Problem 8 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

A metal on combution in excess of air forms X. X upon hydrolysis with water yields \(\mathrm{H}_{2} \mathrm{O}_{2}\) and \(\mathrm{O}_{2}\) along with another product. The metal is: [Main Jan. 12, 2019 (I)] (a) \(\mathrm{Na}\) (b) \(\mathrm{Rb}\) (c) \(\mathrm{Mg}\) (d) \(\mathrm{Li}\)

Step-by-Step Solution

Answer

The metal is Li (Lithium).

Step 1: Combustion of Alkali Metals

When alkali metals like sodium (\(\mathrm{Na}\)) combust in excess air,\(\mathrm{Na}_{2}\mathrm{O}_{2}\) is formed instead of just \(\mathrm{Na}_{2}\mathrm{O}\). Similarly, lithium (\(\mathrm{Li}\)) forms lithium peroxide (\(\mathrm{Li}_{2}\mathrm{O}_{2}\)). In both cases, the product is a peroxide.

Step 2: Hydrolysis of Peroxide

Upon hydrolysis with water, peroxides (\(\mathrm{M}_{2}\mathrm{O}_{2}\)) typically yield \(\mathrm{H}_{2}\mathrm{O}_{2}\) (hydrogen peroxide) and other products. For instance, \(\mathrm{Na}_{2}\mathrm{O}_{2}\) will hydrolyze to produce \(\mathrm{H}_{2}\mathrm{O}_{2}, \mathrm{O}_{2}\), and a caustic solution, \(\mathrm{NaOH}\).

Step 3: Identify the Metal

In the given context, if the substance formed after the combustion and subsequent hydrolysis is a common behavior for either Na or Li while forming peroxides. Among the given options, \(\mathrm{Rb}\) and \(\mathrm{Mg}\) are less reactive in the way described and less typical to form \(\mathrm{H}_{2}\mathrm{O}_{2}\) directly. Lithium fits the described formation best.

Key Concepts

Combustion of Metals

Combustion is a chemical reaction where a substance combines with oxygen, liberating light and heat in the process. When metals, especially alkali metals, undergo combustion in the presence of excess air, they form metal oxides or peroxides. For example:

• Sodium (\(\mathrm{Na}\)) and lithium (\(\mathrm{Li}\)) combust to form sodium peroxide (\(\mathrm{Na}_{2}\mathrm{O}_{2}\)) and lithium peroxide (\(\mathrm{Li}_{2}\mathrm{O}_{2}\)) respectively.
• This peroxide formation occurs rather than forming simple metal oxides like \(\mathrm{Na_{2}O}\).

These alkali metals are highly reactive due to their single electron in their outer shell, making them eager to bond with oxygen. This explains why they form stable peroxide compounds in excess air conditions. Understanding this fundamental behavior explains why specific metals react more vigorously during combustion, resulting in unique products.

Hydrolysis Reaction

Hydrolysis involves breaking a chemical bond using water. Peroxides, like those formed from alkali metals, undergo hydrolysis when interacting with water. Let's look at how this works:

• When peroxides such as \(\mathrm{Na_{2}O_{2}}\) encounter water, they break down into hydrogen peroxide (\(\mathrm{H}_{2}\mathrm{O}_{2}\)), oxygen gas (\(\mathrm{O}_{2}\)), and a hydroxide (\(\mathrm{NaOH}\), in the case of sodium).
• This reaction can be represented as:\[\mathrm{Na_{2}O_{2} + 2H_{2}O \rightarrow 2NaOH + H_{2}O_{2} + O_{2}}\]

The formation of \(\mathrm{H}_{2}\mathrm{O}_{2}\) points to the unstable and reactive nature of peroxides. Hydrolysis is significant in chemistry as it shows how substances interact with water, leading to the production of new compounds, enhancing our understanding of chemical transformations.

Alkali Metals Peroxides

Alkali metals, such as sodium and lithium, are known for forming peroxides. These peroxides have distinct properties and reactivity due to their composition:

• Made up of an oxygen-oxygen single bond, a feature that gives them their unique reactive characteristics.
• For instance, lithium forms lithium peroxide (\(\mathrm{Li}_{2}\mathrm{O}_{2}\)) rather than the simpler lithium oxide.
• This peroxide formation is an exception compared to other metals and is attributed to the stability achieved when alkali metals bind with extra oxygen in peroxide form.

These properties make alkali metal peroxides useful in various applications, such as in oxygen generation in confined spaces. Understanding the behavior of these peroxides provides insight into the fascinating world of chemical reactions and the notable characteristics of alkali metals.