Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

The mainoxides formed on combusting 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

Option (c): Li₂O, Na₂O₂, KO₂.

Step 1: Identify the Oxide Formed by Lithium

Lithium ( Li ) tends to form a simple oxide when combusted in air. The reaction typically results in lithium oxide which is Li_2O . This happens because lithium is more reactive with oxygen to form the simple oxide rather than superoxide or peroxide.

Step 2: Determine the Oxide of Sodium

Sodium ( Na ), when burned in excess air, tends to form a peroxide rather than a simple oxide because it is less reactive with oxygen compared to lithium. The predominant compound formed is sodium peroxide, which is Na_2O_2 .

Step 3: Deduce the Oxide of Potassium

Potassium ( K ), being even more reactive, forms a superoxide ( KO_2 ) when combusted in excess air. This is due to its high reactivity, which allows it to stabilize the extra oxygen in the superoxide form.

Step 4: Match the Oxides to the Options

Based on the analyses, the oxides for lithium, sodium, and potassium are Li_2O , Na_2O_2 , and KO_2 respectively. Compare these with the given options: The correct option is (c) Li_2O , Na_2O_2 , KO_2 .

Key Concepts

Lithium Oxide

Lithium oxide, denoted as \(\text{Li}_2\text{O}\), is the simple oxide formed when lithium is combusted in the presence of air. Lithium is unique among the alkali metals because it reacts with oxygen to form simple oxides rather than more complex peroxides or superoxides. In this context, lithium’s reactivity means it combines directly with oxygen to create \(\text{Li}_2\text{O}\).

• Synthesis: When lithium burns in oxygen, the chemical reaction can be represented as: \[4\ \text{Li}\ +\ \text{O}_2\ \rightarrow\ 2\ \text{Li}_2\text{O}\]
• Properties: Lithium oxide is white or pale yellow and has a high melting point due to strong ionic bonds. It is also a basic oxide, meaning it can react with water to form alkaline solutions, like lithium hydroxide.
• Applications: It is used in ceramics and glass industries, often to improve the durability and thermal shock resistance of materials.

Understanding the behavior of lithium in forming lithium oxide is crucial for grasping how elemental reactivity and stability differ within the alkali metal family.

Sodium Peroxide

Sodium peroxide, expressed as \(\text{Na}_2\text{O}_2\), forms when sodium reacts with oxygen, showcasing its tendency to create peroxide instead of a simple oxide. This property is unique due to sodium's particular reactivity profile compared to lithium and potassium. Sodium does not reach the reactivity level needed to form a superoxide.

• Formation: When sodium is combusted in excess oxygen, the reaction produces sodium peroxide:\[2\ \text{Na}\ +\ \text{O}_2\ \rightarrow\ \text{Na}_2\text{O}_2\]
• Characteristics: This compound is yellow and has oxidative properties, making it useful in some chemical reactions and processes. It also reacts with water to liberate oxygen gas.
• Uses: Sodium peroxide has applications in the bleaching and textile industries, and it serves as an oxidizing agent in laboratories.

Comprehending sodium's ability to form sodium peroxide helps explain the variability in alkali metal oxide formation.

Potassium Superoxide

Potassium superoxide, represented as \(\text{KO}_2\), forms when potassium, an extremely reactive metal, burns in air. Unlike lithium and sodium, potassium's reactivity extends to the ability to stabilize additional oxygen atoms, resulting in the formation of a superoxide.

• Reaction: The formation of potassium superoxide can be simplified in the equation:\[\text{K}\ +\ \frac{1}{2}\ \text{O}_2\ \rightarrow\ \text{KO}_2\]
• Properties: Potassium superoxide is bright yellow and is particularly reactive with moisture, decomposing to create oxygen gas and potassium hydroxide.
• Applications: Due to its oxygen-releasing capability, it is valuable in respiratory equipment where it helps regenerate breathable air in closed environments, such as submarines and space capsules.

Studying potassium superoxide provides insights into the advanced reactivity levels of heavier alkali metals.