Problem 18 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\([\mathrm{X}]+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow[\mathrm{Y}]\) a colourless gas with irritating smell, \([\mathrm{Y}]+\) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow\) green solution. \([\mathrm{X}]\) and \([\mathrm{Y}]\) are: (a) \(\mathrm{SO}_{3}^{2-}, \mathrm{SO}_{2}\) (b) \(\mathrm{Cl}^{-}, \mathrm{HCl}\) (c) \(\mathrm{S}^{2-}, \mathrm{H}_{2} \mathrm{~S}\) (d) \(\mathrm{CO}_{3}^{2-}, \mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

The pair is (a) \(\text{SO}_3^{2-}\), \(\text{SO}_2\).

Step 1: Identify the reaction with H2SO4 to produce a gas

The exercise states that a colorless gas with an irritating smell, denoted by \([Y]\), is formed when \([X]\) reacts with \(\text{H}_2\text{SO}_4\). This is commonly seen in sulfur-containing compounds, such as sulfites \(\text{SO}_3^{2-}\) reacting with sulfuric acid to produce \(\text{SO}_2\), a colorless gas with a pungent odor.

Step 2: Determine if [Y] reacts with K2Cr2O7 to form a green solution

\([Y]\) is then described as reacting with \(\text{K}_2\text{Cr}_2\text{O}_7\) and \(\text{H}_2\text{SO}_4\) to form a green solution. This typically identifies a reduction reaction where \(\text{SO}_2\) reduces \(\text{Cr}_2\text{O}_7^{2-}\) to a green \(\text{Cr}^{3+}\) ion. This is characteristic of \(\text{SO}_2\) reacting with dichromate.

Step 3: Identify the pair [X] and [Y]

Considering both the gas production and its reduction of dichromate, the only pair of substances that fit both reactions described is \([X] = \text{SO}_3^{2-}\) and \([Y] = \text{SO}_2\). The \(\text{SO}_3^{2-}\) ion reacts with \(\text{H}_2\text{SO}_4\) to produce \(\text{SO}_2\), which can then react with \(\text{K}_2\text{Cr}_2\text{O}_7\) to form the green solution.

Key Concepts

Irritating Odors in Gases

Sulfur dioxide \(\text{SO}_2\) is a colorless gas that is known for its pungent and irritating odor. This gas is often associated with the smell of burning sulfur or burnt matches, which makes it quite recognizable. It is a significant component of air pollution, especially in urban areas, contributing to the formation of acid rain.
This gas can result from the combustion of fossil fuels or the oxidation of sulfites. In many laboratory settings, reacting sulfur-containing compounds with strong acids, such as \(\text{H}_2\text{SO}_4\), can release \(\text{SO}_2\).
Due to its irritating properties, \(\text{SO}_2\) can cause respiratory discomfort in humans and is harmful if inhaled in large amounts. Understanding the behavior of \(\text{SO}_2\) in reactions helps in predicting its occurrence and managing its impact in both natural and human-made environments.

Dichromate Ion Reduction

The reaction of sulfur dioxide \(\text{SO}_2\) with potassium dichromate \(\text{K}_2\text{Cr}_2\text{O}_7\) under acidic conditions is an excellent example of a redox reaction. In this context, \(\text{SO}_2\) acts as a reducing agent, converting \(\text{Cr}_2\text{O}_7^{2-}\) to \(\text{Cr}^{3+}\) ions, which are green in color.
This process occurs because the dichromate ion is a powerful oxidizing agent. It can accept electrons from \(\text{SO}_2\), resulting in the oxidation of sulfur. The resulting change in chromium's oxidation state from +6 in \(\text{Cr}_2\text{O}_7^{2-}\) to +3 in \(\text{Cr}^{3+}\) is characterized by the noticeable color shift to green.

• This reaction is significant in analytical chemistry as it helps in detecting the presence of reducing agents like \(\text{SO}_2\) in mixtures.
• It has practical applications in purification and testing processes.

Such reactions are crucial in industries and laboratories where controlled oxidation-reduction processes are required.

Sulfur Compounds in Chemistry

Sulfur compounds play a foundational role in various chemical processes and industries due to their unique properties and reactivity. Among these, sulfites \(\text{SO}_3^{2-}\), sulfates, and sulfur dioxide \(\text{SO}_2\) are some of the most commonly referenced.
These compounds often serve as preservatives, reagents, or even pollutants, depending on their application. For instance, sulfites are used as preservatives in food and wine to prevent oxidation and spoilage.
In chemistry labs, sulfites reacting with acids like \(\text{H}_2\text{SO}_4\) can generate sulfur dioxide, which is useful for demonstrating gas evolution reactions. Sulfates, like \(\text{\text{K}_2\text{SO}_4}\), are integral in fertilizers and other industrial applications because of their solubility and nutrient value.
The study of sulfur compounds often involves understanding their cyclic breakdown and synthesis, their significance in environmental chemistry, such as in acid rain formation, and their role in biological systems as trace nutrients.