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

Question

The main oxides formed on comustion of Li, 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

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

Step 1: Identify the Metal Oxides Formed by Lithium

When lithium (Li) is burned in air, it typically forms lithium oxide: \( \mathrm{Li}_{2} \mathrm{O} \). This is due to lithium's strong affinity for oxygen and lower reactivity compared to other alkali metals, which leads to the formation of the simple oxide.

Step 2: Identify the Metal Oxides Formed by Sodium

When sodium (Na) is burned in air, it predominantly forms sodium peroxide: \( \mathrm{Na}_{2} \mathrm{O}_{2} \). Sodium is more reactive than lithium and tends to form higher oxides such as peroxides.

Step 3: Identify the Metal Oxides Formed by Potassium

When potassium (K) burns in air, it forms potassium superoxide: \( \mathrm{KO}_{2} \). Potassium is highly reactive and forms superoxides due to the large size of the potassium ion and its ability to stabilize the larger oxide ions in superoxides.

Step 4: Determine the Correct Option

Based on the oxides formed, we expect \( \mathrm{Li}_{2} \mathrm{O} \), \( \mathrm{Na}_{2} \mathrm{O}_{2} \), and \( \mathrm{KO}_{2} \). Comparing these to the options provided, we see that option (c) \( \mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}, \mathrm{KO}_{2} \) matches our findings.

Key Concepts

Lithium Oxide

When lithium is exposed to air and burns, it forms a compound known as lithium oxide, represented chemically as \( \mathrm{Li}_{2} \mathrm{O} \). This reaction occurs because lithium has a strong attraction to oxygen, which is a key aspect of its chemical behavior. Unlike some other elements, lithium tends to form a simple oxide rather than more complex forms like peroxides or superoxides.

- **Why Simple Oxide?**
- Lithium is the smallest and least reactive in the alkali metal group. This means it doesn't have as much energy to interact with the larger amounts of oxygen necessary for forming more complex oxides like peroxides or superoxides.
- It uses its electrons to bond directly with one oxygen molecule, creating a stable compound.
- **Practical Applications**
- Lithium oxide is used in ceramics and glass, improving the durability and thermal stability of these materials. - In some battery types, such as lithium-ion batteries, a form of oxide plays a crucial role in the battery's chemistry.

Understanding lithium oxide gives insight into the fundamental chemistry of alkali metals and their oxide-formation tendencies.

Sodium Peroxide

When sodium burns in air, it forms sodium peroxide, written as \( \mathrm{Na}_{2} \mathrm{O}_{2} \). Sodium is more reactive than lithium, leading it to form a different type of oxide. This behavior is characteristic of sodium and explains why we seldom find it as a simple oxide like lithium.

- **Why Peroxide?**
- Sodium is larger and more reactive than lithium, which allows it to interact more avidly with oxygen to form the peroxide structure.
- Peroxides contain an extra oxygen compared to simple oxides. This additional oxygen is crucial to creating the unique bonding and structural characteristics of sodium peroxide.
- **Applications**
- Sodium peroxide is used as a bleaching agent and plays a role in cleaning and disinfecting processes.
- It also serves as an oxidizer in certain types of chemical reactions.

Sodium's reactivity and the resulting formation of sodium peroxide instead of a simple oxide, demonstrate the broader chemical principles that govern the interactions of alkali metals with air.

Potassium Superoxide

When potassium is exposed to air and burns, it forms the compound known as potassium superoxide, denoted as \( \mathrm{KO}_{2} \). Potassium's reactivity and larger atomic size enable it to stabilize different types of oxides than its lighter alkali metal counterparts.

- **Why Superoxide?**
- Potassium is much more reactive than both lithium and sodium, allowing it to form a superoxide, which has more complex characteristics.
- The large ion size of potassium can accommodate and stabilize the larger superoxide ion.
- The formation of superoxide requires the capture of more oxygen molecules, which potassium can energetically support.
- **Uses of Potassium Superoxide**
- In closed-circuit breathing apparatuses, potassium superoxide serves to absorb carbon dioxide and generate oxygen, making it vital in rebreathers.
- It also acts as an oxygen source in spacecraft and submarines, due to its ability to release oxygen when needed.

Potassium superoxide exemplifies how varying reactivity and size among alkali metals lead to the formation of different oxide types. This reactivity plays a crucial role in both practical applications and theoretical chemistry.