Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is produced when concentrated cinnabar is roasted in the excess of air? (a) \(\mathrm{Hg}\) (b) \(\mathrm{Fe}\) (c) \(\mathrm{Zn}\) (d) \(\mathrm{Al}\)

Step-by-Step Solution

Answer

The answer is (a) Hg.

Step 1: Understanding the Problem

We need to find out what is produced when cinnabar, which is mainly mercury(II) sulfide (HgS), is roasted in excess air.

Step 2: Identifying the Reaction

When cinnabar (HgS) is roasted in air, it reacts with oxygen (O_2) to form mercury (Hg) and sulfur dioxide (SO_2). The chemical reaction is: \[2HgS + 3O_2 ightarrow 2Hg + 2SO_2\]

Step 3: Analyzing the Products

From the reaction equation \((2HgS + 3O_2 ightarrow 2Hg + 2SO_2)\), one of the products is mercury (Hg). Sulfur dioxide is also produced, but it's not an option in the given choices.

Step 4: Selecting the Correct Answer

We found the product of roasting cinnabar is mercury (Hg), which corresponds to option (a).

Key Concepts

Cinnabar

Cinnabar is a bright red mineral that is primarily composed of mercury(II) sulfide (HgS). It has been historically important as a source of mercury, which is a valuable metal with various applications in science and industry. Cinnabar occurs naturally in a crystalline form, which can be found in areas with volcanic activity, metamorphic rocks, and hot springs.

Being the chief ore of mercury, cinnabar is extracted through mining operations. One of its most intriguing features is its vibrant red hues, which have made it a sought-after pigment for artists and designers. In the field of chemistry, cinnabar is vital for studying the conversion of mercury sulfide into elemental mercury through chemical reactions. This conversion process, known as roasting, involves heating cinnabar in the presence of air or oxygen.

When discussing cinnabar, remember:

• It is primarily composed of mercury sulfide.
• It is the largest source of elemental mercury.
• It has a distinctive red color, historically used for pigments.

Mercury Production

The process of mercury production from cinnabar involves a chemical reaction called roasting. In this context, roasting refers to heating cinnabar in the presence of oxygen, converting it into elemental mercury and sulfur dioxide (SO₂). The chemical equation for this reaction is:

\[2HgS + 3O_2 \rightarrow 2Hg + 2SO_2\]

This simplification helps us understand how mercury is obtained from its sulfide ore.

The roasting process is significant because it allows for the extraction of mercury in a relatively pure form. During this reaction, heat drives the formation of mercury droplets that can be collected and further purified if needed. Ensuring the right conditions—primarily an excess of oxygen—is critical for efficient mercury yield.

Key points regarding mercury production from cinnabar are:

• The process is a thermal decomposition involving heating with oxygen.
• It produces mercury and sulfur dioxide as end products.
• Efficiency relies on the controlled supply of oxygen and temperature.

Chemical Reactions Analysis

Understanding the chemical reactions involved in the roasting of cinnabar is essential for appreciating the changes occurring during mercury production. A chemical reaction involves the reorganization of atoms, where reactants are transformed into products through breaking and forming of bonds.

In this example, cinnabar (HgS) undergoes a reaction with oxygen (O₂) to produce mercury (Hg) and sulfur dioxide (SO₂). This reaction involves the following crucial steps:

1. The bond between mercury and sulfur in HgS breaks.
2. Mercury atoms rearrange and consolidate to form elemental mercury.
3. Sulfur atoms react with oxygen to produce sulfur dioxide.

Through this process, the total mass and number of each type of atom are conserved, reflecting the Law of Conservation of Mass. This means that the mass of reactants equals the mass of products, an important principle in chemical reactions.

Key aspects of chemical reactions analysis include:

• Reactants and products must be balanced—atoms and mass are conserved.
• Understanding bond changes is essential to predict products.
• Reactions often require energy; here, heat facilitates the breakdown and formation.