Problem 96 - A Complete Resource Book in Chemistry for JEE Main

Question

An orange solid (A) on heating gives a green residue (B), a colourless gas (C) and water vapours. The dry gas (C) on passing over heated magnesium gave a white solid (D). (D) on reaction with water gives a gas (E) which gives dense white fumes with HCl. Here, (E) will be (a) \(\mathrm{N}_{2}\) (b) \(\mathrm{NO}_{2}\) (c) \(\mathrm{NH}_{3}\) (d) \(\mathrm{N}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(E) is \\(\mathrm{NH_{3}}\). Choice (c).

Step 1: Identify known chemical processes

We start by reviewing the given transformations: when an orange solid (A) is heated, it decomposes into green residue (B), water vapor, and a colorless gas (C). The key ingredient here is that (C) reacts with magnesium to form a white solid (D). Thus, we deduce that (A) could be a hydrated or oxidized compound that decomposes upon heating.

Step 2: Analyze the reaction of (C) with magnesium

Given that the dry gas (C) reacted with heated magnesium, we know it forms a white solid (D). Magnesium typically reacts with non-metal oxides, especially nitrogen oxides. Hence, (C) might be a nitrogen oxide. A reasonable hypothesis is that (C) is \(\mathrm{N_{2}O}\), as it is colorless and would yield a white solid on reacting with magnesium.

Step 3: Examine (D) reacting with water

When the white solid (D) is added to water, it produces gas (E). This aligns well with ammonia (
0\mathrm{NH_{3}}), especially knowing that ammonia is commonly produced from magnesium nitride (\mathrm{Mg_{3}N_{2}}). So, (E) is likely \(\mathrm{NH_{3}}\).

Step 4: Confirm (E) by chemical properties

The gas (E) forms dense white fumes with HCl, indicative of ammonia, which reacts with hydrochloric acid to form ammonium chloride (\(\mathrm{NH_4Cl}\)). This confirms that \(\mathrm{NH_3}\) is indeed the gas (E).

Key Concepts

Decomposition Reaction

A decomposition reaction is a type of chemical reaction where a single compound breaks down into two or more simpler substances. This type of reaction often involves heating a compound, which leads to its breakdown. In the context of the exercise, an orange solid, referred to as compound (A), undergoes decomposition when heated. The products of this reaction are a green residue, probably a new compound, a colorless gas, and water vapor. This is a classic example of a thermal decomposition reaction, where heat is required to initiate the breakdown.

• This reaction is usually marked by the production of gases and may involve a change in color or the formation of a residue.
• The identification of the decomposition products provides insights into the nature of the original compound.

Understanding decomposition reactions is crucial as they occur in a variety of natural and industrial processes.

Nitrogen Oxides

Nitrogen oxides are chemical compounds made up of nitrogen and oxygen. They are commonly found as pollutants in the atmosphere, but they also play significant roles in chemical reactions. In the exercise, the colorless gas (C) that evolves from the decomposition of the orange solid could be a nitrogen oxide. Magnesium is known to react with non-metal oxides, and nitrogen oxides fit this category well.

• Common nitrogen oxides include nitrous oxide ( _2O), nitrogen dioxide (NO_2), and nitric oxide (NO).
• Among these, nitrous oxide ( _2O) is colorless and can react with magnesium to produce solid magnesium nitride.

Nitrogen oxides have distinct properties, such as color and reactivity, which help in their identification.

Ammonia Formation

Ammonia ( NH_3) is a colorless gas with a pungent smell, commonly used as a fertilizer. It is also valuable in various industrial applications. In the exercise, ammonia is inferred as the gas produced when the solid (D) reacts with water.

• The formation of ammonia from magnesium nitride ( Mg_3N_2) occurs when it reacts with water, releasing the gas.
• This reaction is an example of how nitrides react with water to produce ammonia gas.

Ammonia's interaction with other chemicals, such as hydrochloric acid, can be used for identification, as it produces dense white fumes of ammonium chloride.

Magnesium Reaction

Magnesium is a highly reactive metal, particularly with oxygen and nitrogen compounds. In the student exercise, magnesium reacts with the colorless gas (C) to form a white solid (D).

• This solid is likely magnesium nitride ( Mg_3N_2), which forms when magnesium reduces nitrogen oxides or reacts directly with nitrogen.
• The reactivity of magnesium can create nitrides that react with water, releasing ammonia gas.

Understanding magnesium's reactions is essential, especially for recognizing compound formation and decomposition in chemical processes.

Hydrated Compounds

Hydrated compounds contain water molecules integrated into their structure. These compounds often undergo decomposition upon heating, releasing water vapor. In the exercise context, the orange solid (A) likely begins as a hydrated or oxidized form that releases water as one of the decomposition products when heated.

• Dehydration reactions are typical of hydrates, where heating leads to the loss of water and a potential color change or residue formation.
• This process is essential in many areas, including mineral processing and chemical manufacturing.

Understanding hydration and dehydration reactions aids in analyzing chemical changes and compound identities.