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: [2016] (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

(c) \(\mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\)

Step 1: Understand the elements involved

In this problem, we are dealing with the alkali metals: lithium (Li), sodium (Na), and potassium (K). When these metals are exposed to oxygen, they form different types of oxides depending on the available oxygen and the specific properties of the metals.

Step 2: Review oxide formations of alkali metals

Alkali metals can form different types of oxides: normal oxides, peroxides, and superoxides. The type of oxide formed is influenced by the position of the metal in the periodic table and the conditions (such as the amount of air/oxygen available).

Key Concepts

Li Oxides

Lithium, a soft silvery-white metal, is the lightest member of the alkali metal group. When lithium reacts with oxygen, it predominantly forms lithium oxide (\(\text{Li}_2\text{O}\)). This compound is referred to as a normal oxide. Lithium forms normal oxides due to its high electronegativity compared to other alkali metals.
The formation of \(\text{Li}_2\text{O}\) can be understood as:

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

Due to its stoichiometry, lithium oxide is a very stable compound, and lithium seldom forms other kinds of oxides (such as peroxides or superoxides) under normal conditions.

Applications of Lithium Oxide

Lithium oxide is quite valuable in various applications such as ceramics and glass production due to its ability to increase thermal shock resistance.

Na Oxides

Sodium is another alkali metal that reacts with oxygen to form a variety of oxides. The most common oxides that sodium forms are sodium oxide \(\text{Na}_2\text{O}\) and sodium peroxide \(\text{Na}_2\text{O}_2\).
When exposed to excess oxygen, sodium primarily forms sodium peroxide:

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

Sodium peroxide is a yellowish solid that releases oxygen when it reacts with moisture, thus it is often used in applications where oxygen is needed.

Uses of Sodium Peroxide

Sodium peroxide is used for bleaching textiles and wood pulp, as well as in chemical laboratories as a reagent and oxidizing agent to supply oxygen.

K Oxides

Potassium reacts with oxygen to form multiple oxides. Unlike lithium and sodium, which form normal oxides or peroxides, potassium forms superoxides such as potassium superoxide \(\text{KO}_2\).
The reaction can be represented as:

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

Potassium superoxide is a potent oxidizer and holds additional oxygen in the form of the superoxide ion \(\text{O}_2^-\). This compound is especially useful for creating oxygen in closed environments, such as submarines, space vehicles, or aircraft.

Significance of Potassium Superoxide

Due to its ability to release oxygen on reacting with carbon dioxide and moisture, \(\text{KO}_2\) is extensively used in life-support systems.

Periodic Table

The periodic table plays a crucial role in understanding the properties and behaviors of elements, including alkali metals like lithium, sodium, and potassium. Alkali metals are located in Group 1 of the periodic table. This positioning is key to their similar chemical behaviors.
Alkali metals have one electron in their outer shell, which makes them highly reactive, especially with oxygen. Each element's position in the periodic table helps predict the type of oxide formed:

• Lithium's smaller size and comparatively higher electronegativity allows the formation of \(\text{Li}_2\text{O}\).
• As we move down the group, sodium forms \(\text{Na}_2\text{O}_2\) in a reaction with oxygen.
• Potassium, being larger and more reactive, forms superoxides like \(\text{KO}_2\).

Understanding the periodic table's trends aids in predicting not only oxide formation but a wide range of chemical behaviors.

Oxide Formation

Oxide formation involves the combination of elements with oxygen, resulting in various compounds. In the case of alkali metals, the type of oxide formed depends largely on the metal's size and its reaction conditions, especially the surplus of oxygen.

Types of Oxides

Alkali metals can form different oxides:

• Normal oxides, such as \(\text{Li}_2\text{O}\), are basic oxides that react with water to form hydroxides.
• Peroxides, like \(\text{Na}_2\text{O}_2\), contain an extra oxygen molecule and can release oxygen.
• Superoxides, such as \(\text{KO}_2\), hold more oxygen than other oxides and provide oxygen-intensive reactions.

Recognizing the conditions and the tendencies of each metal helps in understanding how different oxides are formed and their respective uses in industry and everyday applications.