Problem 98 - A Complete Resource Book in Chemistry for JEE Main

Question

A substance on treatment with dilute \(\mathrm{H}_{2} \mathrm{SO}_{4}\) liberates a colourless gas which produces (i) turbidity with baryta water and (ii) turns acidified dichromate solution green. These reactions indicate the presence of (a) \(\mathrm{CO}_{3}^{2-}\) (b) \(\mathrm{S}^{2-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{NO}_{2}^{-}\)

Step-by-Step Solution

Answer

The substance contains \(\mathrm{SO}_{3}^{2-}\) (option c).

Step 1: Identify the Gas

The problem states that the substance liberates a colorless gas when treated with dilute \(H_2SO_4\). This is typical of anions like \(CO_3^{2-}\) and \(SO_3^{2-}\), which can release \(CO_2\) and \(SO_2\) respectively upon reacting with acids.

Step 2: Reaction with Baryta Water

The colorless gas produces turbidity with baryta water. Baryta water (a solution of \(Ba(OH)_2\)) reacts with \(CO_2\) forming \(BaCO_3\), an insoluble white precipitate, causing turbidity. \(SO_2\) would also react but does not form an insoluble precipitate with baryta water.

Step 3: Reaction with Acidified Dichromate Solution

The gas turns an acidified dichromate solution green. Dichromate ions \(Cr_2O_7^{2-}\) are reduced to \(Cr^{3+}\) (which is green) when treated with \(SO_2\). This is a key identifying reaction for \(SO_2\), distinguishing \(SO_2\) from \(CO_2\) which does not reduce dichromate.

Step 4: Conclusion Based on Reactions

Given the two critical reactions, the gas is identified as \(SO_2\) because it causes turbidity with baryta water (although not forming an insoluble carbonate) and turns acidified dichromate green. This points to \(SO_3^{2-}\) as the original anion, which upon acid treatment became \(SO_2\).

Key Concepts

Anion Identification

Anion identification involves detecting specific negatively charged ions (anions) in a substance. It's crucial for understanding the composition of a compound.
A systematic approach is used. This begins with treating the sample with specific reagents to observe its reaction behavior. In our case, the sample is treated with dilute \( \mathrm{H}_2\mathrm{SO}_4 \), and the resulting gas can give us clues about which anion is present:

• Carbonate ions (\( \mathrm{CO}_3^{2-} \)) release \( \mathrm{CO}_2 \), a neutral and non-reactive gas with certain metals and solutions.
• Sulfite ions (\( \mathrm{SO}_3^{2-} \)) release \( \mathrm{SO}_2 \), a reactive gas that can cause color changes in specific reagents, like turning dichromate solutions green.

By identifying these characteristics of gases, we determine the likely anion in the original compound.

Chemical Reactions

Chemical reactions involve the transformation of substances through bond breaking and forming. In the original exercise, the anions react with dilute \(\mathrm{H}_2\mathrm{SO}_4\). This results in the release of a gas that provides diagnostic clues about the anion present.
The reaction is essentially an acid-base interaction where:

• Carbonates neutralize sulfuric acid releasing \(\mathrm{CO}_2\).
• Sulfites react to release \(\mathrm{SO}_2\), which is more reactive and can participate in further identifiable reactions.

These reactions not only produce gas but also serve as a gateway to further tests, as demonstrated by how the gases interact with baryta water and acidified dichromate solution.

Gas Evolution

Gas evolution is a reaction process where gaseous products are released. It provides valuable insight into both the presence and properties of specific ions.
In our exercise, the release of a colorless gas is a key factor in identifying the original anion. The reaction with sulfuric acid liberates either \(\mathrm{CO}_2\) or \(\mathrm{SO}_2\), depending on the anion involved:

• \(\mathrm{CO}_2\) is typically involved with carbonates and results in a simple acid-base reaction.
• \(\mathrm{SO}_2\) is released by sulfites and undergoes further reactions, like reducing dichromate solutions.

Observing the resultant gas helps us conclude which anion was initially present in the compound.

Reduction Reaction

Reduction reactions involve the gain of electrons by a molecule, atom, or ion. They are one half of a redox (reduction-oxidation) process in chemistry.
In this scenario, the colorless gas \(\mathrm{SO}_2\) plays a role in reducing dichromate ions \(\mathrm{Cr}_2\mathrm{O}_7^{2-}\):

• Upon interaction with \(\mathrm{SO}_2\), dichromate ions reduce to chromium ions \(\mathrm{Cr}^{3+}\).
• \(\mathrm{Cr}^{3+}\) is responsible for turning the solution green, signaling a reduction has occurred.

Recognizing these changes, the reaction provides a clear indication of the type of gas and ultimately the anion present, pinpointing \(\mathrm{SO}_3^{2-}\) as the initial anion.