Problem 4 - Chemistry in 30 days

Question

\(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{CrO}_{7}\) 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 \(\mathrm{Mg}_{3} \mathrm{~N}_{2}\) with \(\mathrm{H}_{2} \mathrm{O}\) (d) heating \(\mathrm{H}_{2} \mathrm{O}_{2}\) on \(\mathrm{NaNO}_{2}\)

Step-by-Step Solution

Answer

The gas is nitrogen. The same gas is obtained by heating \\(\mathrm{NH}_4 \mathrm{NO}_2\\) (option a).

Step 1: Identify the Reaction

When \((\mathrm{NH}_4)_2 \mathrm{CrO}_7\) is heated, it decomposes to form chromium(III) oxide, water vapor, and nitrogen gas. The reaction can be written as: \((\mathrm{NH}_4)_2 \mathrm{CrO}_7 \rightarrow \mathrm{Cr}_2\mathrm{O}_3 + 4\mathrm{H}_2\mathrm{O} + \mathrm{N}_2\). The gas liberated is nitrogen.

Step 2: Examine Each Option

We need to find which of the options liberates nitrogen gas upon reaction. Let's examine each option:(a) Heating \(\mathrm{NH}_4 \mathrm{NO}_2\), which decomposes to \((\mathrm{NH}_4 \mathrm{NO}_2 \rightarrow 2\mathrm{H}_2\mathrm{O} + \mathrm{N}_2)\).(b) Heating \(\mathrm{NH}_4 \mathrm{NO}_3\), which decomposes to \((\mathrm{NH}_4 \mathrm{NO}_3 \rightarrow \mathrm{N}_2\mathrm{O} + 2\mathrm{H}_2\mathrm{O})\).(c) Treating \(\mathrm{Mg}_3 \mathrm{N}_2\) with \(\mathrm{H}_2 \mathrm{O}\) gives \((\mathrm{Mg}_3 \mathrm{N}_2 + 6\mathrm{H}_2\mathrm{O} \rightarrow 3\mathrm{Mg}(\mathrm{OH})_2 + 2\mathrm{NH}_3)\).(d) The reaction of \(\mathrm{H}_2\mathrm{O}_2\) and \(\mathrm{NaNO}_2\) does not produce nitrogen gas.

Step 3: Determine the Correct Answer

From the analysis above, heating \(\mathrm{NH}_4 \mathrm{NO}_2\) is the reaction that also produces pure nitrogen gas, similar to the decomposition of \((\mathrm{NH}_4)_2 \mathrm{CrO}_7\). Hence, option (a) is the correct answer.

Key Concepts

Decomposition Reactions

Decomposition reactions are a type of chemical reaction where one compound breaks down into two or more simpler substances. They are essential in both natural and industrial chemical processes. These reactions typically involve the addition of energy, usually in the form of heat, to break chemical bonds.

In the context of this exercise, heating ammonium dichromate \((\mathrm{NH}_4)_2 \mathrm{CrO}_7\) is an example of a decomposition reaction. \((\mathrm{NH}_4)_2 \mathrm{CrO}_7\) breaks down into chromium(III) oxide (\(\mathrm{Cr}_2\mathrm{O}_3\)), water (\(\mathrm{H}_2\mathrm{O}\)), and nitrogen gas (\(\mathrm{N}_2\)).

Another example given in the exercise is the decomposition of ammonium nitrite (\(\mathrm{NH}_4 \mathrm{NO}_2\)), which also yields nitrogen gas and water. Both reactions highlight how complex molecules can disintegrate into simpler common substances.

• Ammonium dichromate decomposition: \( (\mathrm{NH}_4)_2 \mathrm{CrO}_7 \rightarrow \mathrm{Cr}_2\mathrm{O}_3 + 4\mathrm{H}_2\mathrm{O} + \mathrm{N}_2 \)
• Ammonium nitrite decomposition: \({\mathrm{NH}_4 \mathrm{NO}_2 \rightarrow 2\mathrm{H}_2\mathrm{O} + \mathrm{N}_2} \)

These equations show how energy is fundamental in facilitating these transformations.

Nitrogen Gas

Nitrogen gas (\(\mathrm{N}_2\)) is a diatomic molecule, meaning it consists of two nitrogen atoms bonded together. It is colorless, odorless, and mostly inert, making up about 78% of the Earth's atmosphere. Its inert nature is due to the strong triple bond between the nitrogen atoms, which requires a significant amount of energy to break.

In chemical reactions, nitrogen gas is often liberated as a stable byproduct due to its inert nature. In the decomposition reactions of ammonium dichromate and ammonium nitrite, nitrogen gas is produced.

The release of nitrogen gas is noteworthy because of its stability and the significant energy required to dissociate nitrogen bonds. It's used industrially in processes such as the Haber process for ammonia synthesis and is crucial in preserving perishable goods due to its inertness.

Ammonium Dichromate

Ammonium dichromate (\((\mathrm{NH}_4)_2 \mathrm{CrO}_7\)) is a bright orange-red crystalline solid. It is known for its dramatic decomposition reaction, which is often used in laboratory demonstrations to simulate volcanic eruptions.

When heated, ammonium dichromate undergoes decomposition, releasing nitrogen gas and leaving behind chromium(III) oxide, a green residue. The process is highly exothermic, meaning it releases a great deal of energy, making it visually impressive.

• Formula: \( (\mathrm{NH}_4)_2 \mathrm{CrO}_7 \)
• Color: Bright orange-red
• Decomposition products: Nitrogen gas and chromium(III) oxide among others

Due to its oxidizing properties, it is rarely used outside of educational settings. The reaction is also a powerful demonstration of the concepts of catalytic decomposition and transformation of matter.