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 \(\mathrm{CO}_{3}^{2-}\) ions.

Step 1: Identify the Reactions Occurring

When the substance is treated with dilute \(\mathrm{H}_2\mathrm{SO}_4\), it produces a colorless gas. The colorless gas reacts with baryta water \((\mathrm{Ba(OH)_2})\) to produce turbidity, indicating the formation of an insoluble barium compound.

Step 2: Analyze the Gas's Reaction with Baryta Water

The turbidity observed when the gas reacts with baryta water suggests the formation of barium carbonate \((\mathrm{BaCO_3})\), which is insoluble in water. This indicates the gas is likely carbon dioxide \((\mathrm{CO_2})\). This matches the reaction: \(\mathrm{CO_2} + \mathrm{Ba(OH)_2} \rightarrow \mathrm{BaCO_3} + \mathrm{H_2O}\).

Step 3: Examine the Reaction with Acidified Dichromate

The colorless gas also turns an acidified dichromate solution green. This color change indicates the reduction of dichromate ions \((\mathrm{Cr_2O_7^{2-}})\) to chromium ions \((\mathrm{Cr^{3+}})\), a typical reaction when reducing agents like \(\mathrm{CO_2}\) (with \(\mathrm{S}^{2-}\) as a possible intermediate) are involved.

Step 4: Conclude Based on Observations

The observations match the behavior of carbonate ions \((\mathrm{CO_3^{2-}})\), as they would release \(\mathrm{CO_2}\) upon acid treatment. This \(\mathrm{CO_2}\) reacts with baryta water to form \(\mathrm{BaCO_3}\) (turbidity) and also turns acidified dichromate solution green, confirming the presence of carbonate ions.

Step 5: Determine the Correct Option

Given the observations with baryta water and acidified dichromate, the reactions are consistent with the presence of carbonate ions \((\mathrm{CO_3^{2-}})\). Therefore, the correct option is \((a)\ \mathrm{CO}_{3}^{2-}\).

Key Concepts

Gas Identification

Gas identification in chemistry involves analyzing the properties and reactions of a gas to determine its identity. Here, the substance, when treated with dilute \(\mathrm{H}_{2}\mathrm{SO}_{4}\), releases a colorless gas. A vital technique in gas identification is to observe its behavior upon interaction with other substances. For instance, the gas's reaction with baryta water, which results in turbidity, suggests the formation of an insoluble compound, giving a clue about the gas's identity.
This gas, when interacting with baryta water \((\mathrm{Ba(OH)_2})\), results in turbidity due to the formation of barium carbonate \((\mathrm{BaCO_3})\). This observation provides critical evidence pointing towards the gas being carbon dioxide \((\mathrm{CO_2})\).
Thus, gas identification often relies on the chemical reaction outcomes with known compounds to ascertain the chemical nature of an unknown gas.

Chemical Reactions

Chemical reactions describe the process where reactants transform into different products. In this specific exercise, the compound treated with \(\mathrm{H}_{2}\mathrm{SO}_{4}\) undergoes a chemical transformation, liberating a gas. The chemical reactions involved illustrate the conversion of carbonate (from the compound) into carbon dioxide.
The main steps involve the reaction of carbonates with acids like sulfuric acid, described by:
\[\mathrm{CO_3^{2-}} + \mathrm{2H^+} \rightarrow \mathrm{CO_2} + \mathrm{H_2O}\]
Additionally, the gas's reaction with baryta water is represented by:
\[\mathrm{CO_2} + \mathrm{Ba(OH)_2} \rightarrow \mathrm{BaCO_3} + \mathrm{H_2O}\]
These reactions illustrate both the release of carbon dioxide and its subsequent conversion into an insoluble carbonate, a typical chemical reaction pathway.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of protons \((\mathrm{H^+})\) between reactants. When the substance is treated with dilute sulfuric acid, an acid-base reaction occurs. The acid \((\mathrm{H}_{2}\mathrm{SO}_{4})\) donates protons to the carbonate ions \((\mathrm{CO_3^{2-}})\), leading to the formation of carbon dioxide \((\mathrm{CO_2})\) and water.
This reaction demonstrates a crucial acid-base concept: acids donate protons that enhance the process of gas evolution. Here, this is clearly seen when carbonate ions react with hydrogen ions to produce carbon dioxide and water, illustrating the fundamental concept of acids generating protons to drive chemical changes.
Acid-base reactions are pivotal in various chemical processes, including environmental systems, industrial applications, and everyday life, playing a significant role in creating solutions and reactions necessary for diverse chemical phenomena.