Problem 134 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following radicals are decomposed by conc. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) only? (a) \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) (b) \(\mathrm{HCO}_{3}^{-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)

Step-by-Step Solution

Answer

\(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) is decomposed by conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\) only.

Step 1: Understand the Role of Concentrated Sulfuric Acid

Concentrated sulfuric acid (conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\)) is a strong dehydrating and oxidizing agent. When selecting radicals that it can decompose, look for ions that can be oxidized or dehydrated effectively by \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Step 2: Analyze Each Radical

Examine each option to determine its susceptibility to decomposition by conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\).(a) \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\): Oxalate ions can be oxidized by \(\mathrm{H}_{2}\mathrm{SO}_{4}\) to carbon dioxide \((\mathrm{CO}_2)\) and water.(b) \(\mathrm{HCO}_{3}^{-}\): Bicarbonate ions can be decomposed by \(\mathrm{H}_{2}\mathrm{SO}_{4}\) to carbon dioxide \((\mathrm{CO}_2)\) and water, but it is not completely decomposed by \(\mathrm{H}_{2}\mathrm{SO}_{4}\) alone.(c) \(\mathrm{SO}_{3}^{2-}\): Sulfite ions can be readily oxidized by \(\mathrm{H}_{2}\mathrm{SO}_{4}\) to form sulfur dioxide \((\mathrm{SO}_2)\).(d) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\): Acetate ions are not significantly oxidized or decomposed by \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Step 3: Identify Radicals Decomposed Only by Conc. Sulfuric Acid

From the analysis, \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) and \(\mathrm{SO}_{3}^{2-}\) are decomposed by conc. \(\mathrm{H}_{2}\mathrm{SO}_{4}\). While both exhibit reactions with it, only \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) undergoes a complete decomposition solely due to the action of \(\mathrm{H}_{2}\mathrm{SO}_{4}\).

Key Concepts

Oxidizing Agents

An oxidizing agent is a substance that has the ability to oxidize other substances, which means it can accept electrons during a chemical reaction. Oxidizing agents are essential in redox reactions, as they help in transferring electrons from the reducing agent.

• The role of an oxidizing agent is crucial in many chemical processes, as it contributes to breaking down or transforming various compounds.
• Substances like atmospheric oxygen, halogens, and hydrogen peroxide are well-known oxidizing agents.

Concentrated sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\) ), mentioned in our exercise, also acts as a powerful oxidizing agent. It can oxidize certain radicals, making it helpful in specific decomposition reactions. In cases of radicals like \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) (oxalate ion) and \(\mathrm{SO}_{3}^{2-}\) (sulfite ion), sulfuric acid facilitates their oxidation and induces decomposition.

Sulfuric Acid Reactions

Concentrated sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\) ) is known for its aggressive chemical behavior, making it invaluable in many reactions that involve dehydration and oxidation. First, it's important to understand its behavior as a dehydrating agent:

• Because of its affinity for water, sulfuric acid can effectively remove water (dehydrate compounds) from other substances.
• This trait is particularly useful when dealing with organic compounds or during the concentration of acids.

In the context of our exercise, sulfuric acid shows its strength as an oxidizing agent. It supplies oxygen internally to oxidize other compounds or ions. This is evident in its reaction with the oxalate ion to produce carbon dioxide (\(\mathrm{CO}_2\) ) and water. In conclusion, knowledge of sulfuric acid's capabilities allows for better understanding when predicting or analyzing how specific chemical reactions proceed.

Ions and Radicals

In chemistry, ions and radicals are two concepts often encountered during the study of reactions. Let's break them down for clarity:

• Ions: These are charged particles that result from the gain or loss of electrons. For instance, \(\mathrm{C}_{2} \mathrm{O}_{4}^{2-}\) is an anion (negatively charged ion).
• Radicals: Unlike ions, radicals have an unpaired electron. They are highly reactive and play crucial roles in various chemical transformations.

It is common for radicals to partake in decomposition reactions - processes in which a single compound breaks down into two or more simple substances. While ions and radicals have different characteristics, their behaviors often interplay in chemical reactions, driving significant transformations within those reactions.

Chemical Reactions in Acidic Medium

Chemical reactions taking place in an acidic medium involve acids providing certain properties that can influence reaction pathways. The acidic medium, provided by acids like sulfuric acid, facilitates such reactions for specific reasons:

• Acids supply protons (\(\mathrm{H}^+\) ) that can interact with other reactants, often leading to improved reaction rates or different reaction pathways.
• The generation of positive hydrogen ions in acidic solutions can stabilize certain intermediates throughout the reaction process.

In the exercise, concentrated sulfuric acid is an example of an acidic medium. Its presence not only speeds up the oxidation process of radicals like \(\mathrm{SO}_{3}^{2-}\), leading to by-products such as sulfur dioxide (\(\mathrm{SO}_2\) ), but also allows for complete reactions and further decomposition without additional compounds.