Problem 34 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

. On heating ammonium dichromate, the gas evolved is [1999-2 Marks] (a) oxygen (b) ammonia (c) nitrous oxide (d) nitrogen

Step-by-Step Solution

Answer

(d) nitrogen

Step 1: Chemical Reaction

When ammonium dichromate is heated, it decomposes into chromium(III) oxide, nitrogen gas, and water in the form of steam. The chemical equation for this reaction is:\[(NH_4)_2Cr_2O_7 (s) \rightarrow Cr_2O_3 (s) + N_2 (g) + 4H_2O (g)\]

Step 2: Identify the Gas

From the balanced chemical equation, it is clear that the gases produced during the decomposition of ammonium dichromate are nitrogen gas \(N_2\) and water vapor \(H_2O\). Since the question asks for the gas evolved, we focus on the diatomic gas \(N_2\) that is released in the gaseous state.

Key Concepts

Thermal Decomposition

Thermal decomposition is a process where a chemical compound breaks down into simpler substances when it is heated. This reaction happens because the heat provides energy required to overcome the chemical bonds in the compound.
This process is essential to many industrial and laboratory applications. Let's break down how it happens:

• Heat energy is absorbed by the compound.
• The absorbed energy causes the chemical bonds in the compound to vibrate and eventually break.
• This results in the formation of simpler, often more stable, products.

The temperature at which a compound decomposes can vary widely, depending on the strength of its chemical bonds. For instance, ammonium dichromate decomposes at a relatively moderate temperature when heated, making it a common example used for educational purposes.

Ammonium Dichromate

Ammonium dichromate is an orange crystalline compound, known for its vivid color and its use in chemical demonstrations. This compound has fascinating properties that make it a favorite in various experiments, such as the popular 'volcano' demonstration.
When it decomposes upon heating, a few things occur:

• The compound breaks down into chromium(III) oxide, nitrogen gas, and water vapor.
• This reaction is highly exothermic, meaning it releases a significant amount of heat energy.
• The decomposition produces spectacular bright green chromium(III) oxide ash, which is a vibrant contrast to its original color.

This transformation not only illustrates the concept of decomposition but also shows changes in color and state, making it a captivating demonstration of chemical reactions.

Gas Evolution

Gas evolution is a common phenomenon in many chemical reactions where a gas is released during the process. In the example of ammonium dichromate decomposition, nitrogen gas evolves as a product.
Here are some key points to know about gas evolution:

• The release of gas can often be observed as bubbling or effervescence in a reaction mixture.
• In thermal decomposition, gases like nitrogen or carbon dioxide are commonly produced.
• The evolution of gas is an important indicator of reaction progress and helps in identifying the products formed.

In the reaction we're examining, nitrogen gas is the primary evolved gas. It is released in significant quantities, demonstrating not only the decomposition of the original compound but also the physical change from solid to gas.