Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Ammonium dichromate is used in some fire works. The green coloured powder blown in the air is (a) \(\mathrm{CrO}_{3}\) (b) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{Cr}\) (d) \(\mathrm{CrO}\left(\mathrm{O}_{2}\right)\)

Step-by-Step Solution

Answer

The green colored powder is \( \text{Cr}_2\text{O}_3 \), option (b).

Step 1: Understanding Ammonium Dichromate formula

Ammonium dichromate is represented by the formula \( ext{(NH}_4)_2 ext{Cr}_2 ext{O}_7\). Upon heating, it decomposes.

Step 2: Decomposition reaction

When ammonium dichromate \(( ext{(NH}_4)_2 ext{Cr}_2 ext{O}_7)\) is ignited, it undergoes a decomposition reaction that is strongly exothermic and produces sparks. The chemical reaction is: \[ ( ext{NH}_4)_2 ext{Cr}_2 ext{O}_7 ightarrow ext{Cr}_2 ext{O}_3 + ext{N}_2 + 4 ext{H}_2 ext{O} \].

Step 3: Identifying the green powder

The decomposition of ammonium dichromate results in the formation of chromium(III) oxide \( ext{(Cr}_2 ext{O}_3)\), which is a green powder that is blown into the air.

Step 4: Matching with given options

Among the options provided, \( ext{Cr}_2 ext{O}_3\) corresponds to option (b). It is the compound that forms as the green powder after ammonium dichromate decomposes.

Key Concepts

Chromium(III) Oxide

In the context of the decomposition of ammonium dichromate, chromium(III) oxide \(\mathrm{Cr}_2\mathrm{O}_3\) plays a crucial role. When ammonium dichromate is heated, it breaks down and releases various products, one of which is \(\mathrm{Cr}_2\mathrm{O}_3\). This compound is characterized by its distinctive green color. Because of this unique coloration, \(\mathrm{Cr}_2\mathrm{O}_3\) is often referred to as a component associated with fireworks. As it is dispersed into the air, the green powder creates an eye-catching visual effect.
Chromium(III) oxide is not commonly found in everyday situations. However, it is important as an industrial compound used in pigments due to its stable, intense color. Besides the fireworks applications, \(\mathrm{Cr}_2\mathrm{O}_3\) is used in the manufacturing of glass and ceramics, contributing to the distinctive shading of these materials. Understanding the properties of this compound and its formation can be very beneficial for students studying chemical reactions, particularly those involving decomposition.

Exothermic Reaction

An exothermic reaction is defined by the release of heat during the process. In the case of ammonium dichromate decomposition, this is exactly what happens. Upon initiation, the reaction quickly generates heat, which contributes to the vivid sparks noticeable during the process. Such a release of energy into the surrounding environment is typical for exothermic reactions.
In practical terms, exothermic reactions are often used to harness energy rapidly. This is one reason why ammonium dichromate has found a niche application in fireworks; the heat and light released add to the spectacle. Students should remember that not all chemical reactions release heat. This highlights the importance of understanding the energy changes involved in various reaction types.

Chemical Decomposition Reaction

A chemical decomposition reaction involves breaking down a single compound into multiple products. For ammonium dichromate, the decomposition reaction is expressed as: \[(\mathrm{NH}_4)_2\mathrm{Cr}_2\mathrm{O}_7 \rightarrow \mathrm{Cr}_2\mathrm{O}_3 + \mathrm{N}_2 + 4\mathrm{H}_2\mathrm{O}\].
This reaction showcases how a complex compound can split into simpler forms. The products of this particular decomposition are chromium(III) oxide, nitrogen gas \(\mathrm{N}_2\), and water vapour. As each product has its own set of properties, and \(\mathrm{Cr}_2\mathrm{O}_3\) is particularly significant due to its visual effect, this reaction demonstrates the transformability inherent in chemical substances.
Decomposition reactions like this one are fundamental in both natural processes and industrial applications. Understanding them allows students to gain insight into broader principles that govern chemistry, emphasizing the transformation and redistribution of matter.