Problem 13 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

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

Step-by-Step Solution

Answer

Option (b): \( \mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2}, \mathrm{KO}_{2} \).

Step 1: Identify Oxides Formed by Li

Lithium combines with oxygen to form lithium oxide, which can be represented by the formula \( \text{Li}_2\text{O} \). This is due to lithium having a valency of +1 and combining with oxygen which has a valency of -2, resulting in \( \text{Li}_2\text{O} \).

Step 2: Identify Oxides Formed by Na

Sodium in excess oxygen typically forms sodium peroxide \( \text{Na}_2\text{O}_2 \). The presence of excess oxygen favors the formation of the peroxide rather than the simple oxide \( \text{Na}_2\text{O} \).

Step 3: Identify Oxides Formed by K

Potassium reacts with excess oxygen to form potassium superoxide \( \text{KO}_2 \). The large size and low ionization energy of potassium favor the formation of the superoxide when oxygen is in excess.

Step 4: Match with Given Options

The combination of oxides \( \text{Li}_2\text{O} \), \( \text{Na}_2\text{O}_2 \), and \( \text{KO}_2 \) corresponds to option (b). Therefore, (b) \( \mathrm{Li}_{2} \mathrm{O}, \mathrm{Na}_{2} \mathrm{O}_{2} \text{ and } \mathrm{KO}_{2} \) is the correct answer.

Key Concepts

Lithium Oxide Formation

Lithium is a member of the alkali metals and is known for forming a stable compound with oxygen called lithium oxide. The formula for this compound is \( \text{Li}_2\text{O} \). This formation is primarily a result of lithium's valency of +1, which reflects its tendency to lose one electron during chemical reactions. Oxygen, on the other hand, has a valency of -2, meaning it tends to gain two electrons to complete its outer shell. Thus, the formation of lithium oxide involves two lithium atoms each donating an electron to one oxygen atom. This creates a balanced compound, represented by \( \text{Li}_2\text{O} \).

This oxide forms when lithium combusts in air, particularly under conditions where there is sufficient oxygen. Lithium oxides tend to be quite simple due to the small size and high ionization energy of the lithium ion, which encourages the formation of just the basic oxide \( \text{Li}_2\text{O} \).

Sodium Peroxide Formation

Sodium reacts differently compared to lithium when exposed to excess oxygen. In such conditions, it tends to form a peroxide rather than a simple oxide. The compound formed is sodium peroxide, denoted by the formula \( \text{Na}_2\text{O}_2 \). This distinct formation is influenced by sodium's chemical properties, such as its +1 valency and the moderate size of its atomic structure compared to lithium or potassium.

In the presence of excess oxygen, the environment promotes a peroxide over an oxide. This is because peroxides can accommodate two extra oxygen atoms, creating a unique chemical structure. In \( \text{Na}_2\text{O}_2 \), two sodium atoms each relinquish one electron, partially filling the oxygen's electron needs and forming a bond where pairs of oxygen share electrons—thus resulting in a peroxide rather than a basic oxide.

• Sodium's ability to form peroxides is significant in various chemical applications, especially where oxidative agents are needed.

Potassium Superoxide Formation

Potassium exhibits a fascinating behavior in excess oxygen by forming a superoxide, specifically potassium superoxide with the formula \( \text{KO}_2 \). The formation of superoxides is an interesting trait of potassium, largely attributed to its lower ionization energy and larger atomic size relative to other alkali metals. These properties make the capture of extra oxygen atoms more favorable.

The reaction involves a single potassium atom that pairs with an oxygen molecule (\( \text{O}_2 \)), leading to the creation of superoxide ions. This differs from the typical oxide or peroxide, with superoxides featuring an odd number of electrons, making them paramagnetic and quite reactive. Potassium superoxide is primarily used as a potent oxygen source, making it highly valuable in applications like life-support systems and oxygen masks.

• This unique ability to form superoxides is a hallmark of heavier alkali metals like potassium, revealing a complexity in the reactivity of these elements with oxygen.