Problem 40 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Ammonium dichromate is used in some fireworks. 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

(b) \(Cr_2O_3\)

Step 1: Recall Decomposition of Ammonium Dichromate

Ammonium dichromate (\((NH_4)_2Cr_2O_7\)) decomposes when heated. The chemical equation for its decomposition is:\[(NH_4)_2Cr_2O_7 ightarrow Cr_2O_3 + N_2 + 4H_2O\]This reaction releases nitrogen gas and water vapor and forms chromium(III) oxide, \(Cr_2O_3\).

Step 2: Identify the Resulting Compound

Based on the chemical equation, the solid product formed is \(Cr_2O_3\), which is a green powder. This is characteristic of chromium(III) oxide, giving the green color in the fireworks reaction.

Step 3: Match the Product with the Options

Compare the resulting compound from the decomposition of ammonium dichromate with the given options. The green powder, chromium(III) oxide, corresponds to option (b) \(Cr_2O_3\).

Key Concepts

Chromium(III) Oxide

Chromium(III) oxide, with the chemical formula \(Cr_2O_3\), is a compound that forms when ammonium dichromate decomposes. This substance is noteworthy for its rich green hue, which makes it popular in various applications, such as pigments for ceramics and paints. Not only does it add vibrant color to fireworks, but it also finds usage in polishing compounds and as a catalyst in certain reactions.

Recognizing the color of chromium(III) oxide is critical in understanding its presence in chemical reactions. Green chromium(III) oxide forms as one of the products when ammonium dichromate is subjected to heat. This visual identifier can help differentiate it from other chromium compounds, which may exhibit different colors.

If you're studying the decomposition of ammonium dichromate, remembering that \(Cr_2O_3\) is a distinctive green powder that results from the reaction is useful. This attribute is especially handy for matching it to multiple-choice questions or when identifying the compounds produced in a laboratory setting.

Chemical Reactions

Chemical reactions are transformative processes where one set of chemical substances changes into another. When ammonium dichromate decomposes, it undergoes a specific type of chemical reaction. Here, heat acts as the catalyst to initiate and propel the reaction forward.

Understanding chemical reactions involves recognizing the reactants and products. In our example, ammonium dichromate \((NH_4)_2Cr_2O_7\) serves as the reactant, which breaks down into simpler substances:

• Chromium(III) oxide \(Cr_2O_3\) as the solid product.
• Nitrogen gas \(N_2\) released into the atmosphere.
• Water vapor \(H_2O\) that also escapes.

This example illustrates how chemical reactions can lead to the transformation of a compound into different states or phases, such as solid, gas, or vapor.

By balancing the chemical equation, one understands the conservation of mass and the arrangement of atoms in products and reactants. This symmetry reflects that no atoms are lost, only rearranged, during the reaction.

Decomposition Reactions

A decomposition reaction is a specific type of chemical reaction where one compound breaks down into two or more simpler compounds or elements. In the case of ammonium dichromate, the decomposition reaction is triggered by the application of heat, leading to the breakdown of the compound.

General characteristics of decomposition reactions include:

• Transformation of a single compound into multiple distinct products.
• Commonly requires energy input, such as heat or light.
• Turns more complex substances into simpler ones.

For ammonium dichromate, its decomposition produces chromium(III) oxide, nitrogen gas, and water vapor. The reaction can be summarized by the equation:\[(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O\]

The decomposition of ammonium dichromate is an iconic example, often seen in scientific demonstrations for its dramatic flame and color change. Not only does it visually demonstrate the transformation but also reinforces the understanding of how compounds can break apart and form new substances upon decomposition.