Problem 37 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following statement(s) is (are) correct when a mixture of \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is gently warmed with conc. \(\mathrm{H}_{2} \mathrm{SO}_{4} ?\) (a) A deep red vapour is evolved (b) The vapours when passed into \(\mathrm{NaOH}\) solution gives a yellow solution of \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\) (c) Chlorine gas is evolved (d) Chromyl chloride is formed

Step-by-Step Solution

Answer

Statements (a), (b), and (d) are correct.

Step 1: Understanding the Reactants and Reaction

When a mixture of \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is warmed with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\), the limited solubility of chloride ions can lead to the formation of chromyl chloride vapors. The chemical reaction is as follows: \( \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} + 4 \mathrm{NaCl} + 6 \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow 2 \mathrm{CrO}_{2} \mathrm{Cl}_{2} + 2 \mathrm{KHSO}_{4} + 4 \mathrm{NaHSO}_{4} + 3 \mathrm{H}_{2} \mathrm{O} \). The \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) formed is a deep red vapour.

Step 2: Analysis of Each Statement

Now, evaluate each option given the understanding of the reaction. (a) Correct: A deep red vapour, which is \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\), is evolved. (b) Correct: When \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) is passed into \(\mathrm{NaOH}\), it forms \(\mathrm{Na}_{2} \mathrm{CrO}_{4}\), which is yellow in color. (c) Incorrect: Chlorine gas is not evolved in this reaction.(d) Correct: Chromyl chloride (\(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\)) is the deep red vapor that is formed.

Step 3: Summary of Findings

Based on the analysis, statements (a), (b), and (d) are correct. These describe the formation and behavior of \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\) during and after the reaction. Statement (c) is incorrect as no chlorine gas is produced.

Key Concepts

Chemical Reactions

In chemistry, reactions are processes where substances, known as reactants, transform into different substances called products. This transformation involves breaking and forming chemical bonds.
Understanding chemical reactions requires knowing the reactants, products, and conditions of the reaction.

• A reactant is the starting material, and a product is what you end up with.
• Conditions such as temperature, pressure, and the presence of catalysts can affect the rate and outcome of the reaction.

The reaction described in the exercise involves \(\mathrm{NaCl}\) and \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) mixed with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\). When warmed, these react to produce chromyl chloride, \(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\), as red vapors.
This specific reaction exemplifies how different reactants can combine under specific conditions to form a characteristic product, here signified by the deep red color of the vapor.
Knowing the equations and predicting the products are crucial skills for mastering chemical reactions.

Chromyl Chloride

Chromyl chloride (\(\mathrm{CrO}_{2} \mathrm{Cl}_{2}\)) is a chemical compound that appears as deep red vapors. It is produced when certain chlorides are treated with potassium dichromate and sulfuric acid.
Its formation can be summarized in the following chemical equation:
\[\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7} + 4 \mathrm{NaCl} + 6 \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow 2 \mathrm{CrO}_{2} \mathrm{Cl}_{2} + 2 \mathrm{KHSO}_{4} + 4 \mathrm{NaHSO}_{4} + 3 \mathrm{H}_{2} \mathrm{O}\]
This reaction is a useful qualitative test for chloride ions in inorganic chemistry.
Some properties of chromyl chloride include:

• A distinct deep red color, making it easily identifiable.
• It is a corrosive liquid and should be handled with caution.

Formation of chromyl chloride demonstrates the intricate ways in which reaction conditions and selected reactants can determine the products formed in a chemical reaction.
Post-reaction, chromyl chloride vapors can be further reacted with sodium hydroxide, producing yellow sodium chromate (\(\mathrm{Na}_{2} \mathrm{CrO}_{4}\)).
This step implies its practical use in generating other chromium-containing compounds.

Inorganic Chemistry

Inorganic chemistry is a branch of chemistry dealing with inorganic compounds, usually those without carbon-hydrogen bonds. It covers a vast array of materials, including metals, salts, minerals, and compounds like chromyl chloride.
Inorganic chemistry focuses on the behavior and synthesis of these compounds.

• It involves the study of colorful compounds like chromyl chloride, which can indicate the presence of certain elements.
• Key reactions include redox reactions, acid-base reactions, and complexation reactions.

Studying inorganic chemistry helps us understand natural materials and develop new compounds with practical uses.
In the context of the exercise, the interesting property of chromyl chloride, being a distinct blood-red vapor, showcases how inorganic chemistry often involves visually striking reactions.
Understanding reactions like the one producing chromyl chloride aids in identifying substances and driving innovations in various fields such as catalysis, material science, and industrial chemistry.