium oxide, lithium metal is combusted in the presence of excess oxygen. The reaction is straightforward and can be expressed by the equation:

• \( 4\mathrm{Li} + \mathrm{O}_2 \rightarrow 2\mathrm{Li}_2\mathrm{O} \)

This balanced equation indicates that four lithium atoms react with one oxygen molecule to produce two units of lithium oxide. Lithium oxide is an example of a simple binary oxide since it contains only oxygen and lithium. The structure of \( \mathrm{Li}_2\mathrm{O} \) is quite simple—each oxygen atom is surrounded by a pair of lithium atoms, making it an ionic compound. Besides being a product of combustion, lithium oxide is notably used in ceramics and glass production as it contributes to the flexibility and durability of these materials.

Sodium Peroxide

Sodium peroxide, \( \mathrm{Na}_2\mathrm{O}_2 \), is the main oxide of sodium formed when sodium is burned in an excess of air. Here, sodium and oxygen combine to form this compound, which occurs through the reaction:

• \( 2\mathrm{Na} + \mathrm{O}_2 \rightarrow \mathrm{Na}_2\mathrm{O}_2 \)

This balanced chemical equation states that two sodium atoms react with one oxygen molecule to yield a compound of sodium peroxide. Sodium peroxide is a yellowish compound that can also work as a bleaching agent due to its ability to release oxygen upon contact with water. It is also utilized in some chemical processes and in emergency oxygen systems, where it serves as an oxygen source by decomposing to release air. The formation of sodium peroxide is distinct from simple oxides since it involves a peroxide ion \(\mathrm{O}_2^{2-}\). This makes it a useful oxidizing agent.

Potassium Superoxide

Potassium superoxide, represented as \( \mathrm{KO}_2 \), is particularly interesting among alkali metal oxides due to its unique properties. This superoxide is formed when potassium reacts with oxygen, as depicted in the equation:

• \( \mathrm{K} + \mathrm{O}_2 \rightarrow \mathrm{KO}_2 \)

Unlike simpler oxides, potassium superoxide contains the superoxide anion \( \mathrm{O}_2^- \). This equation states that for each potassium atom, a superoxide molecule is created. Potassium superoxide is not only unusual due to its composition but also for its applications. Its ability to release oxygen and absorb carbon dioxide makes it particularly useful in breathing apparatus, such as those used in submarines or space missions. These unique characteristics of \( \mathrm{KO}_2 \) make it an essential compound in specific industrial and safety applications.