Problem 114 - A Complete Resource Book in Chemistry for JEE Main

Question

In the following sequence of reaction the product formed \(\mathrm{A}\) and the necessary conditions \(\mathrm{B}\) are \(\mathrm{PbS} \stackrel{\text { Heat }}{\longrightarrow} \mathrm{A}+\mathrm{PbS} \longrightarrow{\mathrm{B}}{\longrightarrow} \mathrm{Pb}+\mathrm{SO}_{2}\), in air (a) \(\mathrm{PbSO}_{4}\) and high temperature, absence of air (b) \(\mathrm{PbO}\) and high temperature, absence of air (c) \(\mathrm{PbO}\) and low temperature (d) both a and \(b\)

Step-by-Step Solution

Answer

The correct answer is (b): PbO and high temperature, absence of air.

Step 1: Analyze the Reaction Sequence

The reaction involves the decomposition of lead sulfide (PbS) upon heating. The initial product formed ('A') when PbS is decomposed in air is likely lead oxide (PbO), because sulfur dioxide (SO₂) is given off in subsequent reactions.

Step 2: Determine the Role of the Conditions 'B'

The conditions 'B' refer to the environment necessary for PbS and 'A' (PbO) to further yield lead (Pb) and sulfur dioxide (SO₂). Given these products, it suggests the reduction of lead oxide to lead metal, a process that could occur under high temperature and absence of air to prevent further oxidation.

Step 3: Evaluate the Answer Choices

Based on the analysis, choice (b), which identifies 'A' as PbO and conditions 'B' as high temperature and absence of air, matches the described conditions. Choice (a) and 'both a and b' cannot be correct because 'A' is not lead sulfate (PbSO₄) in air, where PbO commonly forms.

Key Concepts

Decomposition

Decomposition is a type of chemical reaction where a single compound breaks down into two or more simpler substances. In the context of this reaction sequence, decomposition begins with lead sulfide (PbS). When PbS is heated, it decomposes to form lead oxide (PbO) and sulfur dioxide (SO₂). This process exemplifies thermal decomposition, as heat is used to break down the compound, PbS.

The decomposition reaction can be visually simplified as follows:

• PbS (solid) → PbO (solid) + SO₂ (gas)

Decomposition is frequently driven by the application of heat, making it a core process in many industrial applications where breaking down compounds is essential. Understanding decomposition helps predict the products formed when a compound is heated, and knowing these products is crucial for controlling reactions in chemical manufacturing. Recognizing thermal decomposition is important as it signals that heat alone facilitates the breakdown.

In our context, PbS decomposing to PbO and SO₂ sets the stage for the subsequent chemical transformations, which are vital for students learning about stepwise reactions.

Oxidation-Reduction

Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between substances, leading to changes in oxidation states. This reaction sequence involves an oxidation-reduction process where lead sulfide eventually forms elemental lead.

In redox reactions, one component is oxidized (loses electrons) while the other is reduced (gains electrons). Let’s look at the redox changes in this exercise:

• Oxidation: Sulfur from PbS is oxidized as it is converted to SO₂.
• Reduction: Lead (Pb) is reduced from PbO to elemental lead (Pb). It gains electrons as the oxygen is removed.

For the reaction to proceed efficiently, certain conditions such as high temperature and the absence of air are necessary. High temperatures facilitate the energy transfer required to break and form chemical bonds. The absence of air (oxygen) is crucial to prevent lead from being oxidized again after formation. This careful manipulation of conditions to promote redox reactions ensures that the desired product, elemental lead, is obtained.

Chemical Conditions

Chemical conditions are the specific environmental factors and external parameters that affect how chemical reactions proceed. In this exercise, the conditions referred to as 'B' include high temperature and absence of air, which are critical for successfully completing the reaction sequence.

Understanding why these conditions are necessary involves recognizing the sensitive balance required to guide reactions to desired products.

• High Temperature: Provides the energy needed to initiate and sustain the reaction. For example, it supports the decomposition of PbS and the subsequent reduction of PbO.
• Absence of Air: Ensures that no unwanted side reactions occur. Exposure to oxygen can lead to the undesired re-oxidation of lead, preventing its formation as an elemental metal.

In controlled chemical processes, managing these conditions allows for precise reaction control and optimization of product yield. In educational settings, understanding the role of chemical conditions highlights their significance in laboratory and industrial applications, influencing both the direction and outcome of chemical transformations. This insight is essential for students to master manipulation and control in practical chemistry.