Problem 33 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) on heating liberates a gas. The same gas will be obtained by (a) heating \(\mathrm{NH}_{4} \mathrm{NO}_{2}\) (b) heating \(\mathrm{NH}_{4} \mathrm{NO}_{3}\) (c) treating \(\mathrm{H}_{2} \mathrm{O}_{2}\) with \(\mathrm{NaNO}_{2}\) (d) treating \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) with \(\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

The same gas as \((NH_4)_2 Cr_2 O_7\) is obtained by heating \(NH_4 NO_2\). (Option A)

Step 1: Identify Reaction for \((NH_4)_2 Cr_2 O_7\)

On heating, \((NH_4)_2 Cr_2 O_7\) decomposes in the following manner: \((NH_4)_2 Cr_2 O_7\) \(\rightarrow\) \( N_2 + Cr_2 O_3 + 4 H_2O\). The gas liberated in this reaction is \(N_2\) (nitrogen gas).

Step 2: Analyze Option A

Option (a) involves heating \(NH_4 NO_2\). The decomposition reaction is \(NH_4 NO_2 \rightarrow N_2 + 2 H_2O\). This also produces nitrogen gas.

Step 3: Analyze Option B

Option (b) involves heating \(NH_4 NO_3\). It dissolves into \(N_2 O + 2 H_2O\), releasing nitrous oxide \(N_2 O\), not nitrogen gas \(N_2\).

Step 4: Analyze Option C

Option (c) involves treating \(H_2O_2\) with \(NaNO_2\). This reaction does not produce nitrogen gas but results in products like \(NO\), \(NaNO_3\), and \(H_2O\).

Step 5: Analyze Option D

Option (d) is about treating \(Mg_3N_2\) with water, which yields \(Mg(OH)_2\) and \(NH_3\) (ammonia gas). This does not produce nitrogen gas.

Step 6: Conclude the Correct Option

Since heating \(NH_4 NO_2\) (Option A) produces nitrogen gas, it is the correct option. Options B, C, and D result in other gases.

Key Concepts

Thermal Decomposition Reactions

In chemistry, thermal decomposition reactions occur when a compound breaks down into simpler substances due to the application of heat. This type of reaction is endothermic, meaning it requires energy to proceed.
Essentially, decomposition reactions involve the breakdown of a single compound into two or more chemical species. Often, these reactions involve the release of gases, which can be a key point for identifying what substances are produced.
For students preparing for JEE Chemistry, understanding thermal decomposition is crucial as it forms the basis for predicting the products in reactions and knowing what energy changes occur. When a compound undergoes thermal decomposition, it may transform into elements, simpler compounds, or a mixture of both.

• An example of thermal decomposition is:

  (NH_4)_2 Cr_2 O_7 
  ightarrow N_2 + Cr_2 O_3 + 4 H_2O

  
  where ammonium dichromate breaks down on heating to produce nitrogen gas (\(N_2\)), chromic oxide (\(Cr_2 O_3\)), and water (\(H_2O\)).

Ammonium Dichromate

Ammonium dichromate, \((NH_4)_2 Cr_2 O_7\), is an inorganic compound known for its vibrant orange crystals and dramatic decomposition reaction, often observed in chemistry demonstrations as the 'volcano reaction.'
Upon heating, ammonium dichromate undergoes thermal decomposition, liberating nitrogen gas (\(N_2\)), water vapor (\(H_2O\)), and a green chromium(III) oxide residue (\(Cr_2 O_3\)). This reaction illustrates the significant gas release phenomenon associated with certain thermal decompositions.
For JEE Chemistry aspirants, understanding the components and behavior of ammonium dichromate during decomposition provides insight into predicting reaction products.

• Key aspects include:
  - Ammonium dichromate is used in demonstrations because its decomposition is exothermic beyond its activation energy, effectively sustaining itself once initiated.
  - The formation of \(N_2\) gas is characteristic, making this reaction a reference point when considering the production of nitrogen gas in related decomposition reactions.

Gaseous Products in Reactions

Many chemical reactions result in the formation of gaseous products, which can dramatically influence the course of a reaction and its practical applications.
During JEE Chemistry Preparation, students must learn to identify which gases result from specific reactions, as this can be a determining factor in solving problems or predicting experimental outcomes.
In the decomposition of ammonium dichromate, nitrogen gas (\(N_2\)) is liberated. Recognizing the gaseous products helps determine reaction types and potential chemical pathways.

• Consider the following reactions that release different gases:
  - \((NH_4)_2 Cr_2 O_7\) decomposition produces nitrogen gas.
  - \(NH_4 NO_2\) decomposition also yields \(N_2\) gas.
  - \(NH_4 NO_3\), when heated, produces nitrous oxide \(N_2O\) rather than \(N_2\).
• Key points for students include:
  - Identifying reaction conditions and substrates to predict whether nitrogen, oxygen, carbon dioxide, or another gas will be produced.
  - Remembering that the gaseous output is an important indicator of the type of reaction occurring and the role of thermal dynamics in this process.