Problem 26 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The compound(s) formed upon combustion of sodium metal in excess air is (are) (a) \(\mathrm{Na}_{2} \mathrm{O}_{2}\) (b) \(\mathrm{Na}_{2} \mathrm{O}\) (c) \(\mathrm{NaO}_{2}\) (d) \(\mathrm{NaOH}\)

Step-by-Step Solution

Answer

The compound formed is sodium peroxide ( Na_2O_2 ).

Step 1: Understand Sodium Combustion

When sodium metal is burned in excess air, it reacts readily with oxygen to form oxides. Sodium typically forms peroxides or superoxides when an excess of oxygen is present.

Step 2: Reaction with Excess Oxygen

In the presence of excess air, sodium tends to form sodium peroxide ( Na_2O_2 ). This is the most common product because the reaction with excess oxygen favors the formation of peroxides rather than simple oxides or superoxides.

Step 3: Evaluate Other Possible Compounds

Sodium metal can form simple oxide ( Na_2O ) but this is less likely in the presence of excess oxygen. Sodium will not typically form superoxide ( NaO_2 ) unless it reacts under high pressure or with pure oxygen. Sodium hydroxide ( NaOH ) is not formed during combustion in air but rather through reaction with water.

Step 4: Conclusion

Among the given options, the only compound that forms in significant amounts upon the combustion of sodium in excess air is sodium peroxide ( Na_2O_2 ). Therefore, the correct answer is option (a).

Key Concepts

Sodium Oxides

When sodium combusts in air, it can form different oxides depending on the conditions of the reaction. An oxide of sodium is a chemical compound of sodium with oxygen. Among these oxides, we typically have three main types:

• Sodium Oxide \(\text{(Na}_2\text{O)}\): This is a simple oxide of sodium and forms when there is a limited supply of oxygen. In a controlled laboratory setting with restricted oxygen, sodium oxide is a possible product. However, it is less common in natural scenarios where air, abundant in oxygen, is present.
• Sodium Peroxide \(\text{(Na}_2\text{O}_2)}\): Sodium peroxide is more commonly formed when sodium combusts, especially with excess air. It appears as a white or yellowish powder and can react further when exposed to moisture. This form is often seen in applications where sodium is exposed to atmospheric conditions.
• Sodium Superoxide \((\text{NaO}_2)\): This compound forms under specific conditions like high pressure or pure oxygen. It's not as commonly formed when sodium is combusted in open air.

Understanding which oxide forms during sodium combustion helps explain the behavior of sodium in different environments. Sodium peroxide is the major product in excess air because the reaction conditions favor its more stable formation.

Sodium Peroxide Formation

Sodium peroxide, with the chemical formula \(\text{Na}_2\text{O}_2\), plays a significant role in the combustion process of sodium. It is the most prevalent byproduct when sodium burns in air, primarily due to the presence of excess oxygen. This elevated oxygen concentration promotes the formation of peroxide rather than simple oxides.

The formation process involves sodium metal reacting with diatomic oxygen \((\text{O}_2)\). The balanced chemical equation for this reaction is:

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

One can observe that sodium peroxide serves multiple purposes. Not only is it a product of combustion, but it's also used as an oxidizing agent in various chemical processes.
Additionally, sodium peroxide can further react with water to form sodium hydroxide and oxygen gas, illustrating its reactive nature:

• \(\text{2Na}_2\text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{NaOH} + \text{O}_2\)

Understanding the formation and reactions involving sodium peroxide highlights its importance and its typical occurrence in chemical reactions involving sodium in air.

Sodium Reaction with Oxygen

The reaction between sodium and oxygen is an exothermic process, meaning it releases heat. In the presence of air, which contains oxygen, sodium vigorously reacts to form a variety of oxides. The type of oxide formed is influenced by the

conditions, particularly the amount of oxygen available and the environment's pressure conditions:

• With limited oxygen, a simple oxide \((\text{Na}_2\text{O})\) may form. However, this scenario requires specific conditions such as reduced oxygen supply or a controlled laboratory environment.
• In an excess of oxygen, sodium predominantly forms a peroxide \((\text{Na}_2\text{O}_2)\). This is the case when sodium is burned in open air, favorably leading to the creation of peroxides, as they are more stable and dominant in oxygen-rich conditions.
• Under high pressure or in pure oxygen environments, sodium can further oxidize to form superoxides \((\text{NaO}_2)\), but this is rare in everyday scenarios.

The reaction of sodium with oxygen showcases its highly reactive nature and significant role in producing compounds that have practical uses, such as in chemical synthesis and production.