Problem 1 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

On combustion of Li, \(\mathrm{Na}\) and \(\mathrm{K}\) in excess of air, the major oxides formed, respectively, are: [Main Sep. 04, 2020 (I)] (a) \(\mathrm{Li}_{2} \mathrm{O}_{2}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{K}_{2} \mathrm{O}_{2}\) (b) \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\) (c) \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}\) and \(\mathrm{K}_{2} \mathrm{O}_{2}\) (d) \(\mathrm{Li}, \mathrm{O}, \mathrm{Na}, \mathrm{O}\), and \(\mathrm{K}, \mathrm{O}\)

Step-by-Step Solution

Answer

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

Step 1: Understand the Problem

We need to determine the major oxides formed when lithium (Li), sodium (Na), and potassium (K) are burned in excess of air. The possible options are given, and we must identify which one correctly represents the major oxides formed during combustion.

Step 2: Consider Lithium (Li) Combustion

When lithium combusts in air, it forms lithium oxide. Therefore, the major oxide formed from lithium is \( \mathrm{Li}_2 \mathrm{O} \).

Step 3: Consider Sodium (Na) Combustion

Sodium, when burned in excess of oxygen, forms sodium peroxide. So, the major oxide is \( \mathrm{Na}_2 \mathrm{O}_2 \).

Step 4: Consider Potassium (K) Combustion

Potassium reacts with excess oxygen to form potassium superoxide. Therefore, the major oxide is \( \mathrm{KO}_2 \).

Step 5: Verify Against Options

From the information collected in Steps 2, 3, and 4, the major oxides are \( \mathrm{Li}_2 \mathrm{O} \), \( \mathrm{Na}_2 \mathrm{O}_2 \), and \( \mathrm{KO}_2 \). Comparing this to the provided options, option (b) accurately represents the major oxides formed.

Key Concepts

Lithium Oxide Formation

When lithium burns in air, it reacts primarily with oxygen to form lithium oxide. In the reaction, two lithium atoms (Li) combine with one oxygen molecule (\( ext{O}_2 \)) to produce lithium oxide (\( ext{Li}_2 ext{O} \).The chemical equation for this reaction is:\[4 ext{Li} + ext{O}_2 \rightarrow 2 ext{Li}_2 ext{O}\]Lithium oxide (\( ext{Li}_2 ext{O} \)) is a white, ionic compound formed through the transfer of electrons from lithium, a metal with a low electronegativity, to oxygen, a more electronegative non-metal. * Key Points about Lithium Combustion: * Lithium generally forms stable oxides due to strong ionic bonds. * It burns with a distinctive crimson flame indicating its presence. * The formation of \( ext{Li}_2 ext{O} \)is typical for Group 1 elements as they usually form oxides when burned in sufficient oxygen.This ease of forming a stable oxide makes lithium distinct among the alkali metals.

Sodium Peroxide Formation

Sodium reacts differently under combustion compared to lithium. When sodium is exposed to excess oxygen, it predominantly forms sodium peroxide rather than its simple oxide form.The balanced chemical equation for this reaction is:\[2 ext{Na} + ext{O}_2 \rightarrow ext{Na}_2 ext{O}_2\]Sodium peroxide (\( ext{Na}_2 ext{O}_2 \)) is an inorganic compound characterized by its pale yellow color and strong oxidizing properties. This peroxide formation involves two sodium atoms reacting with a single oxygen molecule but creates a different linkage than a simple oxide.- Important Aspects of Sodium Peroxide: * It is a powerful oxidizing agent and can be hazardous in certain reactions. * Reacts more aggressively with acids to form sodium salts and hydrogen peroxide. *While sodium peroxide can serve multiple industrial uses, its potential reactivity means it needs careful handling, especially when interacting with organic compounds or flammable materials.

Potassium Superoxide Formation

Potassium shows yet another unique behavior when it combusts in excess air. It primarily forms potassium superoxide (\( ext{KO}_2 \)) instead of simple oxide or peroxide.The equation representing this combustion is:\[4 ext{K} + ext{O}_2 \rightarrow 2 ext{KO}_2\]Potassium superoxide is a bright yellow or orange solid. It forms through the interaction of potassium with oxygen, resulting in a compound containing the superoxide ion (\( ext{O}_2^- \)). This ion has two oxygen atoms bonded together with an extra electron, making it highly reactive.- Key Features of Potassium Superoxide: * Uses include oxygen generation and carbon dioxide absorption, useful in breathing apparatus or spacecraft. * Potassium superoxide is more stable compared to peroxides but is still reactive. *Due to potassium's reactivity and the propensity to form superoxide, special care is necessary when handling, storing, or disposing of \( ext{KO}_2 \). Its ability to react with moisture can pose additional risks, releasing oxygen and heat.