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

Question

The mainoxides formed on combusting of Li, 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

The correct option is (c) \( \text{Li}_2 \text{O}, \text{Na}_2 \text{O}_2, \text{KO}_2 \).

Step 1: Understanding the Oxidation Tendency of Alkali Metals

Alkali metals such as lithium (Li), sodium (Na), and potassium (K) have a strong tendency to react with oxygen. The type of oxide formed depends on the specific alkali metal and its position in the group on the periodic table.

Step 2: Lithium Oxidation Product

Lithium (Li), being the smallest alkali metal, tends to form normal oxides rather than peroxides or superoxides. Therefore, when lithium combusts in excess air, it forms lithium oxide, represented as \( \text{Li}_2 \text{O} \).

Step 3: Sodium Oxidation Product

Sodium (Na), which is larger than lithium, tends to form peroxides in excess air. When sodium combusts, it forms sodium peroxide, represented as \( \text{Na}_2 \text{O}_2 \).

Step 4: Potassium Oxidation Product

Potassium (K), being considerably larger, often forms superoxides when combusted in excess air. Thus, the main oxide from the combustion of potassium is potassium superoxide, represented as \( \text{KO}_2 \).

Step 5: Analyzing the Options

Given the oxide types for Li, Na, and K as determined, we match them to the options: Li forms \( \text{Li}_2 \text{O} \), Na forms \( \text{Na}_2 \text{O}_2 \), and K forms \( \text{KO}_2 \). These correspond to option (c) \( \text{Li}_2 \text{O} \text{Na}_2 \text{O}_2 \) and \( \text{KO}_2 \).

Key Concepts

Lithium Oxidation

Lithium, with the chemical symbol Li, is the lightest and smallest of the alkali metals. Its position in the periodic table reflects its unique chemical properties. When lithium is exposed to air, particularly in conditions with excess oxygen, it reacts to form lithium oxide. Lithium oxide has the chemical formula \( \text{Li}_2 \text{O} \). This is the most stable oxide form that lithium takes on due to its compact size and lower tendency to form more complex oxides like peroxides or superoxides.

The tendency to form a simple oxide like \( \text{Li}_2 \text{O} \) is attributed to lithium's limited ability to stabilize extra oxygen atoms in its crystal structure due to its small atomic size. This simplicity makes lithium a key component in many applications, such as in lithium-ion batteries.

• Lithium forms a simple oxide with formula \( \text{Li}_2 \text{O} \).
• Its small size limits its ability to stabilize complex oxides.
• Lithium's stability is crucial in applications like batteries.

Sodium Oxidation

Sodium, denoted by \( \text{Na} \), occupies a position in the periodic table that allows it to exhibit behaviors different from lithium. When sodium is burned in an atmosphere with an abundance of oxygen, it tends to form sodium peroxide, which has the chemical formula \( \text{Na}_2 \text{O}_2 \).

Because sodium is larger than lithium, its atoms can accommodate more oxygen atoms. This ability to form peroxides gives sodium unique oxidative characteristics compared to lithium. Sodium's atomic structure can stabilize two oxygen atoms between every two sodium atoms, constituting the peroxide ion \( \text{O}_2^{2-} \).

• Sodium forms a peroxide \( \text{Na}_2 \text{O}_2 \) when combusted.
• Larger atomic size strengthens its capacity for additional oxygen.
• Peroxides offer distinct oxidative properties versus other alkali metals.

Potassium Oxidation

Potassium, represented as \( \text{K} \), is considerably larger than both lithium and sodium. This increased size allows it to form even more complex oxides when it reacts with excess oxygen. Potassium primarily forms potassium superoxide \( \text{KO}_2 \) upon combustion.

Superoxides, like \( \text{KO}_2 \), are characterized by the presence of the superoxide ion \( \text{O}_2^- \). This ion has a unique electron configuration, distinguishable from peroxides and simple oxides. Potassium's ability to form superoxide results from its capacity to stabilize the extra electrons involved in this ion due to its large atomic radius.

• Potassium typically forms superoxide \( \text{KO}_2 \).
• Its large size helps stabilize superoxide ions.
• Superoxides have unique electron configurations and properties.