Problem 60 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

An orange solid \((A)\) on heating gave a green residuce \((B)\), a colourless gas \((C)\) and water vapour. The dry gas \((C)\) on passing over heated \(\mathrm{Mg}\) gave a white solid \((D) .(D)\) on reaction with water gave a gas \((E)\) which formed dense white fumes with \(\mathrm{HCl}\). Identify \((A)\) to \((E)\) and give reactions involved.

Step-by-Step Solution

Answer

(A) is ammonium dichromate; (B) is chromium(III) oxide; (C) is oxygen; (D) is magnesium oxide; (E) is ammonia.

Step 1: Identify gas (C)

The gas \(C\) is likely oxygen, \(O_2\), since it can be collected as a colorless gas and is commonly released from compounds when heated to leave behind a residue.

Step 2: Analyze reaction with Mg

The reaction of gas \(C\) (\(O_2\)) with heated \(\text{Mg}\) produces a white solid \(D\), magnesium oxide \(\text{MgO}\), by the equation: \[2Mg + O_2 \rightarrow 2MgO\]

Step 3: Reaction of D with water

Magnesium oxide \(\text{MgO}\) (solid \(D\)) reacts with water to produce magnesium hydroxide \(\text{Mg(OH)}_2\) and forms a gas \(E\) which corresponds to this reaction:\[MgO + H_2O \rightarrow Mg(OH)_2\]

Step 4: Identify gas E forming dense white fumes

The gas \(E\), being likely ammonia \(NH_3\), can form dense white fumes when reacted with hydrochloric acid \(HCl\). The white fumes are due to ammonium chloride \(NH_4Cl\):\[NH_3 + HCl \rightarrow NH_4Cl\]

Step 5: Identify orange solid A and green residue B

Since the gas \(C\) was identified as oxygen, compound \(A\) is ammonium dichromate \(\text{(NH}_4\text{)}_2\text{Cr}_2\text{O}_7\), which upon heating gives green chromium(III) oxide \(B\), with the reaction:\[(NH_4)_2Cr_2O_7 \rightarrow Cr_2O_3 + N_2 + 4H_2O\]Gas \(E\) resulting as nitrogen, supports previous steps.

Key Concepts

Gas Evolution Reactions

Gas evolution reactions are fascinating processes where gases are released during chemical reactions. These reactions often occur when a solid or liquid substance is heated or reacts with another chemical to form gas as a product. In our example, the orange solid (A), identified as ammonium dichromate \((NH_4)_2Cr_2O_7\), upon heating, undergoes decomposition to produce a green residue (B) and gases, including oxygen (C) and nitrogen. Here's a breakdown of what happens:

• Ammonium dichromate decomposes into chromium(III) oxide \(Cr_2O_3\), releasing water vapor, nitrogen, and oxygen gas.
• The oxygen gas is collected as the colorless gas identified in the problem.

These reactions are essential because they often represent steps in combustion engines, volcanic eruptions, and various industrial processes.
Understanding the way gases evolve in such reactions helps in better grasping concepts like atmospheric chemistry and pollution control.

Thermal Decomposition

Thermal decomposition is a type of chemical reaction where a compound breaks down into simpler substances when heated. In the given problem, the orange solid \(A\), ammonium dichromate \((NH_4)_2Cr_2O_7\), when heated, undergoes thermal decomposition. Here’s how thermal decomposition typically works:

• Heat provides the energy needed to break the chemical bonds within a compound.
• This results in the formation of smaller chemical species, such as gases or other substances.
• In our case, the heat breaks down ammonium dichromate into chromium(III) oxide, nitrogen, and water vapor.

Thermal decomposition reactions are significant in various applications, such as the manufacturing of cement where limestone decomposes to quicklime, and in metallurgy, where ores are reduced to pure metals.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, commonly known as redox reactions, involve the transfer of electrons from one substance to another. This concept is integral to understanding many chemical reactions, including the ones in our exercise. Specifically:

• During the reaction with magnesium, the oxygen gas (C) acts as an oxidizing agent.
• Oxygen oxidizes magnesium, resulting in the formation of magnesium oxide (MgO).
• The chemical equation for this oxidation is: \[2Mg + O_2 \rightarrow 2MgO\]

In the second reaction, magnesium oxide reacts with water:

• It forms magnesium hydroxide (Mg(OH)_2), which indicates another transformation through chemical reactions.
• The involvement of oxygen in this series highlights how redox reactions can alter states and forms of matter.

Oxidation-reduction reactions are foundational in biological processes like cellular respiration, as well as in industrial applications like rusting of iron and combustion of fuels.